Fig. 9.1 shows apparatus used to pass steam over a heated sample of metal V. The gas produced is collected over water. Metal V is a middle-of-the-series met...
Question 1 Report
Fig. 9.1 shows apparatus used to pass steam over a heated sample of metal V. The gas produced is collected over water. Metal V is a middle-of-the-series metal.
(a) State the purpose of heating the wet mineral wool, and explain why the gas produced is usually burned off in a real experiment. [2]
(b) Metal V is iron. Write the word equation and the balanced symbol equation for the reaction of iron with steam. (Iron forms Fe3O4.) [3]
(c) Explain why sodium is never tested for its reaction with steam using this apparatus. [2]
(d) Describe two observations that would be made in the tube during the reaction of metal V with steam. [2]
(e) Write the ionic half-equations for the oxidation of the metal and the reduction that forms the hydrogen gas (from H+ supplied by water). [2]
(f) Predict what would happen if copper were used in place of metal V. Give a reason. [2]
(g) Copper, iron and calcium are three metals. Place them in order of reactivity and explain your order in terms of the ease of loss of electrons. [3]
Answer Details
This question tests how a middle-of-the-series metal reacts with steam, and how the reactivity series lets you predict and explain such reactions using oxidation and reduction.
(a) Heating the wet mineral wool boils the water it holds, producing the steam (water vapour) that is carried along the tube and passes over the hot metal [1]. The hydrogen gas made in the reaction is burned off because hydrogen is flammable and forms an explosive mixture with air, so burning it as it is produced is a safety precaution [1].
(b) In words: iron + steam \(\rightarrow\) iron oxide + hydrogen [1]. The balanced symbol equation is \[3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2\] Correct formulae score one mark and correct balancing scores the second [2]. Check the balance: 3 Fe each side, 8 H each side (four H2O give 8 H, four H2 give 8 H) and 4 O each side.
(c) Sodium is far too reactive to be tested this way [1]. It reacts violently, even explosively, with steam and would be extremely dangerous in a heated tube, so its reaction is only ever demonstrated with cold water under controlled conditions [1].
(d) Two observations in the tube: the metal glows and gets hot because the reaction is exothermic [1]; a solid oxide forms so the metal changes appearance to a dark/black solid, and bubbles of gas are given off and collected over the water [1].
(e) The metal atoms lose electrons (oxidation) and the hydrogen ions supplied by water gain them (reduction):
\[Fe \rightarrow Fe^{2+} + 2e^-\ \text{(oxidation)}\]\[2H^+ + 2e^- \rightarrow H_2\ \text{(reduction)}\]
One mark for each correct half-equation [2]. The electrons lost by the metal are exactly the electrons gained by the hydrogen ions, which is why the charges and electrons balance.
(f) Copper would give no reaction [1], because copper lies below hydrogen in the reactivity series and is too unreactive to displace hydrogen from steam [1].
(g) In order of decreasing reactivity: calcium, then iron, then copper [1]. Calcium loses its outer-shell electrons most easily and copper loses them least easily [1], so the tendency to form positive ions falls from calcium to copper, which is exactly what "more reactive" means for a metal [1].
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