(iii) Give two uses of tetraoxosulphate (VI) acid.
(b)(i) Describe a suitable laboratory procedure for preparing a sample of zinc tetraoxosulphate (VI) crystals, starting from zinc oxide.
(ii) Write two equations to show why zinc oxide is classified as an amphoteric oxide.
(c) Zinc reacts with copper (II) tetraoxosulphate (VI) according to the following equation: Zn + CuSO\(_4\) -> ZnSO\(_4\) + Cu
(ii) Determine the mass of zinc that would react completely with 8.0g of copper (II) tetraoxosulphate (VI). [CuSO\(_4\) = 160; Zn = 65]
(a)(i) An acid is a substance that ionizes in water (or other suitable solvent) to produce hydrogen ions, \(H^+\), as the only positive ions. (More generally, a proton, \(H^+\), donor.)
(ii) Three chemical properties of tetraoxosulphate(VI) acid, \(H_2SO_4\):
- It reacts with bases and metallic oxides (neutralization) to form a salt (tetraoxosulphate(VI)) and water only, e.g. \(H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O\).
- It reacts with reactive metals to liberate hydrogen gas, e.g. \(H_2SO_4 + Zn \rightarrow ZnSO_4 + H_2\).
- It reacts with trioxocarbonate(IV) salts to give carbon(IV) oxide, e.g. \(H_2SO_4 + Na_2CO_3 \rightarrow Na_2SO_4 + H_2O + CO_2\). (Hot concentrated \(H_2SO_4\) also acts as an oxidizing and a dehydrating agent.)
(iii) Two uses of \(H_2SO_4\): in the manufacture of fertilizers (e.g. ammonium tetraoxosulphate(VI)); as the electrolyte in lead-acid accumulators (car batteries). (Also used to make detergents and as a drying/dehydrating agent.)
(b)(i) Laboratory preparation of zinc tetraoxosulphate(VI) crystals from zinc oxide (insoluble base + acid method):
- Warm some dilute tetraoxosulphate(VI) acid in a beaker.
- Add zinc oxide (the insoluble base) a little at a time, stirring, until some remains undissolved (i.e. add the oxide in excess to ensure all the acid reacts).
- Filter to remove the excess zinc oxide; the filtrate is zinc tetraoxosulphate(VI) solution.
- Evaporate the filtrate to the point of crystallization (until a hot saturated solution is obtained).
- Allow it to cool slowly so that crystals form; filter off the crystals and dry them between filter papers.
\[ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O\]
(ii) Zinc oxide is amphoteric because it reacts with both acids and bases:
As a base (with acid): \[ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O\]
As an acid (with alkali): \[ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O\]
(c)(i) The reaction \(Zn + CuSO_4 \rightarrow ZnSO_4 + Cu\) is a displacement (redox) reaction (zinc, being more reactive, displaces copper from its salt).
(ii) Mass of zinc that reacts with 8.0 g of \(CuSO_4\):
\[\text{moles of } CuSO_4 = \frac{8.0}{160} = 0.05\,mol\]
From the equation, 1 mole \(CuSO_4\) reacts with 1 mole \(Zn\), so moles of \(Zn = 0.05\,mol\).
\[\text{mass of } Zn = 0.05 \times 65 = 3.25\,g\]
