3Cu + 6HNO3 = 3Cu(NO3)2 + 6H, 3HNO3 + 6H = 3HNO2 + 3H2O, 3HNO2 = HNO3 + 2NO + H2O, which gives, on summing up, 3Cu + 8HNO3 = 3Cu(NO3)2 + 2NO + 4H2O. Divers considered that metals may be divided into two classes with reference to their action on nitric acid. In the first class are placed the four metals copper, silver, mercury, and bismuth, and the primary products are nitrite, nitrate, and water: 1) 2Ag + HO.NO2 = AgOH + AgNO2; 2) AgOH + HNO3 = AgNO3 + H2O. Further action of the nitric acid on the nitrite produces nitrous acid and nitrate: 3) AgNO2 + HNO3 = AgNO3 + HNO2; while the nitrous acid reacts to produce nitrogen peroxide or nitric oxide according as the nitric acid is concentrated or dilute, as shown in the following equations: - 4) HNO2 + HNO3 = 2NO2 + H2O; 5) 3HNO2 = HNO3 + 2NO + H2O. Divers assumed that a very small quantity of nitrous acid is necessary to initiate the reaction and functions in a catalytic manner. There is no formation of ammonia or hydroxylamine at any stage of the reaction. The second class of metals includes zinc, magnesium, aluminium, cadmium, tin, lead, iron, and the alkali metals, and no nitrous acid is required to start their reaction with nitric acid. According to Divers, nitrous acid is not produced in appreciable amounts, because further reduction occurs which is due to the action of nascent hydrogen: HNO3 + 2H = HNO2 + H2O; (Nitrous acid.) 2HNO3 + 8H = H2N2O2 + 4H2O; (Hyponitrous acid.) HNO3 + 6H = NH2OH + 2H2O; (Hydroxylamine.) HNO3 + 8H = NH3 + 3H2O. (Ammonia.) The production of hydroxylamine and ammonia occurs chiefly with tin and zinc, although under suitable conditions other metals may give traces. The nature and quantity of both primary and secondary products depend upon the concentration of the acid. Secondary products result from the following reactions: - 3HNO2 = HNO3 + 2NO + H2O; H2N2O2 = N2O + H2O; HNO2 + NH2OH = N2O + 2H2O; HNO2 + NH3 = N2 + 2H2O. Thus the reduction products of nitric acid produced by zinc under various conditions may be shown in the following reactions: - 3Zn + 7HNO3 = 3Zn(NO3)2 + NH2OH + 2H2O; 4Zn + 9HNO3 = 4Zn(NO3)2 + NH3 + 3H2O; 4Zn + 10HNO3 = 4Zn(NO3)2 + N2O + 5H2O; 5Zn + 12HNO3 = 5Zn(NO3)2 + N2 + 6H2O. Veley suggests that nitrous acid is necessary to initiate the reaction between all metals and nitric acid, and his experiments go to prove that really pure nitric acid has no action on pure metals. There is considerable difficulty in obtaining complete purity of the reacting substances, and traces of nitrous acid are produced probably by electrolytic action set up by the most minute quantities of impurities in the metals. The very slight action of pure nitric acid of 30 per cent, concentration on pure metals was almost entirely prevented if the metals were agitated in the solution, because it was not possible for nitrous acid to concentrate round the metal. No action occurred at all if the formation of nitrous acid was inhibited by adding such substances as urea, hydrogen peroxide, potassium chlorate, etc. While Veley agrees that the metals copper, silver, mercury, and bismuth differ in their action from the rest of the metals, yet he maintains that in all cases nitrous acid is necessary to start the reaction, and that nitrous acid is the primary product in all cases. Thus, in the case of copper the reaction can be represented by the equations Cu + 4HNO2 = Cu(NO2)2 + 2NO + 2H2O, Cu(NO2)2 + 2HNO3 = Cu(NO3)2 + 2HNO2. The decomposition or formation of nitrous acid is shown by the reversible reaction 3HNO2 ⇔ HNO3 + 2NO + H2O. In the case of both classes of metals Veley found that rapid solution occurred in nitrous acid, less rapid in a mixture of nitrous and nitric acids, and much slower action still in the case of pure nitric acid solution. The only metal which produces free hydrogen is magnesium, and the nitric acid must be very dilute. As magnesium is next to the alkali metals with regard to its high solution tension, Webb suggests that hydrogen is liberated first by all metals with a higher solution tension than hydrogen, while those with a lower solution tension (copper, mercury, silver) are polarised in a solution of nitric acid. The function of nitrous acid in starting the reaction between these metals and nitric acid is that of a depolariser.

• published: 22 Jun 2014
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