A series of voltage metals how to use. Chemistry preparation for zno and dpa complex edition

Attention! The description below is a reference material, it is not listed in this vinyl chart!

A SMALL COURSE OF ELECTROCHEMISTRY OF METALS

We have already become acquainted with the electrolysis of solutions of alkali metal chlorides and the production of metals using melts. Now let's try on a few simple experiments to study some of the laws of the electrochemistry of aqueous solutions, galvanic cells, and also get acquainted with the production of protective galvanic coatings.
Electrochemical methods are used in modern analytical chemistry and serve to determine the most important quantities in theoretical chemistry.
Finally, the corrosion of metal objects, which causes great damage national economy, in most cases is an electrochemical process.

VOLTAGE RANGE OF METALS

The fundamental link for understanding electrochemical processes is the voltage series of metals. Metals can be arranged in a row that starts with reactive and ends with the least reactive noble metals:
Li, Rb, K, Ba, Sr, Ca, Mg, Al, Be, Mn, Zn, Cr, Ga, Fe, Cd, Tl, Co, Ni, Sn, Pb, H, Sb, Bi, As, Cu, Hg, Ag, Pd, Pt, Au.
This is how, according to the latest ideas, a series of voltages for the most important metals and hydrogen. If electrodes of a galvanic cell are made from any two metals of a row, then a negative voltage will appear on the material preceding in the row.
Voltage value ( electrochemical potential) depends on the position of the element in the voltage series and on the properties of the electrolyte.
We will establish the essence of the voltage series from a few simple experiments, for which we need a current source and electrical measuring instruments.

Metal coatings, "trees" and "ice patterns" without current

Let's dissolve about 10 g of crystalline copper sulfate in 100 ml of water and immerse a steel needle or a piece of iron sheet into the solution. (We recommend that you first clean the iron to a shine with a fine emery cloth.) Through a short time iron will be covered with a reddish layer of liberated copper. The more active iron displaces the copper from the solution, with the iron dissolving as ions and the copper liberated as a metal. The process continues as long as the solution is in contact with the iron. As soon as the copper covers the entire surface of the iron, it will practically stop. In this case, a rather porous copper layer is formed, so that protective coatings cannot be obtained without the use of current.
In the following experiments, we will lower small strips of zinc and lead tin into the copper sulfate solution. After 15 minutes, take them out, rinse and examine under a microscope. We can see beautiful, ice-like patterns that are red in reflected light and consist of liberated copper. Here, too, more active metals transferred copper from the ionic to the metallic state.
In turn, copper can displace metals that are lower in the series of voltages, that is, less active. On a thin strip of sheet copper or on a flattened copper wire (having previously cleaned the surface to a shine), we apply a few drops of a solution of silver nitrate. With the naked eye, it will be possible to notice the formed blackish coating, which under a microscope in reflected light looks like thin needles and plant patterns (the so-called dendrites).
To isolate zinc without current, it is necessary to use a more active metal. Excluding metals that violently interact with water, we find magnesium in the series of stresses above zinc. We place a few drops of zinc sulfate solution on a piece of magnesium tape or on a thin chip of an electron. zinc sulfate solutionwe get it by dissolving a piece of zinc in dilute sulfuric acid. Simultaneously with zinc sulfate, add a few drops of denatured alcohol. On magnesium, after a short period of time, we notice, especially under a microscope, zinc that has separated out in the form of thin crystals.
In general, any member of the voltage series can be forced out of solution, where it is in the form of an ion, and transferred to the metallic state. However, when trying all sorts of combinations, we may be disappointed. It would seem that if a strip of aluminum is immersed in solutions of salts of copper, iron, lead and zinc, these metals should stand out on it. But this, however, does not happen. The reason for the failure lies not in an error in the series of voltages, but is based on a special inhibition of the reaction, which in this case is due to a thin oxide film on the aluminum surface. In such solutions, aluminum is called passive.

LET'S LOOK BEYOND THE SCENE

In order to formulate the patterns of the ongoing processes, we can restrict ourselves to considering cations, and exclude anions, since they themselves do not participate in the reaction. (However, the type of anions affects the rate of deposition.) If, for simplicity, we assume that both the liberated and dissolved metals give doubly charged cations, then we can write:

Me 1 + Me 2 2+ = Me 1 2+ + Me 2

moreover, for the first experiment Me 1 = Fe, Me 2 = Сu.
So, the process consists in the exchange of charges (electrons) between atoms and ions of both metals. If we separately consider (as intermediate reactions) the dissolution of iron or the precipitation of copper, we get:

Fe = Fe 2+ + 2 e --

Сu 2+ + 2 e--=Cu

Now consider the case when the metal is immersed in water or in a salt solution, with the cation of which the exchange is impossible due to its position in the series of voltages. Despite this, the metal tends to go into solution in the form of an ion. In this case, the metal atom gives up two electrons (if the metal is divalent), the surface of the metal immersed in the solution is charged negatively with respect to the solution, and a double electric layer is formed at the interface. This potential difference prevents further dissolution of the metal, so that the process soon stops.
If two different metals are immersed in a solution, then they will both be charged, but the less active one is somewhat weaker, due to the fact that its atoms are less prone to splitting off electrons.
Connect both metals with a conductor. Due to the potential difference, the flow of electrons will flow from the more active metal to the less active one, which forms the positive pole of the element. A process takes place in which the more active metal goes into solution, and the cations from the solution are released on the more noble metal.

Essence of a galvanic cell

Let us now illustrate with a few experiments the above somewhat abstract reasoning (which, moreover, is a gross simplification).
First, fill a beaker with a capacity of 250 ml to the middle with a 10% sulfuric acid solution and immerse not too small pieces of zinc and copper into it. We solder or rivet a copper wire to both electrodes, the ends of which should not touch the solution.
As long as the ends of the wire are not connected to each other, we will observe the dissolution of zinc, which is accompanied by the release of hydrogen. Zinc, as follows from the voltage series, is more active than hydrogen, so the metal can displace hydrogen from the ionic state. Both metals form an electric double layer. The potential difference between the electrodes is easiest to detect with a voltmeter. Immediately after turning on the device in the circuit, the arrow will indicate approximately 1 V, but then the voltage will quickly drop. If you connect a small light bulb to the element that consumes a voltage of 1 V, then it will light up - at first quite strongly, and then the glow will become weak.
By the polarity of the terminals of the device, we can conclude that the copper electrode is a positive pole. This can be proved even without a device by considering the electrochemistry of the process. Let us prepare a saturated solution of table salt in a small beaker or in a test tube, add about 0.5 ml of an alcohol solution of the phenolphthalein indicator and immerse both electrodes closed with a wire into the solution. Near the negative pole, a slight reddish coloration will be observed, which is caused by the formation of sodium hydroxide at the cathode.
In other experiments, one can place various pairs of metals in the cell and determine the resulting voltage. For example, magnesium and silver will give a particularly large potential difference due to the significant distance between them in a series of voltages, while zinc and iron, on the contrary, will give a very small one, less than a tenth of a volt. Using aluminum, we will not get practically any current due to passivation.
All these elements, or, as electrochemists say, circuits, have the disadvantage that when a current is taken, the voltage drops very quickly on them. Therefore, electrochemists always measure the true value of the voltage in a de-energized state using the voltage compensation method, that is, by comparing it with the voltage of another current source.
Let us consider the processes in the copper-zinc element in more detail. At the cathode, zinc goes into solution according to the following equation:

Zn = Zn2+ + 2 e --

Sulfuric acid hydrogen ions are discharged on the copper anode. They attach electrons coming through the wire from the zinc cathode and as a result, hydrogen bubbles are formed:

2H + + 2 e-- \u003d H 2

After a short period of time, copper will be covered with a thin layer of hydrogen bubbles. In this case, the copper electrode will turn into a hydrogen electrode, and the potential difference will decrease. This process is called electrode polarization. The polarization of the copper electrode can be eliminated by adding a little potassium dichromate solution to the cell after the voltage drop. After that, the voltage will increase again, since potassium dichromate will oxidize hydrogen to water. Potassium dichromate acts in this case as a depolarizer.
In practice, galvanic circuits are used, the electrodes of which are not polarized, or circuits, the polarization of which can be eliminated by adding depolarizers.
As an example of a non-polarizable element, consider the Daniell element, which was often used in the past as a current source. This is also a copper-zinc element, but both metals are immersed in different solutions. The zinc electrode is placed in a porous clay cell filled with dilute (about 20%) sulfuric acid. The clay cell is suspended in a large beaker containing a concentrated solution of copper sulfate, and at the bottom there is a layer of copper sulfate crystals. The second electrode in this vessel is a cylinder of copper sheet.
This element can be made from a glass jar, a commercially available clay cell (in extreme cases, use a flower pot, closing the hole in the bottom) and two electrodes of suitable size.
During the operation of the element, zinc dissolves with the formation of zinc sulfate, and copper ions are released on the copper electrode. But at the same time, the copper electrode is not polarized and the element gives a voltage of about 1 V. Actually, theoretically, the voltage at the terminals is 1.10 V, but when taking the current, we measure a slightly lower value, due to the electrical resistance of the cell.
If we do not remove the current from the cell, we must remove the zinc electrode from the sulfuric acid solution, because otherwise it will dissolve to form hydrogen.
A diagram of a simple cell, which does not require a porous partition, is shown in the figure. The zinc electrode is located in the glass jar at the top, and the copper electrode is located near the bottom. The entire cell is filled with a saturated sodium chloride solution. At the bottom of the jar we pour a handful of copper sulfate crystals. The resulting concentrated solution of copper sulfate will mix with the common salt solution very slowly. Therefore, during the operation of the cell, copper will be released on the copper electrode, and zinc in the form of sulfate or chloride will dissolve in the upper part of the cell.
Batteries now use almost exclusively dry cells, which are more convenient to use. Their ancestor is the Leclanchet element. The electrodes are a zinc cylinder and a carbon rod. The electrolyte is a paste that mainly consists of ammonium chloride. Zinc dissolves in the paste, and hydrogen is released on coal. To avoid polarization, the carbon rod is lowered into a linen bag with a mixture of coal powder and pyrolusite. The carbon powder increases the surface of the electrode, and the pyrolusite acts as a depolarizer, slowly oxidizing the hydrogen.
True, the depolarizing ability of pyrolusite is weaker than that of the previously mentioned potassium dichromate. Therefore, when current is received in dry cells, the voltage drops rapidly, they " get tired"due to polarization. Only after some time does the oxidation of hydrogen occur with pyrolusite. Thus, the elements" rest", if you do not pass current for some time. Let's check this on a flashlight battery, to which we connect a light bulb. Parallel to the lamp, that is, directly to the terminals, we connect a voltmeter.
At first, the voltage will be about 4.5 V. (Most often, three cells are connected in series in such batteries, each with a theoretical voltage of 1.48 V.) After a while, the voltage will drop, the light bulb will weaken. By reading the voltmeter, we can judge how long the battery needs to rest.
A special place is occupied by regenerating elements, known as accumulators. Reversible reactions take place in them, and they can be recharged after the cell is discharged by connecting to an external DC source.
Currently, lead-acid batteries are the most common; in them, the electrolyte is dilute sulfuric acid, into which two lead plates are immersed. The positive electrode is coated with lead dioxide PbO 2 , the negative electrode is metallic lead. The voltage at the terminals is approximately 2.1 V. During discharge, lead sulfate is formed on both plates, which again turns into metallic lead and into lead peroxide during charging.

PLATED COATINGS

The precipitation of metals from aqueous solutions with the help of an electric current is the reverse process of electrolytic dissolution, which we met when considering galvanic cells. First of all, let us examine the precipitation of copper, which is used in a copper coulometer to measure the amount of electricity.

Metal is deposited by current

Having bent the ends of two plates of thin sheet copper, we hang them on opposite walls of a beaker or, better, a small glass aquarium. We attach the wires to the plates with terminals.
Electrolyte prepare according to the following recipe: 125 g of crystalline copper sulfate, 50 g of concentrated sulfuric acid and 50 g of alcohol (denatured alcohol), the rest is water up to 1 liter. To do this, first dissolve copper sulfate in 500 ml of water, then carefully, in small portions, add sulfuric acid (Heating! Liquid may splash!), then pour in alcohol and bring water to a volume of 1 liter.
We fill the coulometer with the prepared solution and include a variable resistance, an ammeter and a lead battery in the circuit. With the help of resistance, we adjust the current so that its density is 0.02-0.01 A/cm 2 of the electrode surface. If the copper plate has an area of ​​​​50 cm 2, then the current strength should be in the range of 0.5-1 A.
After some time, light red metallic copper will begin to precipitate at the cathode (negative electrode), and copper will go into solution at the anode (positive electrode). To clean the copper plates, we will pass a current in the coulometer for about half an hour. Then we take out the cathode, dry it carefully with filter paper and weigh it accurately. We install an electrode in the cell, close the circuit with a rheostat and maintain a constant current, for example 1 A. After an hour, we open the circuit and weigh the dried cathode again. At a current of 1 A per hour of operation, its mass will increase by 1.18 g.
Therefore, an amount of electricity equal to 1 ampere-hour, when passing through a solution, can release 1.18 g of copper. Or in general: the amount of substance released is directly proportional to the amount of electricity passed through the solution.
To isolate 1 equivalent of an ion, it is necessary to pass through the solution an amount of electricity equal to the product of the electrode charge e and the Avogadro number N A:
e*N A \u003d 1.6021 * 10 -19 * 6.0225 * 10 23 \u003d 9.65 * 10 4 A * s * mol -1 This value is indicated by the symbol F and is named after the discoverer of the quantitative laws of electrolysis Faraday number(exact value F- 96 498 A * s * mol -1). Therefore, to isolate a given number of equivalents from a solution n e through the solution, an amount of electricity equal to F*n e A * s * mol -1. In other words,
I*t =F*n e here I- current, t is the time it takes for the current to pass through the solution. In chapter " Titration Basics"It has already been shown that the number of equivalents of a substance n e is equal to the product of the number of moles by the equivalent number:
n e = n*Z Hence:

I*t = F*n*Z

In this case Z- ion charge (for Ag + Z= 1, for Cu 2+ Z= 2, for Al 3+ Z= 3, etc.). If we express the number of moles as the ratio of mass to molar mass ( n = m / M), then we get a formula that allows you to calculate all the processes that occur during electrolysis:

I*t =F*m*Z / M

Using this formula, you can calculate the current:

I = F*m*Z/(t*M)\u003d 9.65 * 10 4 * 1.18 * 2 / (3600 * 63.54) A * s * g * mol / (s * mol * g) \u003d 0.996 A

If we introduce a ratio for electrical work W email

W email = U*I*t and W email / U = I*t

then knowing the tension U, you can calculate:

W email = F*m*Z*U/M

You can also calculate how long it takes for the electrolytic release of a certain amount of a substance, or how much of a substance will be released in a certain time. During the experiment, the current density must be maintained within the specified limits. If it is less than 0.01 A / cm 2, then too little metal will be released, since copper (I) ions will be partially formed. If the current density is too high, the adhesion of the coating to the electrode will be weak, and when the electrode is removed from the solution, it may crumble.
In practice, galvanic coatings on metals are used primarily to protect against corrosion and to obtain a mirror finish.
In addition, metals, especially copper and lead, are refined by anodic dissolution and subsequent separation at the cathode (electrolytic refining).
To plate iron with copper or nickel, you must first thoroughly clean the surface of the object. To do this, polish it with elutriated chalk and sequentially degrease it with a dilute solution of caustic soda, water and alcohol. If the object is covered with rust, it is necessary to pickle it in advance in a 10-15% sulfuric acid solution.
We will hang the cleaned product in an electrolytic bath (a small aquarium or a beaker), where it will serve as a cathode.
The solution for applying copper plating contains 250 g of copper sulfate and 80-100 g of concentrated sulfuric acid in 1 liter of water (Caution!). In this case, a copper plate will serve as the anode. The surface of the anode should be approximately equal to the surface of the coated object. Therefore, you must always ensure that the copper anode hangs in the bath at the same depth as the cathode.
The process will be carried out at a voltage of 3-4 V (two batteries) and a current density of 0.02-0.4 A/cm 2 . The temperature of the solution in the bath should be 18-25 °C.
Pay attention to the fact that the plane of the anode and the surface to be coated are parallel to each other. It is better not to use objects of complex shape. By varying the duration of electrolysis, it is possible to obtain a copper coating of different thicknesses.
Preliminary copper plating is often resorted to in order to apply a durable coating of another metal to this layer. This is especially often used in iron chromium plating, zinc casting nickel plating and in other cases. True, very toxic cyanide electrolytes are used for this purpose.
To prepare an electrolyte for nickel plating, dissolve 25 g of crystalline nickel sulfate, 10 g of boric acid or 10 g of sodium citrate in 450 ml of water. Sodium citrate can be prepared by neutralizing a solution of 10 g of citric acid with a dilute solution of caustic soda or a solution of soda. Let the anode be a nickel plate of the largest possible area, and take the battery as a voltage source.
The value of the current density with the help of a variable resistance will be maintained equal to 0.005 A/cm 2 . For example, with an object surface of 20 cm 2, it is necessary to work at a current strength of 0.1 A. After half an hour of work, the object will already be nickel plated. Take it out of the bath and wipe it with a cloth. However, it is better not to interrupt the nickel plating process, since then the nickel layer may passivate and the subsequent nickel coating will not adhere well.
In order to achieve a mirror shine without mechanical polishing, we introduce a so-called brightening additive into the plating bath. Such additives are, for example, glue, gelatin, sugar. You can enter into a nickel bath, for example, a few grams of sugar and study its effect.
To prepare an electrolyte for iron chromium plating (after preliminary copper plating), we dissolve 40 g of CrO 3 chromic anhydride (Caution! Poison!) and exactly 0.5 g of sulfuric acid in 100 ml of water (by no means more!). The process proceeds at a current density of about 0.1 A/cm 2 , and a lead plate is used as the anode, the area of ​​which should be slightly less than the area of ​​the chromium-plated surface.
Nickel and chrome baths are best heated slightly (up to about 35 °C). Let us pay attention to the fact that electrolytes for chromium plating, especially with a long process and high strength current emit fumes containing chromic acid, which are very harmful to health. Therefore, chrome plating should be carried out under draft or outdoors, for example on a balcony.
In chromium plating (and, to a lesser extent, in nickel plating), not all of the current is used for metal deposition. At the same time, hydrogen is released. On the basis of a series of voltages, it would be expected that metals in front of hydrogen should not be released from aqueous solutions at all, but, on the contrary, less active hydrogen should be released. However, here, as in the case of anodic dissolution of metals, the cathodic evolution of hydrogen is often inhibited and is observed only at high voltage. This phenomenon is called hydrogen overvoltage, and it is especially large, for example, on lead. Due to this circumstance, a lead battery can function. When the battery is charged, instead of PbO 2, hydrogen should appear on the cathode, but, due to overvoltage, hydrogen evolution begins when the battery is almost fully charged.

In chemistry textbooks, when presenting the topic "Acids", in one form or another, the so-called displacement series of metals is mentioned, the compilation of which is often attributed to Beketov.

For example, G. E. Rudzitis and F. G. Feldman, the once most widespread textbook for the 8th grade (from 1989 to 1995, it was published with a total circulation of 8.3 million copies), says the following. It is easy to verify from experience that magnesium reacts quickly with acids (using hydrochloric acid as an example), zinc reacts somewhat more slowly, iron even more slowly, and copper does not react with hydrochloric acid. “Similar experiments were carried out by the Russian scientist N. N. Beketov,” the authors of the textbook write further. – On the basis of experiments, he compiled a displacement series of metals: K, Na, Mg, Al, Zn, Fe, Ni, Sn, Pb (H), Cu, Hg, Ag, Pt, Au. In this series, all metals that stand before hydrogen are able to displace it from acids. It is also reported that Beketov is “the founder of physical chemistry. In 1863 he compiled a displacement series of metals, which is named after the scientist. Next, students are told that in the Beketov series, metals to the left displace metals to the right from solutions of their salts. The exception is the most active metals. Similar information can be found in other school textbooks and manuals, for example: “The Russian chemist N. N. Beketov investigated all metals and arranged them according to their chemical activity in a displacement series (activity series)”, etc.

Several questions may arise here.

Question one. Didn't chemists know before Beketov's experiments (that is, before 1863) that magnesium, zinc, iron, and a number of other metals react with acids to release hydrogen, while copper, mercury, silver, platinum, and gold do not possess this property?

Question two. Didn't chemists before Beketov notice that some metals can displace others from solutions of their salts?

Question three. In the book by V. A. Volkov, E. V. Vonsky, G. I. Kuznetsov “Outstanding chemists of the world. Biographical Reference Book (Moscow: Vysshaya shkola, 1991) says that Nikolai Nikolayevich Beketov (1827–1911) is “a Russian physical chemist, academician… one of the founders of physical chemistry… He studied the behavior of organic acids at high temperatures. Synthesized (1852) benzureide and aceturide. Nominated (1865) series theoretical positions on the dependence of the direction of reactions on the state of the reagents and external conditions ... He determined the heats of formation of oxides and chlorides of alkali metals, for the first time received (1870) anhydrous oxides of alkali metals. Using the ability of aluminum to restore metals from their oxides, he laid the foundations of aluminothermy ... President of the Russian Physico-Chemical Society .... ". And not a word about his compilation of a displacement series, which entered (in contrast, for example, from ureides - urea derivatives) into school textbooks published in millions of copies!

It is hardly necessary to blame the authors of the biographical guide for forgetting the important discovery of the Russian scientist: after all, D. I. Mendeleev, who by no means can be reproached for unpatriotism, in his classic textbook "Fundamentals of Chemistry" also never mentions Beketov's displacement series, although 15 times refers to various of his works. To answer all these questions, we will have to make an excursion into the history of chemistry, to figure out who and when proposed the metal activity series, what experiments N. N. Beketov himself conducted and what his displacement series is.

The first two questions can be answered in the following way. Of course, both the release of hydrogen from acids by metals, and various examples their displacement of each other from the salts were known long before the birth of Beketov. For example, in one of the manuals of the Swedish chemist and mineralogist Thornburn Olaf Bergman, published in 1783, it is recommended to displace lead and silver from solutions using iron plates when analyzing polymetallic ores. When carrying out calculations on the iron content in the ore, one should take into account that part of it that passed into the solution from the plates. In the same manual, Bergman writes: “Metals can be displaced from solutions of their salts by other metals, and some consistency is observed. In the series of zinc, iron, lead, tin, copper, silver and mercury, zinc displaces iron, etc.” And, of course, it was not Bergman who first discovered these reactions: such observations date back to alchemical times. The most famous example of such a reaction was used in the Middle Ages by charlatans who publicly demonstrated the "transformation" of an iron nail into red "gold" when they dipped the nail into a solution of copper sulphate. Now this reaction is demonstrated in chemistry classes at school. What is the essence new theory Beketov? Before the advent of chemical thermodynamics, chemists explained the flow of a reaction in one direction or another by the concept of the affinity of some bodies for others. The same Bergman, based on well-known displacement reactions, developed from 1775 the theory of selective affinity. According to this theory, the chemical affinity between two substances under given conditions remains constant and does not depend on the relative masses of the reactants. That is, if bodies A and B are in contact with body C, then the body that has a greater affinity for it will connect with C. For example, iron has a greater affinity for oxygen than mercury, and therefore it will be the first to be oxidized by it. It was assumed that the direction of the reaction is determined solely by the chemical affinity of the reacting bodies, and the reaction goes to the end. Bergman compiled tables of chemical affinity, which were used by chemists until the beginning of the 19th century. These tables included, in particular, various acids and bases.

Almost simultaneously with Bergman, the French chemist Claude Louis Berthollet developed another theory. Chemical affinity was also associated with the attraction of bodies to each other, but other conclusions were drawn. By analogy with the law of universal attraction, Berthollet believed that in chemistry, attraction should also depend on the mass of the reacting bodies. Therefore, the course of the reaction and its result depend not only on the chemical affinity of the reagents, but also on their quantities. For example, if bodies A and B can react with C, then body C will be distributed between A and B according to their affinities and masses, and not a single reaction will reach the end, since equilibrium will come when AC, BC and free A and B coexist simultaneously. It is very important that the distribution of C between A and B can vary depending on the excess of A or B. Therefore, with a large excess, a body with low affinity can almost completely “select” body C from its “rival”. But if one of the reaction products (AC or BC) is removed, then the reaction will go to the end and only the product that leaves the scope is formed.

Berthollet made his conclusions by observing the processes of precipitation from solutions. These conclusions sound surprisingly modern, apart from outdated terminology. However, Berthollet's theory was qualitative; it did not provide a way to measure affinity values.

Further advances in theory were based on discoveries in the field of electricity. Italian physicist Alessandro Volta late XVIII in. showed that when different metals come into contact, an electric charge arises. Conducting experiments with various pairs of metals and determining the sign and magnitude of the charge of some metals in relation to others, Volta established a series of voltages: Zn, Pb, Sn, Fe, Cu, Ag, Au. Using pairs of different metals, Volta designed a galvanic cell, the strength of which was the greater, the farther apart the members of this series were. The reason for this was unknown at the time. True, back in 1797, the German scientist Johann Wilhelm Ritter predicted that metals should be in the series of stresses in order of decreasing their ability to combine with oxygen. In the case of zinc and gold, this conclusion was not in doubt; as for other metals, it should be noted that their purity was not very high, so the Volta series does not always correspond to the modern one.

Theoretical views on the nature of the processes occurring in this case were very vague and often contradictory. The famous Swedish chemist Jöns Jakob Berzelius early XIX in. created an electrochemical (or dualistic, from lat. dualis - "dual") theory chemical compounds. In accordance with this theory, it was assumed that each chemical compound consists of two parts - positively and negatively charged. In 1811, Berzelius, based on the chemical properties of the elements known to him, arranged them in a row so that each term in it was electronegative with respect to the previous one and electropositive with respect to the next. In an abbreviated version, the following were assigned to the electronegative elements (in descending order):

O, S, N, Cl, Br, S, Se P, As, Cr, B, C, Sb, Te, Si.

Then followed the transition element - hydrogen, and after it - electropositive elements (in order of increasing this property):

Au, Pt, Hg, Ag, Cu, Bi, Sn, Pb, Cd, Co, Ni, Fe, Zn, Mn, Al, Mg, Ca, Sr, Ba, Li, Na, K.

This series, if you rewrite all the metals in reverse order, is very close to the modern one. Some differences in the order of the metals in this series are probably due to the insufficient purification of substances in the time of Berzelius, as well as some other properties of the metals that Berzelius was guided by. According to Berzelius, the farther the elements are from each other in this row, the more opposite they are. electric charges and the more durable chemical compounds they form with each other.

Berzelius' theory of dualism in the middle of the 19th century. was dominant. Its failure was shown by the founders of thermochemistry, the French scientist Marcellin Berthelot and the Danish researcher Julius Thomsen. They measured chemical affinity by the work that a chemical reaction can produce. In practice, it was measured by the heat of the reaction. These works led to the creation of chemical thermodynamics, a science that made it possible, in particular, to calculate the position of equilibrium in a reacting system, including equilibrium in an electro chemical processes. The theoretical basis for the activity series (and for the stress series) in solutions was laid at the end of the 19th century. German physical chemist Walter Nernst. Instead of a qualitative characteristic - the affinity or ability of a metal and its ion to certain reactions - an exact quantity, which characterizes the ability of each metal to pass into solution in the form of ions, as well as to recover from ions to metal on the electrode. Such a value is the standard electrode potential of the metal, and the corresponding series, arranged in order of potential changes, is called the series of standard electrode potentials. (The standard state assumes that the concentration of ions in the solution is 1 mol/l, and the gas pressure is 1 atm; most often, the standard state is calculated for a temperature of 25 ° C.)

The standard potentials of the most active alkali metals were calculated theoretically, since it is impossible to measure them experimentally in aqueous solutions. To calculate the potentials of metals at different concentrations of their ions (i.e., in non-standard states) use the Nernst equation. Electrode potentials have been determined not only for metals, but also for many redox reactions involving both cations and anions. This makes it possible to theoretically predict the possibility of a variety of redox reactions occurring under various conditions. It should also be noted that in non-aqueous solutions, the potentials of the metals will be different, so that the sequence of metals in the series may change markedly. For example, in aqueous solutions, the potential of the copper electrode is positive (+0.24 V) and copper is located to the right of hydrogen. In a solution of acetonitrile CH3CN, the copper potential is negative (–0.28 V), i.e., copper is located to the left of hydrogen. Therefore, the following reaction takes place in this solvent: Cu + 2HCl = CuCl2 + H2.

Now it's time to answer the third question and find out what exactly Beketov studied and what conclusions he came to.

One of the most prominent Russian chemists, N. N. Beketov, after graduating (in 1848) from Kazan University, worked for some time at the Medical and Surgical Academy in the laboratory of N. N. Vinin, then at St. Kharkov University. Shortly after receiving the university department of chemistry in 1857, Beketov went abroad for a year “with an appointment of a thousand rubles a year in excess of the salary received” - at that time it was a large amount. During his stay in Paris, he published (on French) the results of his earlier studies in Russia on the displacement of certain metals from solutions by hydrogen and on the reducing effect of zinc vapor. At a meeting of the Paris Chemical Society, Beketov reported on his work on the reduction of SiCl4 and BF3 with hydrogen. These were the first links in the chain of research devoted to the displacement of some elements by others, which Beketov began in 1856 and completed in 1865.

Already abroad, Beketov drew attention to himself. It is enough to quote the words of D. I. Mendeleev, whom Beketov met in Germany: “From Russian chemists abroad, I learned Beketov ... Savich, Sechenov. That's all ... people who do honor to Russia, people with whom I am glad that I got along.

In 1865, Beketov's dissertation "Research on the phenomena of displacement of some elements by others" was published in Kharkov. This work was republished in Kharkov in 1904 (in the collection “In memory of the 50th anniversary of the scientific activity of N. N. Beketov”) and in 1955 (in the collection “N. N. Beketov. Selected Works in Physical Chemistry”) .

Let's get acquainted with this work of Beketov in more detail. It consists of two parts. The first part (it contains six sections) presents the results of the author's experiments in great detail. The first three sections are devoted to the action of hydrogen on solutions of silver and mercury salts at various pressures. Beketov seemed extremely important task finding out the place of hydrogen in a series of metals, as well as the dependence of the direction of the reaction on external conditions - pressure, temperature, concentration of reagents. He conducted experiments both in solutions and with dry substances. It was well known to chemists that hydrogen easily displaces some metals from their oxides at high temperatures, but is inactive at low temperatures. Beketov found that the activity of hydrogen increases with increasing pressure, which he associated with the "greater density" of the reagent (now they would say - with a higher pressure, i.e., gas concentration).

Studying the possibility of displacing metals with hydrogen from solutions, Beketov set up a number of rather risky experiments. For the first time in the history of chemistry, Beketov applied pressures exceeding 100 atm. He conducted experiments in the dark, in sealed glass tubes with several bends (elbows). In one knee he placed a solution of salt, in the other - acid, and at the end of the tube - metallic zinc. By tilting the tube, Beketov made the zinc fall into the acid taken in excess. Knowing the mass of dissolved zinc and the volume of the tube, it was possible to estimate the achieved hydrogen pressure. In some experiments, Beketov specified the pressure by the degree of compression of air by a liquid in a thin capillary soldered to a tube. The opening of the tube was always accompanied by an explosion. In one of the experiments, in which the pressure reached 110 atm, an explosion during the opening of the tube (it was carried out in water under an overturned cylinder) shattered a thick-walled cylinder, the volume of which was a thousand times greater than the volume of the tube with reagents.

Experiments have shown that the action of hydrogen depends not only on its pressure, but also on the "strength of the metallic solution", that is, on its concentration. The reduction of silver from an ammonia solution of AgCl begins even before the complete dissolution of zinc at a pressure of about 10 atm - the transparent solution turns brown (first at the border with the gas, then throughout the mass), and after a few days gray silver powder settles on the walls. No reaction was observed at atmospheric pressure. Silver was also reduced from nitrate and sulfate, and hydrogen acted on silver acetate at atmospheric pressure. Metal balls were released from mercury salts at high pressure, but copper and lead nitrates could not be reduced even at high hydrogen pressure. The reduction of copper was observed only in the presence of silver and platinum at pressures up to 100 atm. Beketov used platinum to speed up the process, that is, as a catalyst. He wrote that platinum is more conducive to the displacement of certain metals than pressure, since hydrogen on the surface of platinum "is subject to greater attraction and should have the greatest density." Now we know that hydrogen adsorbed on platinum is activated due to its chemical interaction with metal atoms.

In the fourth section of the first part, Beketov describes experiments with carbon dioxide. He studied its effect on solutions of calcium acetate at different pressures; discovered that the reverse reaction - the dissolution of marble in acetic acid at a certain gas pressure stops even with an excess of acid.

In the last sections of the experimental part, Beketov described the effect of zinc vapor at high temperatures on compounds of barium, silicon, and aluminum (he calls the latter element clay, as was customary in those years). By reducing silicon tetrachloride with zinc, Beketov was the first to obtain sufficiently pure crystalline silicon. He also found that magnesium reduces aluminum from cryolite (sodium fluoroaluminate "in-house") and silicon from its dioxide. In these experiments, the ability of aluminum to restore barium from oxide and potassium from hydroxide was also established. So, after calcining aluminum with anhydrous barium oxide (with a small addition of barium chloride to lower the melting point), an alloy was formed, which, according to the results of the analysis, is 33.3% barium, the rest is aluminum. At the same time, many hours of calcining aluminum with powdered barium chloride did not lead to any changes.

The unusual reaction of aluminum with KOH was carried out in a curved gun barrel, in the closed end of which pieces of KOH and aluminum were placed. With a strong incandescence of this end, potassium vapor appeared, which condensed in the cold part of the barrel, "from where a few pieces of soft metal were obtained, burning with a violet flame." Rubidium and cesium were later isolated in a similar way.

The second part of Beketov's work is devoted to the theory of the displacement of some elements by others. In this part, Beketov first analyzed numerous experimental data - both his own and those conducted by other researchers, including Breslav Professor Fischer, as well as Davy, Gay-Lussac, Berzelius, Wöhler. Of particular note are "several interesting facts about the precipitation of metals by the wet route" discovered by the English chemist William Odling. At the same time, Beketov considers the cases of displacement of some elements by others "wet way", i.e. in solutions, and "dry way", i.e. during calcination of reagents. This was logical, since it is impossible to experimentally carry out reactions in aqueous solutions involving alkali and alkaline earth metals, since they actively react with water.

Then Beketov sets out his theory, designed to explain the different activity of the elements. Having arranged all the metals in a row according to their specific gravity (i.e., density), Beketov found that it agrees quite well with the known displacement series. “Consequently,” concludes Beketov, “the place of the metal ... in the displacement series can be fairly correctly determined and, so to speak, predicted in advance by its specific gravity.” Some uncertainty is observed only between "metals adjacent in specific gravity". Thus, potassium is usually a "more energetic" element and, for example, displaces sodium from NaCl when calcined, although potassium is more volatile. However, reverse processes are also known: for example, sodium can displace potassium from its hydroxide and acetate. “As for the ratio of the first alkaline group to the second and the ratio of the metals of the second group to each other, they are still little studied,” writes Beketov.

Beketov met with more serious difficulties. For example, he succeeded in reducing zinc with aluminum from a ZnCl2 solution and failed from a ZnSO4 solution. In addition, aluminum "absolutely did not restore iron, nickel, cobalt, cadmium from solutions." Beketov explained this by the fact that aluminum "acts mainly on water", and assumed that these reactions should go in the absence of water - "dry way". Indeed, later Beketov discovered such reactions and actually discovered aluminothermy.

Another difficulty was that some metals fell out of the rule of specific gravity. So, copper (density 8.9) in the activity series is located not before, but after lead (density 11.4 - Beketov's density values ​​are slightly different from modern ones). Such an "anomaly" forced Beketov to try to displace the more active lead with less active copper. He placed copper plates in hot saturated solutions of lead chloride - neutral and acidic, in an ammonia solution of lead oxide, heated copper with dry oxide and lead chloride. All experiments were unsuccessful, and Beketov was forced to admit "retreat from general rule". Other "anomalies" concerned silver (density 10.5) and lead, as well as silver and mercury (density 13.5), since both lead and mercury reduce the "lighter" silver from solutions of its salts. Beketov explained the anomaly with mercury by the fact that this metal is liquid and therefore its activity is higher than follows from the rule of specific gravity.

Beketov extended his rule to non-metals. For example, in the series chlorine (density of liquid chlorine 1.33), bromine (density 2.86), iodine (density 4.54), the lightest element is at the same time the most active (fluorine was obtained by Moissan only 20 years later). The same is observed in the series O, S, Se, Te: oxygen is the most active and quite easily displaces the rest of the elements from their compounds with hydrogen or with an alkali metal.

Beketov explained his rule by analogy with mechanics: the specific gravity is related to the mass of particles (ie, atoms) and the distance between them in a simple substance. Knowing the densities of metals and their relative atomic masses, we can calculate the relative distances between atoms. The greater the distance between them, the easier, according to Beketov, the atoms are separated in chemical processes. This is also connected with the mutual "affinity" of various elements, and the ability to displace each other from compounds. Having calculated the relative distance between atoms in different metals and taking potassium as a standard, Beketov obtained the following values: K - 100, Na - 80, Ca - 65, Mg - 53, Al - 43, etc. up to platinum.

Further summary Beketov’s theory regarding the relative strength of chemical compounds (namely, the ability of some elements to displace others is connected with this) can be found in the textbook “Fundamentals of Chemistry” by D. I. Mendeleev (quoted from the 1947 edition using modern terminology): “... Professor N. N. Beketov, in his work “Investigations on the Phenomena of Repression” (Kharkov, 1865), proposed a special hypothesis, which we will state almost in the words of the author.

For aluminum oxide Al2O3 is stronger than halides AlCl3 and AlI3. In the oxide, the ratio Al: O = 112: 100, for chloride Al: Cl = 25: 100, for iodide Al: I = 7: 100. For silver oxide Ag2O (ratio 1350: 100) is less durable than chloride (Ag: Cl = = 100: 33), and iodide is the most durable (Ag: I = 85: 100). From these and similar examples it can be seen that the most durable are those compounds in which the masses of the connecting elements become almost the same. Therefore, there is a desire of large masses to combine with large ones, and small masses - with small ones, for example: Ag2O + 2KI give K2O + 2AgI. For the same reason, Ag2O, HgO, Au2O3, and similar oxides composed of unequal masses decompose at elevated temperatures, while oxides of light metals, as well as water, do not decompose so easily. The most heat-resistant oxides - MgO, CaO, SiO2, Al2O3 approach the mass equality condition. For the same reason, HI decomposes more easily than HCl. Chlorine does not act on MgO and Al2O3, but acts on CaO, Ag2O, etc.

For understanding true relationship affinities, - concludes Mendeleev, - those additions to the mechanical theory of chemical phenomena that Beketov gives are still far from enough. Nevertheless, in his way of explaining the relative strength of many compounds, one can see a very interesting statement of questions of paramount importance. Without such attempts it is impossible to grasp the complex objects of experiential knowledge.

So, without belittling the merits of the remarkable chemist, it should be recognized that, although the theory of N. N. Beketov played a significant role in the development of theoretical chemistry, one should not attribute to him the establishment of the relative activity of metals in the reaction of displacement of hydrogen from acids and the corresponding series of activity of metals: its mechanical The theory of chemical phenomena remained in the history of chemistry as one of its many stages.

Why, then, in some books, Beketov is credited with something that he did not discover? This tradition, like many others, probably appeared in the late 1940s and early 1950s. of the 20th century, when a campaign to combat “complaining to the West” raged in the USSR, and the authors simply had to attribute all more or less noticeable discoveries in science exclusively to domestic scientists, and even quoting foreign authors was considered sedition (it was in those years that the joke about that “Russia is the birthplace of elephants”). For example, M. V. Lomonosov was credited with the discovery of the law of conservation of energy, which was discovered only in the middle of the 19th century. Here specific example presentation of the history of science of those times. In Vladimir Orlov’s book “On a Courageous Thought” (Moscow: Young Guard, 1953), inventions in the field of electricity are described in the following words: “Foreigners ruined the cradle of electric light ... The Americans stole a wonderful Russian invention ... Edison in America greedily began to improve the Russian invention ... Foreign scientists cripple an electric lamp created by the genius of the Russian people ... The American imperialists disgraced electricity ... Following them, the Yugoslav fascists disgraced electric light ... "- etc., etc. Separate echoes of those bad memory of the times, apparently, remained in some textbooks, and they should be disposed of. As one of the historians of chemistry said, "Lomonosov is great enough not to attribute other people's discoveries to him."

"The candle was burning..."

The phenomena observed during the burning of a candle are such that there is not a single law of nature that would not be affected in one way or another.

Michael Faraday. History of the candle

This story is about "experimental investigation". The main thing in chemistry is experiment. In laboratories all over the world, millions of various experiments have been and continue to be performed, but it is extremely rare for a professional researcher to do it the way some young chemists do: what if something interesting happens? Most often, the researcher has a clearly formulated hypothesis, which he seeks to either confirm or disprove experimentally. But now the experience is over, the result is obtained. If it does not agree with the hypothesis, then it is incorrect (of course, if the experiment is set up correctly and it is reproduced several times). What if it agrees? Does this mean that the hypothesis is correct and it is time to transfer it into the category of theory? A novice researcher sometimes thinks so, but an experienced one does not rush to conclusions, but first thinks firmly whether it is possible to explain the result obtained in some other way.

The history of chemistry knows thousands of examples of how such "thinking" is useful. The next three stories are just devoted to how dangerous it can be to believe that a "successful" experiment proves the correctness of the hypothesis. Sometimes in the classroom they show such an experience. A small wooden or foam circle is allowed to float in a plate of water, on which a burning candle is fixed. An inverted glass jar is lowered onto a circle with a candle and placed in this position on the bottom of the plate. After a while, the candle goes out, and part of the jar is filled with water. This experiment is supposed to show that only a fifth of the air (oxygen) supports combustion. Indeed, at first glance it looks like the water has risen by about a fifth, although more accurate measurements are not usually made. At first glance, the experiment is simple and quite convincing: after all, oxygen in the air is indeed 21% by volume. However, from the point of view of chemistry, it is not all right. Indeed, candles are made from paraffin, and paraffin consists of saturated hydrocarbons composition C n H2 n+2 with 18–35 carbons. The combustion reaction equation can be in general view write like this: n H2 n +2 + (3 n+ 1)/2 O2 → n CO2 + ( n+ 1)H2O. As n is large, then the coefficient in front of oxygen is very close to 1.5 n(for n= 18 difference between (3 n+ +1)/2 and 1.5 n will be less than 2%, for n= 30 it will be even less). Thus, for 1.5 volume of oxygen consumed, 1 volume of CO2 is released. Therefore, even if all the oxygen from the can (it is 0.21 by volume there) is used up, then instead of it, after combustion, 0.21: 1.5 = 0.14 volumes of carbon dioxide should be released. This means that water should not fill a fifth of the jar at all!

But is this reasoning correct? After all carbon dioxide is known to be highly soluble in water. Maybe it will all “go into the water”? However, the process of dissolving this gas is very slow. This was shown by special experiments: pure water in an inverted jar filled with CO2 almost does not rise in an hour. The experiment with the candle lasts less than a minute, therefore, even if oxygen is completely used up, water should enter the jar by only 0.21 - 0.1 = 0.07 of its volume (about 7%).

But that's not all. It turns out that the candle “burns” in the jar not all the oxygen, but only a small part of it. An analysis of the air in which the candle went out showed that it still contained 16% oxygen (interestingly, the oxygen content in a normal human exhalation decreases to about the same level). This means that water should not enter the jar at all! Experience, however, shows that this is not the case. How to explain it?

The simplest assumption: a burning candle heats up the air, its volume increases, and part of the air comes out of the jar. After cooling the air in the jar (this happens quite quickly), the pressure in it decreases, and water enters the jar under the action of external atmospheric pressure. In accordance with the law of ideal gases (and air in the first approximation can be considered an ideal gas), in order for the volume of air to increase by 1/5, its temperature (absolute) must also increase by 1/5, i.e. increase from 293 K (20 ° C) up to 1.2 293 = 352 K (about 80 ° C). Not so much! Heating the air with a candle flame at 60° is quite possible. It remains only to check experimentally whether air comes out of the jar during the experiment.

The first experiments, however, did not seem to confirm this assumption. So, in a series of experiments carried out with a wide-mouth jar with a volume of 0.45 l, there were no signs of “gurgling” of air from under the edge of the jar. Another unexpected observation: the water in the jar, while the candle was burning, almost did not enter.

And only after the candle went out, the water level in the inverted jar quickly rose. How to explain it?

It could be assumed that while the candle is burning, the air in the jar heats up, but at the same time, not its volume increases, but the pressure, which prevents the water from being sucked in. After the combustion stops, the air in the jar cools down, its pressure drops, and the water rises. However, this explanation does not fit. First, water is not heavy mercury, which would keep air from escaping a jar with a slight increase in pressure. (The mercury seal was once used by all physicists and chemists who studied gases.) Indeed, water is 13.6 times lighter than mercury, and the height of the water seal between the edge of the jar and the level of water in the plate is small. Therefore, even a small increase in pressure would inevitably cause air to "bubbling" through the valve.

The second objection is even more serious. Even if the water level in the plate were higher and the water would not release heated air under high pressure from the jar, then after the air in the jar cools, both its temperature and pressure would return to their original values. So there would be no reason for air to enter the jar.

The riddle was solved only by changing a small detail during the experiment. Usually the jar is “put on” on top of the candle. So, maybe this is the reason for the strange behavior of the air in the bank? A burning candle creates an upward flow of heated air, and as the jar moves from above, the hot air displaces colder air from the jar before the edge of the jar touches the water. After that, the air temperature in the jar, while the candle is burning, changes little, so the air does not leave it (and also does not go inside). And after the cessation of combustion and cooling of the hot air in the jar, the pressure in it noticeably decreases, and the external atmospheric pressure drives part of the water into the jar.

To test this assumption, in several experiments, the jar was “put on” on the candle not from above, but from the side, almost touching the edge of the flame to the jar, after which, with a quick downward movement, the jar was placed on the bottom of the plate. And immediately from under the edge of the jar, air bubbles began to rapidly emerge! Naturally, after the burning of the candle stopped, the water was sucked inward - approximately to the same level as in previous experiments.

So this experiment with a candle cannot in any way illustrate the composition of air. But he reiterates wise saying great physicist, rendered in the epigraph.

Getting closer to balance...

Let us consider one more erroneous explanation of the experiment, in which gases are also heated. This explanation has found its way into popular chemistry articles and even college textbooks. So, in a number of foreign textbooks on general chemistry, a beautiful experiment is described, the essence of which we will illustrate with a quote from the textbook by Noel Waite “ Chemical kinetics". Relaxation method. The Eigen method, for which the author was awarded in 1967 Nobel Prize in chemistry, is called the relaxation method. The reacting system reaches a state of equilibrium under certain conditions. These conditions (temperature, pressure, electric field) are then quickly violated - faster than the equilibrium is shifted. The system again comes into equilibrium, but now under new conditions; this is called "relaxing to a new equilibrium position". While relaxation is taking place, a change in some property of the system is monitored...

An experiment demonstrating the phenomenon of relaxation.

In some cases, the state of equilibrium is established so slowly under new conditions that the change in concentration can be followed with the help of ordinary laboratory equipment and thus the phenomenon of relaxation can be observed. As an example, consider the transition of nitrogen dioxide (dark brown gas) to a dimer (colorless gas):

Fill the glass gas syringe with approximately 80 cm3 of gas. Quickly press the plunger of the syringe and compress the gas to 50–60 cm3. Verify that the color of the gas has changed. First there will be a rapid darkening of the gas, as the concentration of NO2 will increase, but then there will be a slow brightening, since high pressure promotes the formation of N2O4, and equilibrium will be reached under new external conditions.

In a number of textbooks, a similar description is given to illustrate Le Chatelier's principle: with increasing gas pressure, the equilibrium shifts towards a decrease in the number of molecules, in this case towards a colorless N2O4 dimer. The text is accompanied by three color photographs. They show how, immediately after compression, the initially yellowish-brown mixture becomes dark brown, and in the third photograph, taken after a few minutes, the gas mixture in the syringe noticeably brightens.

Sometimes they add that the piston must be pressed as quickly as possible so that the balance does not have time to move during this time.

At first glance, this explanation looks very convincing. However, a quantitative examination of the processes in the syringe completely refutes all conclusions. The fact is that the indicated equilibrium between nitrogen dioxide NO2 and its dimer (nitrogen tetroxide) N2O4 is established extremely quickly: in millionths of a second! Therefore, it is impossible to compress the gas in the syringe faster than this equilibrium is established. Even if you move the piston in the steel "syringe" with the help of an explosion, the equilibrium would most likely have time to be established as the piston moves due to its inertia. How else can the phenomenon observed in this experiment be explained? Of course, a decrease in volume and a corresponding increase in the concentration of gases leads to an increase in color. But this is not the main reason. Anyone who has inflated a bicycle tube with a hand pump knows that a pump (especially an aluminum one) gets very hot. The friction of the piston on the pump tube has nothing to do with it - this is easy to verify by making a few idle swings when the air in the pump is not compressed. Heating occurs as a result of the so-called adiabatic compression - when the heat does not have time to dissipate in the surrounding space. This means that when a mixture of nitrogen oxides is compressed, it must also heat up. And when heated, the equilibrium in this mixture shifts strongly towards the dioxide.

How hot will the mixture be when compressed? In the case of air compression in the pump, the heating can be easily calculated using the adiabatic equation for an ideal gas: TVγ–1 = const, where T is the gas temperature (in kelvins), V is its volume, γ = C p / C v is the ratio of the heat capacity of a gas at constant pressure to the heat capacity at constant volume. For monatomic (noble) gases, γ = 1.66, for diatomic (air also belongs to them) γ = 1.40, for triatomic (for example, for NO2) γ = 1.30, etc. The adiabatic equation for air, compressible from volume 1 to volume 2 can be rewritten as T 2/ T 1 = (V 1/ V 2)γ–1. If the piston is sharply pushed to the middle of the pump, when the volume of air in it is halved, then for the ratio of temperatures before and after compression we obtain the equation T 2/ T 1 = = 20.4 = 1.31. And if T 1 \u003d 293 K (20 ° C), then T 2 = 294 K (111 ° C)!

It is impossible to directly apply the equation of ideal gases to calculate the state of a mixture of nitrogen oxides immediately after compression, since in this process not only volume, pressure and temperature change, but also the number of moles (NO2 N2O4 ratio) during the chemical reaction. The problem can be solved only by numerical integration of the differential equation, which takes into account that the work performed at each moment by the moving piston is spent, on the one hand, on heating the mixture, on the other hand, on the dissociation of the dimer. It is assumed that the dissociation energy of N2O4, the heat capacities of both gases, the value of γ for them, and the dependence of the equilibrium position on temperature are known (all these are tabular data). The calculation shows that if the initial mixture of gases at atmospheric pressure and room temperature is quickly compressed to half the volume, then the mixture will heat up by only 13 °C. If you compress the mixture to a threefold decrease in volume, the temperature will increase by 21 ° C. And even a slight heating of the mixture strongly shifts the equilibrium position towards the dissociation of N2O4.

And then there is just a slow cooling of the gas mixture, which causes the same slow shift of the equilibrium towards N2O4 and a weakening of the color, which is observed in the experiment. The cooling rate depends on the material of the syringe walls, their thickness and other conditions of heat exchange with the surrounding air, such as drafts in the room. It is significant that with a gradual shift of the equilibrium to the right, towards N2O4, dimerization of NO2 molecules occurs with the release of heat, which reduces the rate of cooling of the mixture (similar to the freezing of water in large reservoirs at the beginning of winter does not allow the air temperature to drop rapidly).

Why did none of the experimenters feel the heating of the syringe when they pushed the plunger in? The answer is very simple. The heat capacities of the gas mixture and glass (per unit mass) do not differ very much. But the mass of the glass piston is tens and sometimes hundreds of times higher than the mass of the gas. Therefore, even if all the heat of the cooling gas mixture is transferred to the walls of the syringe, these walls will heat up by only a fraction of a degree.

The considered system with equilibrium between two nitrogen oxides has and practical value. At low pressure, the mixture of NO2 and N2O4 liquefies easily. This makes it possible to use it as an effective coolant, despite its high chemical activity and corrosive effect on equipment. Unlike water, which thermal energy, for example, from nuclear reactor, heats up strongly and can even evaporate, the transfer of heat to a mixture of nitrogen oxides leads mainly not to its heating, but to a chemical reaction - breaking the N–N bond in the N2O4 molecule. Indeed, breaking the N–N bond in one mole of a substance (92 g) without heating it requires 57.4 kJ of energy. If such energy is transferred to 92 g of water at a temperature of 20 ° C, then 30.8 kJ will go to heat the water to a boil, and the remaining 26.6 kJ will lead to the evaporation of about 11 g of water! In the case of nitrogen oxides, the mixture does not heat up much, in the colder places of the installation the circulating mixture cools slightly, the equilibrium shifts towards N2O4, and the mixture is again ready to take heat.

What information can be obtained from a series of voltages?

A number of metal stresses are widely used in inorganic chemistry. In particular, the results of many reactions and even the possibility of their implementation depend on the position of some metal in the NRN. Let's discuss this issue in more detail.

The interaction of metals with acids

Metals that are in the series of voltages to the left of hydrogen react with acids - non-oxidizing agents. Metals located in the ERN to the right of H interact only with acids - oxidizing agents (in particular, with HNO 3 and concentrated H 2 SO 4).

Example 1. Zinc is located in the NER to the left of hydrogen, therefore, it is able to react with almost all acids:

Zn + 2HCl \u003d ZnCl 2 + H 2

Zn + H 2 SO 4 \u003d ZnSO 4 + H 2

Example 2. Copper is located in the ERN to the right of H; this metal does not react with "ordinary" acids (HCl, H 3 PO 4 , HBr, organic acids), however, it interacts with oxidizing acids (nitric, concentrated sulfuric):

Cu + 4HNO 3 (conc.) = Cu(NO 3) 2 + 2NO 2 + 2H 2 O

Cu + 2H 2 SO 4 (conc.) = CuSO 4 + SO 2 + 2H 2 O

I draw your attention to an important point: when metals interact with oxidizing acids, not hydrogen is released, but some other compounds. You can read more about this!

Interaction of metals with water

Metals located in the series of voltages to the left of Mg easily react with water already at room temperature with the release of hydrogen and the formation of an alkali solution.

Example 3. Sodium, potassium, calcium easily dissolve in water to form an alkali solution:

2Na + 2H 2 O \u003d 2NaOH + H 2

2K + 2H 2 O = 2KOH + H 2

Ca + 2H 2 O \u003d Ca (OH) 2 + H 2

Metals located in the range of voltages from hydrogen to magnesium (inclusive) in some cases interact with water, but the reactions require specific conditions. For example, aluminum and magnesium begin to interact with H 2 O only after the removal of the oxide film from the metal surface. Iron does not react with water at room temperature, but interacts with water vapor. Cobalt, nickel, tin, lead practically do not interact with H 2 O, not only at room temperature, but also when heated.

The metals located on the right side of the ERN (silver, gold, platinum) do not react with water under any circumstances.

Interaction of metals with aqueous solutions of salts

We will talk about the following types of reactions:

metal (*) + metal salt (**) = metal (**) + metal salt (*)

I would like to emphasize that the asterisks in this case do not indicate the degree of oxidation, not the valence of the metal, but simply allow us to distinguish between metal No. 1 and metal No. 2.

For such a reaction to occur, three conditions must be met simultaneously:

1. the salts involved in the process must be soluble in water (this is easy to check using the solubility table);
2. metal (*) must be in a series of voltages to the left of metal (**);
3. metal (*) should not react with water (which is also easily checked by ERN).

Example 4. Let's look at a few reactions:

Zn + CuSO 4 \u003d ZnSO 4 + Cu

K + Ni(NO 3) 2 ≠

The first reaction is easy to implement, all of the above conditions are met: copper sulfate is soluble in water, zinc is in the ERN to the left of copper, Zn does not react with water.

The second reaction is impossible, because the first condition is not met (copper (II) sulfide is practically insoluble in water). The third reaction is not feasible, since lead is a less active metal than iron (located to the right in the NRN). Finally, the fourth process will NOT result in nickel precipitation as potassium reacts with water; the resulting potassium hydroxide can react with a salt solution, but this is a completely different process.

The process of thermal decomposition of nitrates

Let me remind you that nitrates are salts of nitric acid. All nitrates decompose when heated, but the composition of the decomposition products may be different. The composition is determined by the position of the metal in the series of stresses.

Nitrates of metals located in the NER to the left of magnesium, when heated, form the corresponding nitrite and oxygen:

2KNO 3 \u003d 2KNO 2 + O 2

During the thermal decomposition of metal nitrates, located in a series of voltages from Mg to Cu inclusive, metal oxide, NO 2 and oxygen are formed:

2Cu(NO 3) 2 \u003d 2CuO + 4NO 2 + O 2

Finally, during the decomposition of nitrates of the least active metals (located in the NER to the right of copper), metal, nitrogen dioxide and oxygen are formed.

metals

In many chemical reactions simple substances, in particular metals, are involved. However, different metals exhibit different activity in chemical interactions, and it depends on this whether the reaction will proceed or not.

The greater the activity of a metal, the more vigorously it reacts with other substances. By activity, all metals can be arranged in a series, which is called the activity series of metals, or the displacement series of metals, or the series of metal voltages, as well as the electrochemical series of metal voltages. This series was first studied by the outstanding Ukrainian scientist M.M. Beketov, therefore this series is also called the Beketov series.

The activity series of Beketov's metals has the following form (the most commonly used metals are given):

K > Ca > Na > Mg > Al > Zn > Fe > Ni > Sn > Pb > > H 2 > Cu > Hg > Ag > Au.

In this series, the metals are arranged with decreasing activity. Among these metals, potassium is the most active, and gold is the least active. Using this series, you can determine which metal is more active from another. Hydrogen is also present in this series. Of course, hydrogen is not a metal, but in this series its activity is taken as a reference point (a kind of zero).

Interaction of metals with water

Metals are capable of displacing hydrogen not only from acid solutions, but also from water. Just as with acids, the activity of the interaction of metals with water increases from left to right.

Metals in the activity series up to magnesium are able to react with water under normal conditions. When these metals interact, alkalis and hydrogen are formed, for example:

Other metals that come before hydrogen in the range of activities can also interact with water, but this occurs under more severe conditions. For interaction, superheated water vapor is passed through hot metal filings. Under such conditions, hydroxides can no longer exist, so the reaction products are the oxide of the corresponding metal element and hydrogen:

The dependence of the chemical properties of metals on the place in the activity series

← metal activity increases

Displaces hydrogen from acids

Does not displace hydrogen from acids

Displace hydrogen from water, form alkalis

Displace hydrogen from water at high temperature, form oxides

3 do not interact with water

It is impossible to displace from an aqueous solution of salt

Can be obtained by displacing a more active metal from a salt solution or from an oxide melt

The interaction of metals with salts

If the salt is soluble in water, then a metal atom in it can be replaced by an atom of a more active element. If an iron plate is immersed in a solution of cuprum (II) sulfate, then after a while copper will be released on it in the form of a red coating:

But if a silver plate is immersed in a solution of cuprum (II) sulfate, then no reaction will occur:

Cuprum can be displaced by any metal that is to the left of the metal activity series. However, the metals that are at the very beginning of the series are sodium, potassium, etc. - are not suitable for this, because they are so active that they will interact not with salt, but with water in which this salt is dissolved.

The displacement of metals from salts by more active metals is widely used in industry for the extraction of metals.

Interaction of metals with oxides

Oxides of metallic elements are able to interact with metals. More active metals displace less active ones from oxides:

But, unlike the interaction of metals with salts, in this case, the oxides must be melted for the reaction to occur. For the extraction of metal from oxide, you can use any metal that is located in the activity row to the left, even the most active sodium and potassium, because water is not contained in the molten oxide.

The interaction of metals with oxides is used in industry to extract other metals. The most practical metal for this method is aluminum. It is quite widespread in nature and cheap to manufacture. You can also use more active metals (calcium, sodium, potassium), but, firstly, they are more expensive than aluminum, and secondly, due to their ultra-high chemical activity, it is very difficult to store them in factories. This method of extracting metals using aluminum is called aluminothermy.

Metals that react easily are called active metals. These include alkali, alkaline earth metals and aluminium.

Position in the periodic table

The metallic properties of the elements weaken from left to right in Mendeleev's periodic table. Therefore, elements of groups I and II are considered the most active.

Rice. 1. Active metals in the periodic table.

All metals are reducing agents and easily part with electrons at the external energy level. Active metals have only one or two valence electrons. In this case, the metallic properties are enhanced from top to bottom with an increase in the number of energy levels, because. the farther an electron is from the nucleus of an atom, the easier it is for it to separate.

Alkali metals are considered the most active:

• lithium;
• sodium;
• potassium;
• rubidium;
• cesium;
• francium.

The alkaline earth metals are:

• beryllium;
• magnesium;
• calcium;
• strontium;
• barium;
• radium.

You can find out the degree of activity of a metal by the electrochemical series of metal voltages. The more to the left of hydrogen an element is located, the more active it is. The metals to the right of hydrogen are inactive and can only interact with concentrated acids.

Rice. 2. Electrochemical series of voltages of metals.

The list of active metals in chemistry also includes aluminum, located in III group and standing to the left of hydrogen. However, aluminum is located on the border of active and medium active metals and does not react with certain substances under normal conditions.

Properties

Active metals are soft (can be cut with a knife), light, and have a low melting point.

Main Chemical properties metals are presented in the table.

Reaction	The equation	Exception
Alkali metals ignite spontaneously in air, interacting with oxygen	K + O 2 → KO 2	Lithium reacts with oxygen only at high temperatures.
Alkaline earth metals and aluminum form oxide films in air, and spontaneously ignite when heated.	2Ca + O 2 → 2CaO
React with simple substances, forming salts	Ca + Br 2 → CaBr 2; - 2Al + 3S → Al 2 S 3	Aluminum does not react with hydrogen
React violently with water, forming alkalis and hydrogen	- Ca + 2H 2 O → Ca (OH) 2 + H 2	The reaction with lithium proceeds slowly. Aluminum reacts with water only after the removal of the oxide film.
React with acids to form salts	Ca + 2HCl → CaCl 2 + H 2; 2K + 2HMnO 4 → 2KMnO 4 + H 2
React with salt solutions, first reacting with water and then with salt	2Na + CuCl 2 + 2H 2 O: 2Na + 2H 2 O → 2NaOH + H 2; - 2NaOH + CuCl 2 → Cu(OH) 2 ↓ + 2NaCl

Active metals easily react, therefore, in nature they are found only in mixtures - minerals, rocks.

Rice. 3. Minerals and pure metals.

What have we learned?

Active metals include elements of groups I and II - alkali and alkaline earth metals, as well as aluminum. Their activity is due to the structure of the atom - a few electrons are easily separated from the outer energy level. These are soft light metals that quickly react with simple and complex substances, forming oxides, hydroxides, salts. Aluminum is closer to hydrogen and requires additional conditions for its reaction with substances - high temperatures, the destruction of the oxide film.

Topic quiz

Report Evaluation

Average rating: 4.4. Total ratings received: 334.