If, from the whole series of standard electrode potentials, we single out only those electrode processes that correspond to the general equation

then we get a series of stresses of metals. In addition to metals, hydrogen is always included in this series, which makes it possible to see which metals are capable of displacing hydrogen from aqueous solutions of acids.

Table 19

A number of stresses for the most important metals are given in Table. 19. The position of a metal in a series of voltages characterizes its ability to redox interactions in aqueous solutions under standard conditions. Metal ions are oxidizing agents, and metals in the form of simple substances are reducing agents. At the same time, the further the metal is located in the series of voltages, the stronger the oxidizing agent in an aqueous solution are its ions, and vice versa, the closer the metal is to the beginning of the series, the stronger the reducing properties are exhibited by a simple substance - metal.

Electrode Process Potential

in a neutral medium it is B (see page 273). Active metals at the beginning of the series, having a potential much more negative than -0.41 V, displace hydrogen from water. Magnesium only displaces hydrogen from hot water. Metals located between magnesium and cadmium usually do not displace hydrogen from water. On the surface of these metals, oxide films are formed that have a protective effect.

Metals located between magnesium and hydrogen displace hydrogen from acid solutions. At the same time, protective films are also formed on the surface of some metals, which inhibit the reaction. So, the oxide film on aluminum makes this metal resistant not only in water, but also in solutions of certain acids. Lead does not dissolve in sulfuric acid at its concentration below , since the salt formed during the interaction of lead with sulfuric acid is insoluble and creates a protective film on the metal surface. The phenomenon of deep inhibition of metal oxidation, due to the presence of protective oxide or salt films on its surface, is called passivity, and the state of the metal in this case is called a passive state.

Metals are able to displace each other from salt solutions. The direction of the reaction is determined in this case by their mutual position in the series of voltages. Considering specific cases of such reactions, it should be remembered that active metals displace hydrogen not only from water, but also from any aqueous solution. Therefore, the mutual displacement of metals from solutions of their salts practically occurs only in the case of metals located in the row after magnesium.

The displacement of metals from their compounds by other metals was first studied in detail by Beketov. As a result of his work, he arranged the metals according to their chemical activity in a displacement series, which is the prototype of a series of metal stresses.

The mutual position of some metals in the series of voltages and in the periodic system at first glance does not correspond to each other. For example, according to the position in the periodic system, the reactivity of potassium must be greater than sodium, and sodium must be greater than lithium. In the series of voltages, lithium is the most active, and potassium occupies a middle position between lithium and sodium. Zinc and copper, according to their position in the periodic system, should have approximately equal chemical activity, but in the series of voltages, zinc is located much earlier than copper. The reason for this kind of inconsistency is as follows.

When comparing metals occupying a particular position in the periodic system, the measure of their chemical activity - reducing ability - is taken as the value of the ionization energy of free atoms. Indeed, during the transition, for example, from top to bottom along the main subgroup of group I of the periodic system, the ionization energy of atoms decreases, which is associated with an increase in their radii (i.e., with a large distance of external electrons from the nucleus) and with increasing screening of the positive charge of the nucleus by intermediate electron layers (see § 31). Therefore, potassium atoms exhibit greater chemical activity - they have stronger reducing properties - than sodium atoms, and sodium atoms are more active than lithium atoms.

When comparing metals in a series of voltages, the measure of chemical activity is taken as the work of converting a metal in a solid state into hydrated ions in an aqueous solution. This work can be represented as the sum of three terms: the energy of atomization - the transformation of a metal crystal into isolated atoms, the ionization energy of free metal atoms and the hydration energy of the formed ions. The atomization energy characterizes the strength of the crystal lattice of a given metal. The ionization energy of atoms - the detachment of valence electrons from them - is directly determined by the position of the metal in the periodic system. The energy released during hydration depends on the electronic structure of the ion, its charge and radius.

Lithium and potassium ions, having the same charge but different radii, will create unequal electric fields around them. The field generated near small lithium ions will be stronger than the field near large potassium ions. From this it is clear that lithium ions will hydrate with the release of more energy than potassium nones.

Thus, in the course of the transformation under consideration, energy is spent on atomization and ionization, and energy is released during hydration. The lower the total energy consumption, the easier the whole process will be and the closer to the beginning of the series of voltages the given metal will be located. But of the three terms of the total energy balance, only one - the ionization energy - is directly determined by the position of the metal in the periodic system. Consequently, there is no reason to expect that the mutual position of certain metals in a series of voltages will always correspond to their position in the periodic system. So, for lithium, the total energy consumption is less than for potassium, in accordance with which lithium is in the series of voltages before potassium.

For copper and zinc, the expenditure of energy for the ionization of free atoms and its gain during hydration of the ions are close. But metallic copper forms a stronger crystal lattice than zinc, which can be seen from a comparison of the melting points of these metals: zinc melts at , and copper only at . Therefore, the energy spent on the atomization of these metals is significantly different, as a result of which the total energy costs for the entire process in the case of copper are much greater than in the case of zinc, which explains the relative position of these metals in the voltage series.

When passing from water to non-aqueous solvents, the mutual position of metals in a series of voltages can change. The reason for this lies in the fact that the energy of solvation of ions of various metals varies in different ways when passing from one solvent to another.

In particular, the copper ion is very vigorously solvated in some organic solvents; this leads to the fact that in such solvents copper is located in a series of voltages up to hydrogen and displaces it from acid solutions.

Thus, in contrast to the periodic system of elements, a series of stresses in metals is not a reflection of the general Regularity, on the basis of which it is possible to give a versatile Characteristic of the chemical properties of metals. A series of voltages Characterizes only the redox ability of the electrochemical system "metal - metal ion" under strictly defined conditions: the values ​​\u200b\u200bgiven in it refer to an aqueous solution, temperature and a unit concentration (activity) of metal ions.

Li, K, Ca, Na, Mg, Al, Zn, Cr, Fe, Pb, H 2 , Cu, Ag, Hg, Au

The further to the left the metal is in the series of standard electrode potentials, the stronger the reducing agent it is, the strongest reducing agent is metallic lithium, gold is the weakest, and, conversely, the gold (III) ion is the strongest oxidizing agent, lithium (I) is the weakest .

Each metal is able to restore from salts in solution those metals that are in a series of voltages after it, for example, iron can displace copper from solutions of its salts. However, it should be remembered that alkali and alkaline earth metals will interact directly with water.

Metals, standing in the series of voltages to the left of hydrogen, are able to displace it from solutions of dilute acids, while dissolving in them.

The reducing activity of a metal does not always correspond to its position in the periodic system, because when determining the place of a metal in a series, not only its ability to donate electrons is taken into account, but also the energy expended on the destruction of the metal crystal lattice, as well as the energy expended on the hydration of ions.

Interaction with simple substances

WITH oxygen most metals form oxides - amphoteric and basic:

4Li + O 2 \u003d 2Li 2 O,

4Al + 3O 2 \u003d 2Al 2 O 3.

Alkali metals, with the exception of lithium, form peroxides:

2Na + O 2 \u003d Na 2 O 2.

WITH halogens metals form salts of hydrohalic acids, for example,

Cu + Cl 2 \u003d CuCl 2.

WITH hydrogen the most active metals form ionic hydrides - salt-like substances in which hydrogen has an oxidation state of -1.

2Na + H 2 = 2NaH.

WITH gray metals form sulfides - salts of hydrosulfide acid:

WITH nitrogen some metals form nitrides, the reaction almost always proceeds when heated:

3Mg + N 2 \u003d Mg 3 N 2.

WITH carbon carbides are formed.

4Al + 3C \u003d Al 3 C 4.

WITH phosphorus - phosphides:

3Ca + 2P = Ca 3 P 2 .

Metals can interact with each other to form intermetallic compounds :

2Na + Sb = Na 2 Sb,

3Cu + Au = Cu 3 Au.

Metals can dissolve in each other at high temperature without interaction, forming alloys.

Alloys

Alloys are called systems consisting of two or more metals, as well as metals and non-metals that have characteristic properties inherent only in the metallic state.

The properties of alloys are very diverse and differ from the properties of their components, for example, in order to make gold harder and more suitable for making jewelry, silver is added to it, and an alloy containing 40% cadmium and 60% bismuth has a melting point of 144 °C, i.e. much lower than the melting point of its components (Cd 321 °C, Bi 271 °C).

The following types of alloys are possible:

Molten metals are mixed with each other in any ratio, dissolving in each other without limit, for example, Ag-Au, Ag-Cu, Cu-Ni and others. These alloys are homogeneous in composition, have high chemical resistance, conduct electric current;

The straightened metals are mixed with each other in any ratio, however, when cooled, they delaminate, and a mass is obtained, consisting of individual crystals of components, for example, Pb-Sn, Bi-Cd, Ag-Pb and others.

Many chemical reactions involve simple substances, in particular metals. 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

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. - they 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.

Electrochemical activity series of metals (voltage range, a range of standard electrode potentials) - the sequence in which the metals are arranged in order of increasing their standard electrochemical potentials φ 0 corresponding to the metal cation reduction half-reaction Me n+ : Me n+ + nē → Me

A number of stresses characterize the comparative activity of metals in redox reactions in aqueous solutions.

Story

The sequence of the arrangement of metals in the order of change in their chemical activity in general terms was already known to alchemists. The processes of mutual displacement of metals from solutions and their surface precipitation (for example, the displacement of silver and copper from solutions of their salts by iron) were considered as a manifestation of the transmutation of elements.

Later alchemists came close to understanding the chemical side of the mutual precipitation of metals from their solutions. So, Angelus Sala in his work "Anatomy Vitrioli" (1613) came to the conclusion that the products of chemical reactions consist of the same "components" that were contained in the original substances. Subsequently, Robert Boyle proposed a hypothesis about the reasons why one metal displaces another from solution, based on corpuscular representations.

In the era of the formation of classical chemistry, the ability of elements to displace each other from compounds became an important aspect of understanding reactivity. J. Berzelius, on the basis of the electrochemical theory of affinity, built a classification of elements, dividing them into "metalloids" (now the term "non-metals" is used) and "metals" and putting hydrogen between them.

The sequence of metals according to their ability to displace each other, long known to chemists, was especially thoroughly and comprehensively studied and supplemented by N. N. Beketov in the 1860s and subsequent years. Already in 1859, he made a report in Paris on the topic "Research on the phenomena of the displacement of some elements by others." In this work, Beketov included a number of generalizations about the relationship between the mutual displacement of elements and their atomic weight, linking these processes with " the original chemical properties of the elements - what is called chemical affinity» . Beketov's discovery of the displacement of metals from solutions of their salts by hydrogen under pressure and the study of the reducing activity of aluminum, magnesium and zinc at high temperatures (metallothermy) allowed him to put forward a hypothesis about the relationship between the ability of some elements to displace others from compounds with their density: lighter simple substances are able to displace more heavy (therefore, this series is often also called Beketov displacement series, or simply Beketov series).

Without denying the significant merits of Beketov in the development of modern ideas about the activity series of metals, one should consider the notion of him as the only creator of this series common in Russian popular and educational literature to be erroneous. Numerous experimental data obtained at the end of the 19th century disproved Beketov's hypothesis. Thus, William Odling described many cases of "activity reversal". For example, copper displaces tin from a concentrated acidified solution of SnCl 2 and lead from an acidic solution of PbCl 2; it is also capable of dissolving in concentrated hydrochloric acid with the release of hydrogen. Copper, tin and lead are in the row to the right of cadmium, however, they can displace it from a boiling slightly acidified CdCl 2 solution.

The rapid development of theoretical and experimental physical chemistry pointed to another reason for the differences in the chemical activity of metals. With the development of modern concepts of electrochemistry (mainly in the works of Walter Nernst), it became clear that this sequence corresponds to a "series of voltages" - the arrangement of metals according to the value of standard electrode potentials. Thus, instead of a qualitative characteristic - the "tendency" of a metal and its ion to certain reactions - Nerst introduced an exact quantitative value characterizing the ability of each metal to pass into solution in the form of ions, and also to be reduced from ions to metal on the electrode, and the corresponding series was named a number of standard electrode potentials.

Theoretical basis

The values ​​of electrochemical potentials are a function of many variables and therefore show a complex dependence on the position of metals in the periodic system. Thus, the oxidation potential of cations increases with an increase in the atomization energy of a metal, with an increase in the total ionization potential of its atoms, and with a decrease in the hydration energy of its cations.

In the most general form, it is clear that metals at the beginning of periods are characterized by low values ​​of electrochemical potentials and occupy places on the left side of the voltage series. At the same time, the alternation of alkali and alkaline earth metals reflects the phenomenon of diagonal similarity. Metals located closer to the middle of the periods are characterized by large potential values ​​and occupy places in the right half of the series. A consistent increase in the electrochemical potential (from -3.395 V for a pair of Eu 2+ /Eu [ ] to +1.691 V for the Au + /Au pair) reflects a decrease in the reducing activity of metals (the ability to donate electrons) and an increase in the oxidizing ability of their cations (the ability to attach electrons). Thus, the strongest reducing agent is europium metal, and the strongest oxidizing agent is gold cations Au+.

Hydrogen is traditionally included in the voltage series, since the practical measurement of the electrochemical potentials of metals is carried out using a standard hydrogen electrode.

Practical use of a range of voltages

A number of voltages are used in practice for a comparative [relative] assessment of the chemical activity of metals in reactions with aqueous solutions of salts and acids and for assessing cathodic and anodic processes during electrolysis:

• Metals to the left of hydrogen are stronger reducing agents than metals to the right: they displace the latter from salt solutions. For example, the interaction Zn + Cu 2+ → Zn 2+ + Cu is possible only in the forward direction.
• Metals in the row to the left of hydrogen displace hydrogen when interacting with aqueous solutions of non-oxidizing acids; the most active metals (up to and including aluminum) - and when interacting with water.
• Metals in the row to the right of hydrogen do not interact with aqueous solutions of non-oxidizing acids under normal conditions.
• During electrolysis, metals to the right of hydrogen are released at the cathode; the reduction of metals of moderate activity is accompanied by the release of hydrogen; the most active metals (up to aluminum) cannot be isolated from aqueous solutions of salts under normal conditions.

Table of electrochemical potentials of metals

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 (M.: Vysshaya shkola, 1991) says that Nikolai Nikolaevich 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. Put forward (1865) a number of theoretical provisions on the dependence of the direction of reactions on the state of the reagents and external conditions ... Determined the heat 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 was included (unlike, for example, ureides - urea derivatives) in 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 activity series of metals, 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 of their displacement of each other from 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 of Beketov's new theory? 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 at the end of the 18th century. 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 at the beginning of the 19th century. created an electrochemical (or dualistic, from lat. dualis - "dual") the theory of 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 series, the more opposite electric charges they have and the more durable they form chemical compounds 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 electrochemical 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 quantitative value appeared that characterizes the ability of each metal to pass into solution in the form of ions, and also to be reduced 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), the Nernst equation is used. 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 (in 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. It seemed to Beketov an extremely important task to find 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 the 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. In this case, the cases of displacement of some elements by others by the “wet way”, i.e. in solutions, and by the “dry way”, i.e. during the calcination of reagents, Beketov considers jointly. 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 "a deviation from the 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, one 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.

A further summary of 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 (cited from the 1947 edition using modern terminology): “…Professor N. N. Beketov, in his essay “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.

To understand the true relationships of 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” was raging in the USSR, and the authors simply had to attribute all more or less noticeable discoveries in science exclusively to domestic scientists, and even citing 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 is a concrete example of the presentation of the history of science of those times. In Vladimir Orlov’s book “On a Courageous Thought” (M.: Molodaya Gvardiya, 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 around 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 of composition C n H2 n+2 with 18–35 carbons. The combustion reaction equation can be written in general form as follows: n H2 n +2 + (3 n+ 1)/2 O2 → n CO2 + ( n+ 1)H2O. Because 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, as you know, is 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 the heavy mercury that 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 down, 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 flame with the edge of 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 once again confirms the wise statement of the great physicist, made 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 Noel Waite's textbook "Chemical Kinetics". Relaxation method. The Eigen method, for which the author was awarded the Nobel Prize in Chemistry in 1967, 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 conventional 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 it is added 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 balance 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 ideal gas equation to calculate the state of a mixture of nitrogen oxides immediately after compression, since in this process not only the 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, and 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 an equilibrium between two nitrogen oxides is also of practical importance. At low pressure, a 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, when receiving thermal energy, for example, from a nuclear reactor, becomes very hot 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 molecule N2O4. 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.