Study of Heavy Metals: Zinc

Coating iron with zinc is called galvanisation. Zinc stops rusting in two ways. The zinc layer keeps air and water away from the iron. Even when the layer is scratched, zinc (\(E^\circ=-0.76\ \mathrm{V}\)) is more reactive than iron (\(E^\circ=-0.44\ \mathrm{V}\)), so zinc is oxidised in place of the iron (it acts as the sacrificial anode) and the iron is protected.

A typical transition element has a partly filled d subshell in its atom or in its common ions. Zinc is \([\mathrm{Ar}]3d^{10}4s^2\) and forms only \(\mathrm{Zn^{2+}}\) (\(3d^{10}\)), so its d subshell is completely filled in the atom and in the ion. It therefore does not show variable oxidation states, coloured compounds or paramagnetism.

Colour in transition metal salts comes from d-d electronic transitions, which need a partly filled d subshell. \(\mathrm{Zn^{2+}}\) has the full \(3d^{10}\) configuration, so there is no vacant d orbital for an electron to move into and no visible light is absorbed. Zinc salts are therefore white or colourless.

Spelter is the crude, impure zinc (about 97 to 98% pure) that is obtained when zinc vapour from the retort is condensed. It contains impurities such as iron, lead, cadmium and arsenic.

Purification of spelter:

(i) Fractional distillation. The boiling points are Zn 907 °C, Cd 767 °C, Pb 1620 °C and Fe 3000 °C. When spelter is heated to about 1000 °C, zinc and cadmium distil over and lead and iron stay behind. The distillate is re-distilled at about 800 °C, when cadmium distils first and pure zinc (99.5%) remains.

(ii) Electrolytic refining (modern method). The impure zinc is the anode, a thin sheet of pure zinc is the cathode, and acidified zinc sulphate solution is the electrolyte:

Zinc of 99.9% purity is obtained, and the insoluble impurities settle below the anode as anode mud. An impure rod is the anode because it has to supply the \(\mathrm{Zn^{2+}}\) ions that deposit on the cathode.

Lithopone is a white pigment made of an equimolar mixture of zinc sulphide and barium sulphate, \(\mathrm{ZnS\cdot BaSO_4}\).

Preparation: by mixing solutions of zinc sulphate and barium sulphide. Both products are insoluble white solids:

Application: it is used as a white pigment in interior wall paints, and also in paper coating, rubber and plastics, because it gives a bright, opaque finish.

\(\mathrm{ZnO}\) (zinc oxide).

\(\mathrm{ZnCO_3}\) (zinc carbonate).

\(\mathrm{ZnS}\) (zinc sulphide).

\(\mathrm{ZnSO_4\cdot7H_2O}\) (zinc sulphate heptahydrate).

Prediction: no displacement reaction takes place. Zinc is more reactive than iron (\(E^\circ_{\mathrm{Zn^{2+}/Zn}}=-0.76\ \mathrm{V}\) and \(E^\circ_{\mathrm{Fe^{2+}/Fe}}=-0.44\ \mathrm{V}\)). A less reactive metal cannot displace a more reactive metal from its salt:

The potential for the reaction is \(E^\circ_{cell}=-0.76-(-0.44)=-0.32\ \mathrm{V}\), which is negative, so it is not spontaneous. (This is different from an iron vessel holding copper sulphate, where iron displaces copper.)

Observation: the water evaporates and white crystals of zinc sulphate remain; there is no deposit of zinc on the vessel. However, the zinc sulphate solution is slightly acidic because \(\mathrm{Zn^{2+}}\) hydrolyses, so the hot solution can attack the iron a little: \(\mathrm{Fe+2H^+\rightarrow Fe^{2+}+H_2\uparrow}\). The vessel may rust, and the crystals can be contaminated with iron(II) salts and look slightly yellow-green.

Modification: use a vessel that does not react, an evaporating dish of porcelain or glass (borosilicate), and heat gently on a water bath, then crystallise.

Dissolve zinc in dilute sulphuric acid:

Filter the solution to remove the unreacted zinc and impurities, concentrate it by gentle heating and cool it. Colourless crystals of white vitriol, \(\mathrm{ZnSO_4\cdot7H_2O}\), separate out.

The crystals lose their water of crystallisation and become anhydrous, and then the anhydrous salt decomposes:

Lithopone, a white precipitate of zinc sulphide and barium sulphate, is formed:

Part of the zinc sulphide first changes into zinc sulphate, which then decomposes:

In moist air containing carbon dioxide, zinc is covered by a grey layer of basic zinc carbonate:

When heated in air at 500 °C, zinc burns with a bluish flame to give white zinc oxide (philosopher’s wool):

Introduction: white vitriol is zinc sulphate heptahydrate, \(\mathrm{ZnSO_4\cdot7H_2O}\). It is called seto tutho in Nepali.

Preparation: by the action of dilute sulphuric acid on zinc, ZnO or \(\mathrm{ZnCO_3}\), followed by crystallisation:

Physical properties: it is a colourless (white) crystalline solid, soluble in water, and it is efflorescent, that is, it loses water of crystallisation when exposed to the atmosphere.

Chemical properties:

Uses:

The chief ore of zinc is zinc blende, \(\mathrm{ZnS}\). Zinc is extracted from it by pyrometallurgy in the following steps.

1. Crushing and pulverisation. The big lumps of ore are broken into small pieces by a jaw crusher or hammer and then ground to a powder in a ball mill.

2. Concentration (froth flotation). The powdered ore is mixed with pine oil and water in a tank and agitated with compressed air. The ore particles float in the froth and the earthy impurities (gangue) settle at the bottom.

3. Roasting. The concentrated ore is strongly heated in an excess of air in a reverberatory furnace, and zinc sulphide changes into zinc oxide. Some of the sulphide first becomes sulphate, which then decomposes. The impurities of P, S and As are removed as volatile oxides.

If calamine, \(\mathrm{ZnCO_3}\), is present with the zinc sulphide, it is changed into the oxide by calcination: \(\mathrm{ZnCO_3\xrightarrow{\Delta}ZnO+CO_2\uparrow}\).

4. Reduction (smelting). Briquettes of zinc oxide powder and coke (2 : 1) are fed into a vertical retort furnace. The retort is heated from outside by burning producer gas (CO + N₂), and at 1400 to 1500 °C the oxide is reduced to zinc vapour, which leaves the retort and is condensed into molten zinc called spelter (97.8% pure) while carbon monoxide escapes:

Figure: Reduction of zinc

5. Purification. The spelter contains iron, lead, cadmium and arsenic. It is purified by fractional distillation (99.5% pure zinc) or, by the modern method, by electrolytic refining. Impure zinc is the anode, pure zinc is the cathode and acidified zinc sulphate is the electrolyte:

The zinc obtained is 99.9% pure, and the impurities fall as anode mud.

The metal M is zinc (Zn). Its atomic number is 30 and its configuration is \([\mathrm{Ar}]3d^{10}4s^2\). Roofs of galvanised (zinc-coated) iron sheets are common.

The chief ore is zinc blende, \(\mathrm{ZnS}\).

(The diagrams are given with Short Answer Question 9.)

Zinc lies above hydrogen in the electrochemical series, so it dissolves in dilute sulphuric acid and liberates hydrogen:

When the solution is concentrated and cooled, colourless crystals of white vitriol, \(\mathrm{ZnSO_4\cdot7H_2O}\), are obtained.

Zinc decomposes steam at about 500 °C (red heat). It forms zinc oxide and liberates hydrogen:

The element is zinc (Group 12, oxidation state +2). The sulphide is zinc blende, \(\mathrm{ZnS}\) (the chief ore), and the carbonate is calamine, \(\mathrm{ZnCO_3}\).

Figure: Vertical retort furnace. Zinc oxide briquettes and coke are charged from the top, the retort is heated from outside by producer gas, and zinc vapour is condensed as spelter

(The flow chart is given with Short Answer Question 9.)

The product depends on the concentration of the acid. The more dilute the acid, the more the nitrogen is reduced: