OCR A Jun 2011 Paper 5 Q1 (with explained solutions) |

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OCR A Jun 2011 Paper 5 Q1

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Answer all the questions. 1 BornHaber cycles provide a model that chemists use to determine unknown enthalpy changes from known enthalpy changes. In this question, you will use a BornHaber cycle to determine an enthalpy change of hydration. (a) Magnesium chloride has a lattice enthalpy of 2493 kJ mol1. Define in words the term lattice enthalpy.[2] (b) The table below shows the enthalpy changes that are needed to determine the enthalpy change of hydration of magnesium ions. enthalpy change energy/kJ mol1 lattice enthalpy of magnesium chloride enthalpy change of solution of magnesium chloride enthalpy change of hydration of chloride ions 2493 154 363 (i) Why is the enthalpy change of hydration of chloride ions exothermic?[1]OCR 2011<br />
(ii) In this part, you will use the BornHaber cycle to determine the enthalpy change of hydration of magnesium ions. On the two dotted lines, add the species present, including state symbols. Mg2+(g) + 2Cl (g)MgCl 2(s)[2] (iii) Calculate the enthalpy change of hydration of magnesium ions. answer =kJ mol1 [2] (c) The enthalpy change of hydration of magnesium ions is more exothermic than the enthalpy change of hydration of calcium ions. Explain why. .[2]OCR 2011 [Total: 9] Turn over<br />

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F325 Question Answer 1 (a) (The enthalpy change that accompanies) the formation of one mole of a(n ionic) compound (cid:57) from its gaseous ions (cid:57) (under standard conditions) Mark Scheme June 2011 Mark Guidance IGNORE 'Energy needed' OR energy required ALLOW as alternative for compound: lattice, crystal, substance, solid Note: 1st mark requires 1 mole 2nd mark requires gaseous ions IF candidate response has 1 mole of gaseous ions, award 2nd mark but NOT 1st mark IGNORE: Mg2+(g) + 2Cl(g)MgCl2(s) (question asks for words) ALLOW statement of any type of bond being formed ALLOW (chloride) ions attract water (molecules) ALLOW a response in terms of hydrogen bonds breaking AND bond making DO NOT ALLOW response stating that energy is required DO NOT ALLOW response that refers to ions in H2O, eg H+ Correct species AND state symbols required for both marks Mark each marking point independently ALLOW response on upper line: Mg2+(g) + 2Cl(aq) (ie Cl hydrated before Mg2+) ALLOW MgCl2(aq) (b) (i) Hydration involves bond forming OR bonds are made (cid:57) (ii) Mg2+(aq) + 2Cl(g) (cid:57) Mg2+(aq) + 2Cl(aq) (cid:57)<br />
Mark Scheme June 2011 Mark Guidance use of Cl rather than 2 x Cl signs all reversed OR lack of 2 for 363 sign wrong for 154 sign wrong for 2493 sign wrong for 2 x 363 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: 2284 (+)1921 1613 (+)3065 3373 ORA: Calcium ion OR Ca2+ is larger OR Ca2+ has smaller charge density IGNORE idea of close packing of ions IGNORE atomic and atoms and assume that Mg or Ca refer to ions, ie ALLOW Mg has a smaller (atomic) radius ALLOW Mg has a stronger attraction to H2O ORA: e.g. Ca2+ has less attraction to H2O DO NOT ALLOW Mg atoms have a stronger attraction to H2O DO NOT ALLOW stronger attraction/bonding between ions Note: Response must refer to attraction/bonding with H2O or this must be implied from the whole response F325 Question Answer 1 (b) (iii) FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 1921 (kJ mol1) award 2 marks -------------------------------------------------------------------- (2493) + (154) = (2 x 363) + Hhyd(Mg2+) (cid:57) Hhyd(Mg2+) = (2493) + (154)(2 x 363) = 1921 (kJ mol1) (cid:57) (c) Magnesium ion OR Mg2+ is smaller OR Mg2+ has greater charge density (cid:57) Mg2+ has a stronger attraction to H2O OR Mg2+ has a stronger bonding with H2O (cid:57) Total<br />

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