O-LevelChemistryStoichiometry5070/22 · Oct/Nov 2015 · Q810 Marks

Chlorine reacts with sodium hydroxide. Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H2O(l)

(a)A volume of 144 cm³ of chlorine gas, measured at room temperature and pressure, is passed into 38.0 cm³ of 0.250mol/dm³ sodium hydroxide. Show by calculation which reactant is in excess.[3]
(b)(i)Chlorine displaces bromine from an aqueous solution of potassium bromide. Construct the equation for this reaction.[2]
(b)(ii)Explain why bromine does not react with aqueous potassium chloride.[1]
(b)(iii)Give the charge of a chloride ion and its electronic configuration. charge electronic configuration[2]
(c)Explain why sodium chloride does not conduct electricity when solid but does conduct electricity when molten.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2015). The mark or mark type is shown in brackets.

[3] (a) 6 × 10⁻³ mol Cl2 (1) 9.5 × 10⁻³ mol NaOH (1) mol NaOH required to react with all chlorine = 12 × 10⁻³ or mol Cl2 needed to react with NaOH = 4.7(5) × 10⁻³ AND Cl2 in excess (1)
[2] (b) (i) Cl2 + 2KBr → Br2 + 2KCl (2) 1 mark for correct formulae 1 mark for balancing (dependent on correct formulae)
[1] (b) (ii) chlorine more reactive than bromine/chlorine above bromine in reactivity series (or reverse argument)
[2] (b) (iii) -1 or Cl¯ or – (1) 2,8,8 (1)
[2] (c) in solid, the ions are in fixed positions/ions do not move (1) when molten, the ions can move (1)

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