(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.
(a) A metal that gives hydrogen with each reagent
- (i) Dilute mineral acid: zinc: \(Zn + 2HCl \to ZnCl_2 + H_2\).
- (ii) Cold water: sodium: \(2Na + 2H_2O \to 2NaOH + H_2\).
- (iii) Steam: iron: \(3Fe + 4H_2O \to Fe_3O_4 + 4H_2\).
- (iv) Hot concentrated alkali: aluminium (or zinc): \(2Al + 2NaOH + 2H_2O \to 2NaAlO_2 + 3H_2\).
(b) Effect on the rate of the zinc/HCl reaction (acid originally in excess)
- (i) Using \(1.0\ \text{mol dm}^{-3}\) HCl: higher concentration, so the rate increases.
- (ii) Using 8.0 g of the same granulated zinc: the acid is still in excess, so the rate is essentially unchanged (only less hydrogen is finally produced).
- (iii) Using 10 g of zinc dust: much larger surface area, so the rate increases markedly.
- (iv) Using a higher volume of the same \(0.50\ \text{mol dm}^{-3}\) HCl: concentration unchanged, so the rate is unchanged.
- (v) Vessel dipped in crushed ice: lower temperature, so the rate decreases.
- (vi) Equal volumes of water and acid (dilution to about \(0.25\ \text{mol dm}^{-3}\)): lower concentration, so the rate decreases.
(c) Extraction of aluminium
- (i) The ore is bauxite (impure \(Al_2O_3\)).
- (ii) Molten cryolite lowers the melting point of the alumina and dissolves it, improving conductivity and saving energy.
- (iii) Purification: dissolve the bauxite in hot concentrated \(NaOH\) (the amphoteric \(Al_2O_3\) dissolves as sodium aluminate), filter off insoluble impurities, precipitate \(Al(OH)_3\) by seeding/cooling, then heat it to give pure alumina.
- (iv) Current required: \(Al^{3+} + 3e^- \to Al\). \[n(Al) = \frac{18.0}{27.0} = 0.667\ \text{mol}\]\[Q = 0.667 \times 3 \times 96500 = 1.93\times10^{5}\ \text{C}\]\[t = 1.50 \times 3600 = 5400\ \text{s}\]\[I = \frac{Q}{t} = \frac{1.93\times10^{5}}{5400} = 35.7\ \text{A}\]
(d)(i) Although reactive, aluminium resists corrosion because it forms a thin, tough, adherent layer of aluminium oxide on its surface that seals the metal beneath from further attack.
(d)(ii) Metals are generally good reducing agents because they readily lose (donate) electrons, thereby reducing other species.
(a) A metal that gives hydrogen with each reagent
- (i) Dilute mineral acid: zinc: \(Zn + 2HCl \to ZnCl_2 + H_2\).
- (ii) Cold water: sodium: \(2Na + 2H_2O \to 2NaOH + H_2\).
- (iii) Steam: iron: \(3Fe + 4H_2O \to Fe_3O_4 + 4H_2\).
- (iv) Hot concentrated alkali: aluminium (or zinc): \(2Al + 2NaOH + 2H_2O \to 2NaAlO_2 + 3H_2\).
(b) Effect on the rate of the zinc/HCl reaction (acid originally in excess)
- (i) Using \(1.0\ \text{mol dm}^{-3}\) HCl: higher concentration, so the rate increases.
- (ii) Using 8.0 g of the same granulated zinc: the acid is still in excess, so the rate is essentially unchanged (only less hydrogen is finally produced).
- (iii) Using 10 g of zinc dust: much larger surface area, so the rate increases markedly.
- (iv) Using a higher volume of the same \(0.50\ \text{mol dm}^{-3}\) HCl: concentration unchanged, so the rate is unchanged.
- (v) Vessel dipped in crushed ice: lower temperature, so the rate decreases.
- (vi) Equal volumes of water and acid (dilution to about \(0.25\ \text{mol dm}^{-3}\)): lower concentration, so the rate decreases.
(c) Extraction of aluminium
- (i) The ore is bauxite (impure \(Al_2O_3\)).
- (ii) Molten cryolite lowers the melting point of the alumina and dissolves it, improving conductivity and saving energy.
- (iii) Purification: dissolve the bauxite in hot concentrated \(NaOH\) (the amphoteric \(Al_2O_3\) dissolves as sodium aluminate), filter off insoluble impurities, precipitate \(Al(OH)_3\) by seeding/cooling, then heat it to give pure alumina.
- (iv) Current required: \(Al^{3+} + 3e^- \to Al\). \[n(Al) = \frac{18.0}{27.0} = 0.667\ \text{mol}\]\[Q = 0.667 \times 3 \times 96500 = 1.93\times10^{5}\ \text{C}\]\[t = 1.50 \times 3600 = 5400\ \text{s}\]\[I = \frac{Q}{t} = \frac{1.93\times10^{5}}{5400} = 35.7\ \text{A}\]
(d)(i) Although reactive, aluminium resists corrosion because it forms a thin, tough, adherent layer of aluminium oxide on its surface that seals the metal beneath from further attack.
(d)(ii) Metals are generally good reducing agents because they readily lose (donate) electrons, thereby reducing other species.
