WJEC Chemistry for AS Level: Revision Workbook

Dewi’s answer (a) (i) Ag + (aq) + Br – (aq) — AgBr (s) ضض (ii) cream ض (b) (i) AgBr (s) — Ag (s) + ½ Br 2 (aq) ض (ii) For a redox reaction to occur, electrons must be transferred from one species to another. The one that loses electrons is oxidised, and the one that gains electrons is reduced. In the decomposition reaction the silver is reduced as the reaction occurring is Ag + + e – — Ag. The bromine is oxidised as it loses electrons in 2Br – — Br 2 + 2e – . In the precipitation reaction the silver starts as silver ions and stays as silver ions – they have not lost or gained electrons, so it is not a redox reaction. The same is true for the bromide ions. ضضض (c) (i) 3Ag + 3HNO 3 — 3AgNO 3 + 1 NO + 2H 2 O ض (ii) 1HNO 3 + 3HCl — NOCl + Cl 2 + 2H 2 O ضض MARKER COMMENTARY (a) (i) The equation is correct, and the state symbols are also correct so this gains 2 marks. (ii) Dewi has correctly recalled the colour of this precipitate from previous work and will also see it later in this unit. (b) (i) State symbols are not needed here so the error in these is ignored for the mark – the substances in the equation are correct. (ii) Dewi has chosen to use electron transfer to answer this question, which is an appropriate method. He gains a mark for explaining what is meant by a redox reaction in terms of electrons. He then applied this idea correctly to each of the two reactions and gains a mark for each of these for a total of 3 marks. (c) These equations are more challenging than those encountered at GCSE. Dewi has balanced the equation in (i) correctly. Once he identified the additional product in part (ii) as water by the need to balance the hydrogen atoms, he balances the equation correctly. Overall Dewi gains 10 out of 10 marks as he recalls all the factual content correctly and can apply these ideas to the reactions of silver bromide discussed. Rebecca’s answer (a) (i) Ag + (aq) + Br – (aq) — AgBr (aq) غض (ii) cream ض (b) (i) AgBr — Ag + Br غ (ii) In a redox reaction oxidation states must change showing that oxidation and reduction have happened – oxidation states become more positive during oxidation. In the precipitation the oxidation states don’t change so it isn’t a redox reaction. In the decomposition reaction the silver’s oxidation state decreases to 0 so it is reduced, while the bromide ions are oxidised with the oxidation states becoming less negative (−1 to 0). ضغض (c) (i) 3Ag + 3HNO 3 — 3AgNO 3 + 1 NO + 2H 2 O ض (ii) HNO 3 + HCl — NOCl + Cl 2 + H 2 غغ MARKER COMMENTARY (a) (i) The equation is correct but the state symbol for the precipitate should be (s) so this gains only 1 mark. (ii) Rebecca has correctly recalled the colour of this precipitate. (b) (i) Rebecca has not written the correct formula for bromine as an element (Br ) so the equation is incorrect. (ii) Rebecca has chosen to use oxidation states to answer this question, which is another appropriate method. Her discussion of what is meant by a redox reaction is clear and gains a mark. When discussing the precipitation reaction, she correctly states that oxidation states don’t change, but doesn’t state what they are, so this isn’t enough for the second mark. Her explanation of the decomposition reaction is better and uses correct oxidation states for most substances to gain the third marking point. (c) Rebecca can balance the first equation correctly. She realises there must be a hydrogen-containing product but suggests this is H – it is impossible to balance this equation so she loses the second mark. Overall Rebecca gains 5 out of 10 marks as she has many correct ideas but careless errors on formulae and state symbols have reduced her marks. Her work on oxidation states is correct but not enough to gain all the marks available. 16 | WJEC Chemistry for AS Level: Revision Workbook | Unit 1: The Language of Chemistry, Structure of Matter and Simple Reactions