Chemical Properties of Zinc
|Zinc tarnishes rapidly in air. It burns to the oxide when heated, and zinc wool burns with a brilliant flame when lighted with a Bunsen burner - forming a coherent mass of oxide. |
In dry oxygen oxidation virtually ceases below 150° C. when the surface film has formed. As the temperature rises above this, moist oxygen begins to act more vigorously than dry. Ozone attacks the metal somewhat more actively than oxygen.
Basic carbonates are formed by atmospheric action upon zinc, and hydrogen peroxide can be detected during the rusting.
Steam acts readily on the metal at a red heat, but the reaction
ZnO+H2O ⇔ ZnO+H2
is reversible. Potable waters dissolve zinc, and distilled or rain water dissolves it more readily than harder water, though purified water has been said to have no action on the metal.
Ground waters may take up 5 parts of zinc per million when they pass through galvanised iron pipes.
According to Davies, all waters attack zinc when air is present. Coating the zinc, he adds, does not stop the action, and zinc is found in the viscera of people who have drunk water containing zinc compounds. Moderate hardness in the water favours the action, he also says, extreme hardness does not prevent it, and rain-water has the least action.
The corrosion of zinc by acidic reagents is greater when the metal contains lead, cadmium, or iron than in the refined product. Minute traces of arsenic, antimony, copper, or tin also favour corrosion.
Aqueous halogen acids dissolve zinc to the halide with evolution of hydrogen. The rate of dissolution in hydrochloric acid depends upon the condition of the metal, also upon the impurities present, and the action is very slow if the metal is pure. Dry hydrogen chloride has a solvent action in the following solvents: chloroform, ethyl chloride, amyl alcohol, methyl alcohol, ethyl alcohol, acetone, ethyl ether, and benzene.
Zn+2HCl.200H2O = ZnCl2.400H2O+H2+36.820 Cal.
if the hydrogen is dry. The corresponding figure for moist hydrogen at the same temperature, 20° C., is 36.070 Cal.
Zinc reacts with dilute aqueous sulphuric acid to form hydrogen and zinc sulphate, but if the metal is very pure there is scarcely any action. There is probably no action at all if both acid and metal are exceedingly pure, and the rate of solution varies with the condition of the zinc and the impurities present. Solution proceeds more slowly at first, and this preliminary "induction period" is probably due to a layer of hydrogen bubbles that prevents contact between the metal and the acid. Rise of temperature increases the rate of solution, and its effect increases with the acid concentration, though it has no appreciable effect when the acid is very dilute. Amalgamation protects zinc from attack by acids, because the discharge potential of hydrogen on mercury exceeds the potential of zinc.
Small quantities of sulphur dioxide and hydrogen sulphide are often produced during the action of dilute sulphuric acid on ordinary zinc, though they are not produced if the acid and metal are nearly pure. Sulphur dioxide is the chief gaseous product when the sulphuric acid is concentrated, but hydrogen sulphide is also produced at higher temperatures, and both compounds are formed when the temperature is fairly high (160° C.).
When zinc is heated in a stream of sulphur dioxide some sulphide seems to be formed. When aqueous sulphurous acid acts on zinc the sulphite and salts of other sulphur acids are produced in the solution. If the action occurs at 200° C. in sealed tubes, amorphous zinc sulphide, sulphur, and zinc sulphate result. Zinc hydrosulphite or hyposulphite is formed if dry sulphur dioxide is passed through a suspension of zinc in absolute alcohol, and, according to Bernsthen, the reaction
Zn+2SO2 = ZnS2O4
first occurs when aqueous sulphurous acid acts upon zinc. Secondary reactions then occur. According to Schutzenberger, zinc sulphite and zinc hydrosulphite are the primary products, and no hydrogen is produced.
According to Fordos and Gelis, zinc sulphite and hydrogen are produced first. Then the nascent hydrogen reduces some sulphurous acid to hydrogen sulphide. This may precipitate part of the sulphite (this occurs to a considerable extent with cadmium) as sulphide, and it also interacts with sulphur dioxide to form sulphur. The sulphur finally reacts with some zinc sulphite to produce zinc thiosulphate. Since zinc sulphide reacts with excess of sulphurous acid according to the equation
2ZnS+3SO2 = 2ZnS2O3+S,
as Henderson and Weiser point out, Fordos and Gelis obtained a solution of zinc sulphite and thiosulphate when they acted upon the metal with an excess of sulphurous acid. They noted that the decomposition
2ZnS2O3 = ZnS+ZnS3O6
The reaction between zinc and sulphurous acid may apparently vary with the conditions, but the formation of various thionic acids has usually been explained by the primary formation of zinc sulphite and nascent hydrogen. Zinc sulphide has a tendency to precipitate from solutions resulting from the action of sulphurous acid on zinc from the decomposition of zinc thiosulphate, etc.
According to Schweitzer, zinc sulphite and thiosulphate are first formed, and the liquid also ultimately contains sulphur, zinc sulphide, and trithionate. Nascent hydrogen, he adds, is not produced, and is not responsible for any reactions.
According to Acworth and Armstrong, nitric oxide, nitrous oxide, and nitrogen are always evolved by the action of nitric acid on zinc. Montemartini says that hyponitrous acid, nitric oxide, nitrous oxide, nitrogen, and ammonia are formed at a low temperature with a large excess of acid. Nitrous acid is also formed if the solution does not contain more than 30 per cent, of acid, and nitrogen peroxide if it does. A maximum of ammonia is produced with acid of concentration 40.45 at a temperature of 3°-8° C., a maximum of nitrous oxide at a concentration of 40 and a minimum at 80, a maximum of nitrogen peroxide at a concentration of 80, which then remains constant, and nitrogen is never formed in more than very small quantities. Hydroxylamine is said to be produced during the action of nitric acid on zinc, and to be easily observable if sulphuric or another acid is present. Bijlert detected no hydroxylamine when N/20 to N/10 nitric acid acted upon zinc - ammonia was formed. Nitrous acid has been said to be the primary product of the reaction.
Zinc dissolves slowly in caustic alkalies, forming hydrogen and zincates.
Zinc apparently forms no compound with hydrogen, though the spectrum of an arc between zinc poles in hydrogen has been said to indicate zinc hydride, and the existence of zinc hydride has been affirmed. The metal, however, is apt to occlude hydrogen, and zinc dust has been said to contain thirty-nine times its volume of this gas.
It decomposes nitric oxide slowly but completely at 600° C., and unites directly with most of the negative elements.
Zinc displaces less electropositive metals from solutions of their salts, and a zinc-copper couple, prepared by depositing copper on zinc, decomposes water at ordinary temperatures. This evolution of hydrogen accounts, it seems probable, for the production of this gas when zinc displaces copper and other metals from their solutions. The zinc-copper couple is an effective reducing agent, and will decompose water even at 2° C. A properly prepared couple is suitable for estimating nitrogen (by reducing it to ammonia) in water.
There is a period of induction when metals are replaced in solutions of their salts by zinc, and they may be precipitated partly in the form of hydroxide, nickel, and cobalt, for example. If asbestos is wrapped round a zinc rod that is dipped in a solution of lead acetate, or copper sulphate, or antimony chloride and tartaric acid, a spongy mass of metal settles out on the asbestos. A black deposit of antimony also drops to the bottom of the vessel, that explodes when heated (explosive antimony).
Zinc and copper have been said to be reciprocally replaceable by one another to some extent under appropriate conditions, and a solution of an iron salt was said to be produced by acting on the solution of a normal zinc salt with powdered iron.
Hydrogen is evolved when magnesium is immersed in a solution of zinc chloride, and zinc containing some hydroxide is precipitated.15 Magnesium will only precipitate 50 per cent, of the zinc from a solution of zinc sulphate, and the precipitated zinc contains some oxides of magnesium and zinc. Aluminium reacts with solutions of zinc salts.