(a)(i) Outline a suitable procedure for the preparation of ZnCl\(_2\) crystals stating from zinc granules. (ii) Write a balanced equation for the reaction in (a)(i). (b) Considei' the following solutions: Na\(_2\)SO\(_{4(aq)}\), CH3COOK\(_{(aq)}\), Pb(NO\(_3\))\(_{2(aq)}\) and MgCl\(_2\). Which of them has/have a pH (i) greater than 7; (ii) equal to 7; (iii) less than 7?
(c)(i) Determine the oxidation number of Mn in I. MnO\(_2\); II. KMnO\(_4\). (ii) State one laboratory use of each of the .compounds in (c)(i). (d) Explain why oxidation and reduction processes are complementary. (e) Consider the reaction represented by the equation: Fe\(_{(s)}\) + 2Ag\(^+_{(aq)}\) \(\to\) Fe\(^{2+}_{(aq)}\) + 2Ag\(_{(s)}\) (i) Write a balanced ionic half equations for the reaction. (ii) Which of the species is I. oxidized; II. reduced? (iii) State the change in the oxidation number of silver during the reaction.
Explanation
(a) (i) to some dil. HCI in a beaker, add Zn granules and shake. (ii) continue to add Zn granules un effervescence stops and the Zn is in excess. (iii) filter off undissolved Zn granules (iv) put the filtra in an evaporating dish and heat to concentrate. (v) cool to obtain crystals (vi) filter and dry the crysta between filter papers. (ii) Zn\(_{(s)}\) + 2HCI\(_{(aq)}\) \(to\) ZnCl\(_{2(aq)}\) + H\(_{2(g)}\)
(b)(i) CH\(_3\)COOK\(_{(aq)}\) (ii) Na\(_2\)SO\(_{4(aq)}\) (iii) Pb(NO\(_3\))\(_{2(aq)}\) and MgCl\(_{2(aq)}\)
(c)(i) I. MnO\(_2\) Mn + (-2 x 2) = O Mn — 4 = 0 Mn = + 4 (ii) KMnO\(_4\) 1 + Mn + (– 2 x 4) = 0 1 + Mn – 8 = 0 Mn = + 7
(ii) MnO\(_2\) – as a catalyst – as an oxidizing agent KMnO\(_4\) – as an oxidizing agent
(d) Oxidation is a process of electron loss, while reduction is a process of electron gain. A species can only be oxidized if there is another species that would readily accept the donated electron(s)/ Hence every oxidation must be accompanied by a reduction.
(e) (i) I. Fe\(_{(s)}\) \(\to\) F\(^{2+}_{(aq)}\) + 2e\(^-\) II. Ag\(^+_{(aq)}\) + e\(^-\) \(\to\) Ag\(_{(s)}\) (ii) I. Fe is oxidized II. Ag\(^+\) is reduced (iii) +1 to 0.
