(a) Giving different examples, mention one metal in each case which produces hydrogen on reacting with
(i) dilute mineral acid
(ii) cold water;
(iii) steam;
(iv) hot, concentrated alkali.
(b) In an experiment, excess 0.50 mol dm(^{-3}) HCI was added to 1Og of granulated zinc in a beaker. Other conditions remaining constant, state how the reaction rate would be affected in each case, if the experiment was repeated using:
(i) 1.0 mol dm(^{-3}) HCI;
(ii) 8.0g of granulated zinc;
(iii) 10g of zinc dust;
(iv) a higher volume of 0.50 mol dm HCI;
(v) a reaction vessel dipped in crushed ice;
(vi) equal volumes of water and 0.50 mol dm(^3) HCI.
(c) Aluminium is extracted from its ore by electrolysis.
(i) Name the ore from which the metal is extracted.
(ii) State the role of molten cryolite in the extraction.
(iii) Describe in outline how the ore is purified before electrolysis
(iv) Calculate the current in amperes required to produce 18.0g of aluminium in 1.50 hours. [Al = 27.0; F = 96500C]
(d) Give the reason why
(i) aluminium, which is a reactive metal, is resistant to corrosion.
(ii) metals are generally good reducing agents.
Explanation
(a)(i) Zn; Na, Ca, Mg, etc.
(ii) Na, K, Ca.
(iii) Mg, Al, Zn, Iro
(iv) Sn, Zn, or AI
(b)(i) The forward reaction is favoured and it proceeds faster in that direction.
(ii) The reaction rate is slowed because of the reduction in the concentration of Zn.
(iii) Zinc dust will make available more area of contact and hence it favours the forward reaction speedily.
(iv) Increasing the volume of HCI will increase the concentration of the reactants and hence favours the forward reaction by speeding it up
(v) Since the reaction condition remain constant, dipping the vessel in crushed ice will alter the rate of the reaction.
(vi) Adding water will reduce the concentration of the reactants and this will slow down the reaction rate.
(c)(i) Bauxite – Al(_2)O(_3).2H(_2)O
(ii) The molten cryolite prevents the molten aluminium from atmospheric oxidation.
(iii) The bauxite is first heated with caustic soda under pressure — Al(_2)O(_3) + 2NaOH + 3H(_2)O (to) 2NaAl(OH)(_4). The aluminate
(iii) is then seeded with aluminium hydroxide crystals to induce the precipitation of aluminium hydroxide Na Al(OH)(_4) Al(OH)(_3) + NaOH. The Al(OH)(_2) is then filtered off, washed, dried and heated strongly to yield Al(_2)O(_3).
2Al(OH)(_3) (to) Al(_2)O(_3) + 3H(_2)O
(iv) Al(^{3+}) + 3e(^-) —> Al; 3 x 96500 —> 27; x —>18.
x = (frac{18}{27}) x 3 x 96500 = 19300c
but Q = it; I x t = 193000; but t = 1.50 hrs = 1 x 3600 + 50 x 60 = 6600
I x 6600 = 193000
I = (frac{193000}{6600}) = 29.2A
(d)(i) Aluminium when exposed to air acquires a thin continuous coating of aluminium oxide, which prevents further attack of the metal by atmospheric oxygen and water or steam and it is said to be corrosion-free.
(ii) Metals are generally good reducing agents because they tend to donate their electrons readily during chemical reactions.