Chemical Equilibrium, Law Of Mass-action : Chemical Affinity

In the preceding chapters chemical reactions of various kinds have been considered, without any reference to the possible cause of chemical change. In the earlier history of chemistry it seems to have been assumed that substances which were closely related to one another (e.g., mercury and gold) showed the greatest tendency to combine, hence the name affinity (from affinis, related) was given to the cause of chemical combination. When the mutual action of acids and alkalies was examined it became clear that it is, on the contrary, dissimilar substances which enter most easily into combination, and in the electrochemical theory of Berzelius, in which substances of opposite electrochemical character were regarded as most prone to combination, the antithesis of the older idea found its sharpest expression.

It was assumed by the alchemists (with the exception of Van Hel-mont) that substances were destroyed on combination, so that an acid and alkali, for example, had nothing in common with the salt produced from them. Boyle, in his Sceptical Chymist (1661), however, remarks that: "gold may be so altered, as to help to constitute several bodies, different from itself, and the other ingredients; yet it may be reduced again into the same yellow, fixed, ponderable, and malleable gold it was, before its mixture with them." He also observes that: "notwithstanding, the particles of some bodies are so closely united, yet there are some which may meet with particles of other denomination, which are disposed to be more closely united with some of them than they are amongst themselves." In this the elective character of chemical affinity is clearly expressed.

Mayow (1674) held very clear views on chemical affinity. If ammonia, he says, be added to hydrochloric acid, sal-ammoniac is produced, in which, it is true, neither acid nor alkaline properties are apparent But if this is heated with potash, the ammonia is displaced, " because the acid is capable of entering into closer union " with potash than with ammonia. To show that an acid is not destroyed on neutralisation he refers to the distillation of nitre with sulphuric acid, which displaces the nitric acid and leaves in the retort the same substance as is produced by the direct action of sulphuric acid on potash. Nitre on heating alone does not lose nitric acid, because the acid is kept down by the attraction of the potash; if sulphuric acid is added, the nitric acid comes off, "because the volatile acid ... has been expelled from the society of the alkaline salt by the more fixed vitriolic acid." Mayow gives a number of examples of this kind.

Similar views were held by Newton, who pointed, out that potash becomes moist in the air, whilst nitre remains dry, in Consequence of an attraction for moisture shown by the first substance, but not by the second. Similarly, mercury precipitates silver from its solution in nitric acid, copper in turn precipitates mercury and iron precipitates copper, because of the increasing attractions of these metals for the acid He suggested that the attractions might be electrical in character. There is still very little known of affinity, but it appears that Newton s speculation may be true.

Geoffrey (1718), and Bergman (1775), generalised the results and stated that of three substances, A, B, and C, if A has a stronger attraction for B than C has, then A is able to decompose BC completely, turning out C and forming AB. Tables of affinity were therefore drawn up, giving the order in which acids, for example, displaced each other both in solution and in the state of fusion.

Bergman's theory of elective affinities was called into question by Berthollet (Researches into the Laws of Affinity, Cairo, 1799). He pointed out that the reaction A + BC = AB + C does not always proceed to completion in one direction, as it should according to Bergman's theory. It may proceed in the opposite direction under different conditions, and in general is not complete: "in opposing the body A to the combination BC, the combination AB can never take place [completely], but the body B will be divided between the bodies A and C proportionally to the affinity and the quantity of each."

A chemical reaction, e.g., A + BC = AB + C, may proceed only to a certain point, because the opposed reaction: AB + C = A + BC can often take place under the same conditions and at the same time as the direct reaction. A state of equilibrium is then reached, when the two opposing reactions balance each other, i.e., proceed with equal speeds. This is denoted by: A + BC <=> AB + C.

Many examples of such states have already been given. Thus, steam is reduced by heated iron, giving hydrogen and oxide of iron: 3Fe + 4H₂O -> Fe₃O₄ + 4H₂. But under the same conditions, oxide of iron is reduced by hydrogen, giving iron and steam: Fe₃O₄ + 4H₂ -&gr; 3Fe + 4H₂O. The oxygen is shared between the iron and the hydrogen, and a state of equilibrium is set up when the two reactions are balanced, i.e., as much steam is decomposed as is produced in a given time: 3Fe + 4H₂O <=> Fe₃O₄ + 4H₂. Other examples are the decomposition of barium peroxide by heat: 2BaO₂ <=> 2BaO + O₂; and the dissociation of steam at high temperatures: 2H₂O <=> 2H₂ + O₂. Such reactions as the above, which can proceed in either direction, are called reversible reactions.

Dulong found that if barium sulphate is boiled with successive quantities of potassium carbonate solution it is completely converted into barium carbonate; whilst barium carbonate, when boiled with successive quantities of potassium sulphate solution, is entirely transformed into barium sulphate: the reaction is therefore reversible: BaSO₄ + K₂CO₃ <=> BaCO₃ + K₂SO₄. Both BaSO₄ and BaCO₃ are commonly supposed to be "insoluble"; they are, however, slightly soluble and the reactions proceed in solution.

Expt. 1. - Pour concentrated hydrochloric acid over crystals of 'Glauber's salt (Na_{2SO₄,10H₂O). Filter off the white residue, wash with a little water, dry on a porous plate, and heat with concentrated sulphuric acid: fumes of hydrochloric acid are evolved, hence the precipitate is sodium chloride. The two reactions are: (1) Na_{2SO₄ + 2HCl => 2NaCl+H₂SO₄; (2) 2NaCl + H₂SO₄ => Na_{2SO₄ +2HCl.

Very general statements, to the effect that all reactions are really reversible, must be accepted with reserve. Many chemical reactions appear to be irreversible under all known conditions. Magnesium burns in oxygen to form magnesium oxide: 2Mg + O₂ => 2MgO, and even at the highest temperatures this oxide appears to be stable. The oxidation of mercury in Lavoisier's experiment is a similar reaction but is reversible: 2Hg + O₂ <=> 2HgO. Again, all organic compounds burn in oxygen to produce carbon dioxide and water (if they contain only carbon, hydrogen, and possibly oxygen). Sugar burns in this way: C₁₂H₂₂O₁₁ + 12O₂ => 12CO₂ + 11H₂O. There is no trace of sugar left in equilibrium with CO₂, H₂O, and O₂, and the reaction is irreversible. Nevertheless, the reverse reaction takes place in green plants under the influence of sunlight.}}}

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