(a)i. With the aid of an equation, explain briefly why aluminum metal is not affected by air.
(b) ZnO is an amphoteric oxide. Write equations to illustrate this statement.
(c)i) List three uses of sodium trioxocarbonate(IV).
(ii) Explain briefly why a solution of trioxonitrate(V) acid turns yellowish on storage for some time.
(ii) Describe briefly how trioxonitrate(V) ions could be tested for in the laboratory.
(d) Write balanced chemical equations for the preparation of hydrogen chloride.
(ii) by direct combination of its constituent elements.
(a)(i) Why aluminium is not affected by air
When exposed to air, aluminium quickly forms a thin, tough, coherent and impervious layer of aluminium oxide on its surface. This oxide film sticks firmly to the metal and prevents air and moisture from reaching the metal beneath, so the metal is protected from further attack.
\[4Al_{(s)} + 3O_{2(g)} \to 2Al_2O_{3(s)}\]
(a)(ii) Extraction of aluminium from bauxite
- Substance used for purifying the ore: sodium hydroxide solution (used to dissolve the alumina away from insoluble impurities).
- Composition of the mixture electrolyzed: pure aluminium oxide (alumina, \(Al_2O_3\)) dissolved in molten cryolite (\(Na_3AlF_6\)).
(b) ZnO as an amphoteric oxide
Reacting as a base (with acid):
\[ZnO_{(s)} + 2HCl_{(aq)} \to ZnCl_{2(aq)} + H_2O_{(l)}\]
Reacting as an acid (with alkali):
\[ZnO_{(s)} + 2NaOH_{(aq)} \to Na_2ZnO_{2(aq)} + H_2O_{(l)}\]
(c)(i) Three uses of sodium trioxocarbonate(IV)
Manufacture of glass; manufacture of soap and detergents; softening of hard water (also used in paper manufacture).
(c)(ii) Why trioxonitrate(V) acid turns yellow on storage
On standing, especially in light and warmth, the acid partly decomposes, releasing brown nitrogen(IV) oxide which dissolves back in the acid and gives it a yellow colour:
\[4HNO_{3} \to 4NO_{2} + 2H_2O + O_{2}\]
(c)(iii) Test for trioxonitrate(V) ions
Brown-ring test: to the solution add freshly prepared iron(II) tetraoxosulphate(VI) solution, then carefully pour concentrated tetraoxosulphate(VI) acid down the side of the tilted test tube so it forms a layer below. A brown ring forms at the junction of the two liquids, confirming the presence of nitrate ions.
(d) Preparation of hydrogen chloride
\[\text{(i) } NaCl_{(s)} + H_2SO_{4(l)} \to NaHSO_{4(s)} + HCl_{(g)}\]\[\text{(ii) } H_{2(g)} + Cl_{2(g)} \to 2HCl_{(g)}\]
(iii) One use of hydrogen chloride: it is used to prepare hydrochloric acid and metal chlorides, and industrially in the manufacture of chlorine and in cleaning (pickling) metals.
