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Chemical Properties of Copper

At ordinary temperatures copper is not attacked by dry air, but in presence of moisture and carbon dioxide it becomes coated with a green basic carbonate. When heated in oxygen it is converted into cuprous and cupric oxides. At temperatures above 200° to 250° C. nitrous oxide and nitric oxide convert it into cuprous oxide, and nitrogen peroxide into cupric oxide and possibly a substance Cu2NO2, although more recent work has failed to confirm the formation of the compound last mentioned. The action of air begins at 145° C., and is more energetic than that of oxygen, which begins above 60° to 80° C., a phenomenon probably originating in the formation of oxides of nitrogen. At temperatures below 200° C. dry oxygen is absorbed slightly more rapidly than the moist gas. At ordinary pressure and temperature chlorine combines with the metal to form cupric chloride and a small proportion of the cuprous salt; at a high temperature cuprous chloride is the sole product. When copper is heated in gaseous hydrogen chloride, cuprous chloride is produced, and hydrogen evolved. At 1200° C. the metal reacts slowly with carbon dioxide, forming cuprous oxide and carbon monoxide. An arc between copper poles burns in carbon dioxide almost as well as in air, but very imperfectly in coal-gas or steam.

Copper is not attacked by water at ordinary temperatures, or at 100° C., and only slightly at white heat; but very prolonged immersion in sea-water produces a superficial coating of cuprous oxide. It is insoluble in dilute sulphuric acid of 5 to 10 per cent, strength, but reacts with the concentrated acid in accordance with the equations
  1. Cu+2H2SO4=CuSO4+SO2+2H2O,
    this reaction probably taking place in the two stages
    1. Cu+H2SO4=CuSO4+2H,
    2. H2SO4+2H=SO2+2H2O;
  2. 5Cu+4H2SO4=Cu2S+3CuSO4+4H2O.
The first reaction proceeds between 0° and 270° C., the second accompanying it between 0° and 100° C. When the copper is exhausted, the cuprous sulphide is then decomposed in accordance with the equations

Cu2S+2H2SO4=CuS+CuSO4+SO2+2H2O, CuS+2H2SO4=CuSO4+SO2+S+2H2O.

The black residue formed in the reaction appears to be mainly cuprous sulphide.

Nitric acid dissolves copper, forming cupric nitrate and oxides of nitrogen. The primary process corresponds with the equation

Cu+2HNO3 =Cu(NO3)2+2H,

but hydrogen is not evolved, being oxidized to water by the nitric acid. The action is conditioned by the presence of nitrous acid as a catalyst, since in absence of this acid the velocity of solution of copper in nitric acid of moderate concentration is very small. A similar retardation is produced by addition of substances capable of reacting with nitrous acid, such as perchlorates, permanganates, hydrogen peroxide, or urea. The velocity of the reaction


is much accelerated by the presence of nitrous acid, an example of autocatalysis.

The formation of the oxides of nitrogen must be regarded as a secondary process, due to the interaction of some of the initial products. The nature of the substances formed depends on the experimental conditions, such as the quantity, concentration, and temperature of the nitric acid, and the concentration of the cupric nitrate generated. An example of this influence is afforded by the observation of Ackworth and Armstrong that the proportion of nitrous oxide formed increases with the dilution of the acid, and with the concentration of the cupric nitrate.

By employing 5N-nitric acid, Bagster obtained a gas completely soluble in sodium hydroxide and free from nitric oxide. His explanation of the mechanism of the reaction involves the assumption that nitrous acid oxidizes the hydrogen film on the copper to unimolecular hyponitrous acid, HNO, being itself reduced to the same acid. Simultaneously, copper dissolves to replace the hydrogen. The unimolecular hyponitrous acid formed is assumed to undergo oxidation by nitric acid to nitrous acid, the oxidizer being also reduced to the same acid. Hypo-nitrous acid has been proved by Divers to have the double molecule H2N2O2, and can therefore be assumed to be readily oxidizable on liberation in the nascent state as HNO.

The products formed by the action of nitric acid on copper, and their relative proportions, have been investigated by Montemartini, Freer and Higley, Higley, and Stansbie. The table is given by Higley, each observation being based on the solution of 0.3 gram of copper in not less than ten times the theoretical weight of nitric acid.

Normally, copper does not react with hot concentrated hydrochloric acid, but the periodic addition to the boiling liquid of some drops of nitric acid, or a few crystals of potassium chlorate, induces the ready formation of a clear solution of cuprous chloride. As a result of electrolytic action, the presence of platinum also facilitates the solution of copper in hydrochloric acid, the platinum being the cathode and the copper the anode.

Watts and Whipple have observed the corrosion of copper by acids to be promoted by contact with oxidizers.

In presence of oxygen the metal is rapidly dissolved by solutions containing free ammonia, with formation of complex cupric ammonium derivatives, the ammonia being oxidized to nitrite. At 568° C. sodium hydroxide begins to attack copper. The action of alkali-metal persulphates on copper and its alloys has been investigated by Groschuff.

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