OCR A Jun 2014 Paper 5 Q1 (with explained solutions) |

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OCR A Jun 2014 Paper 5 Q1

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Answer all the questions. 1 BornHaber cycles can be used to calculate enthalpy changes indirectly. (a) The table below shows enthalpy changes for a BornHaber cycle involving potassium sulfide, K2S. Formation of potassium sulfide, K2S 1st electron affinity of sulfur 2nd electron affinity of sulfur Atomisation of sulfur 1st ionisation energy of potassium Atomisation of potassium Enthalpy change / kJ mol1 381 200 +640 +279 +419 +89 (i) The incomplete BornHaber cycle below can be used to determine the lattice enthalpy of potassium sulfide. In the boxes, write the species present at each stage in the cycle. Include state symbols for the species. 2K+(g) + S(g) + 2e 2K(s) + S(g) 2K(s) + S(s) K2S(s) [3]OCR 2014<br />
(ii) Define, in words, the term lattice enthalpy.[2] (iii) Using the BornHaber cycle, calculate the lattice enthalpy of potassium sulfide. (b) Several ionic radii are shown below. lattice enthalpy =kJ mol1 [2] Ion Radius / pm Na+ 95 K+ 133 Rb+ 148 Cl181 Br 195 I 216 Predict the order of melting points for NaBr, KI and RbCl from lowest to highest. Explain your answer. Lowest melting pointHighest melting pointExplanation[3]OCR 2014 [Total: 10] Turn over<br />

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F325 Question (a) (i) Mark Scheme June 2014 Answer 2K+(g) + S2(g) (cid:57) 2K+(g) + S(g) + e (cid:57) 2K(g) + S(g) (cid:57) Marks Guidance Mark each marking point independently Correct species AND state symbols required for each mark For S2, DO NOT ALLOW S2 For e, ALLOW e For e only, IGNORE any state symbols added ALLOW k and s It can be very difficult distinguishing K from k; S from s<br />
F325 (a) (ii) Mark Scheme June 2014 (The enthalpy change that accompanies) the formation of one mole of a(n ionic) compound from its gaseous ions (under standard conditions) (cid:57)(cid:57) Award marks as follows. 1st mark: formation of compound from gaseous ions 2nd mark: one mole for compound only DO NOT ALLOW 2nd mark without 1st mark Note: A definition for enthalpy change of formation will receive no marks IGNORE 'Energy needed' OR energy required ALLOW one mole of compound is formed/made from its gaseous ions ALLOW as alternative for compound: lattice, crystal, substance, solid IGNORE: 2K+(g) + S2(g) (cid:127)(cid:111) K2S(s) (question asks for words) ALLOW 1 mark (special case) for absence of gaseous only, i.e. the formation of one mole of a(n ionic) compound from its ions (under standard conditions) (cid:57)<br />
(iii) FIRST, CHECK THE ANSWER ON ANSWER LINE F325 (a) IF answer = 2116 (kJ mol1) award 2 marks -------------------------------------------------------------------- 381(2 (cid:117) +89 + 279 + 2 (cid:117) +419 200 + 640) (cid:57) 3811735 =2116 (cid:57) (kJ mol1) Mark Scheme June 2014 IF there is an alternative answer, check to see if there is any ECF credit possible using working below. See list below for marking of answers from common errors ------------------------------------------------------------- ALLOW for 1 mark ONE mistake with sign OR use of 2: 2027 (2 (cid:117) 89 not used for K) 1697 (2 (cid:117) 419 not used for K) 2516 (+200 rather than 200 for S 1st electron affinity) (+)2116 (wrong sign) 1354 (+381 instead of 381) (+)1354 (+1735 instead of 1735) 836 (640 instead of +640) 1558 (279 instead of +279) 1760 (2 (cid:117) 89 instead of +2 x 89) 439 (2 (cid:117) 419 instead of +2 x 419) 2120 (rounded to 3SF) For other answers, check for a single transcription error or calculator error which could merit 1 mark DO NOT ALLOW any other answers, e.g. 1608 (2 errors: 2 (cid:117) 89 and 2 x 419 not used for K) 846 (3 errors:)<br />
F325 (b) KI RbCl melting point Lowest Highest melting point NaBr Mark 2nd and 3rd marking points independently Attraction and ionic size linked: Correct order (cid:57) Greater attraction from smaller ions/closer ions/larger charge density (cid:57) Comparison needed Energy AND attraction/breaking bonds linked: More energy/heat to overcome attraction (between ions) OR More energy/heat to break (ionic) bonds (cid:57) Mark Scheme June 2014 FULL ANNOTATIONS MUST BE USED -------------------------------------------------------------------------------- ORA throughout Response must clearly refer to ions for explanation marks 2nd and 3rd marking point must be comparative DO NOT ALLOW incorrect named particles, e.g. atoms, molecules, Na, Cl, Cl2, atomic, etc DO NOT ALLOW responses using nuclear size or attraction DO NOT ALLOW responses linked with loss of electrons IGNORE larger electron density ALLOW smaller sum of radii gives a greater ionic attraction IGNORE NaBr has greater ionic attraction IGNORE NaBr has smallest ionic radius (not focussing on size of each ion) ASSUME bonds broken are ionic unless otherwise stated DO NOT ALLOW incorrect named particles, e.g. atoms, molecules, Na, Cl, Cl2, atomic, etc Note: Comparison for energy only (i.e. link between more energy and breaking bonds/overcoming attraction) Total 10<br />

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