OCR A Jan 2013 Paper 5 Q3 (with explained solutions) |

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OCR A Jan 2013 Paper 5 Q3

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3 Energy changes take place when water dissolves compounds and when water changes its physical state. (a) You are provided with the following information. Ion Na+ Mg2+ OH Hhydration /kJ mol1 405 1926 460 The enthalpy change of solution of Mg(OH)2 is 152 kJ mol1. (i) Define, in words, the terms enthalpy change of solution and enthalpy change of hydration. enthalpy change of solutionenthalpy change of hydration[3] (ii) Explain the difference between the Hhydration values for Na+ and Mg2+.[3]OCR 2013<br />
(iii) A BornHaber cycle can be drawn to link the lattice enthalpy and enthalpy change of solution of Mg(OH)2 with hydration enthalpies. On the two dotted lines, add the species present, including state symbols.lattice enthalpyMg(OH)2(s) Mg2+(aq) + 2OH(aq) [2] (iv) Calculate the lattice enthalpy of Mg(OH)2. lattice enthalpy =kJ mol1 [2]OCR 2013 Turn over<br />
(b) Energy changes for the melting and boiling of H2O are shown below. H = +6.01 kJ mol1 H = +40.7 kJ mol1 H2O(l) H2O(g) H2O(s) H2O(l) Standard entropies of H2O in its three physical states are given in the table below. S o / J K1 mol1 H2O(s) +48.0 H2O(l) +70.0 H2O(g) +188.7 (i) Explain the following:When water melts or boils, H is positiveWhen water melts or boils, S o increases. In your answer, you should explain why the increase in S o is much greater when water boils than when water melts.[3] (ii) Using the data in the table above, show that ice melts at 0 C (at standard pressure).OCR 2013 [3] [Total: 16]<br />
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F325 Mark Scheme January 2013 Question 3 (a) (i) solution: (enthalpy change for) Answer 1 mole of a compound/substance/solid/solute dissolving (cid:57) ---------------------------------------------------- hydration: (enthalpy change for) 1 mole of gaseous ions OR 1 mole of hydrated/aqueous ions (cid:57) gaseous ions forming aqueous/hydrated ions (cid:57) Marks Guidance IGNORE energy released OR energy required For dissolving, ALLOW forms aqueous/hydrated ions DO NOT ALLOW dissolving elements IGNORE ionic OR covalent DO NOT ALLOW response that implies formation of 1 mole of aqueous ions ----------------------------------------------------------------- IGNORE energy released OR energy required For final mark IGNORE gaseous ions are hydrated IGNORE gaseous ions dissolve Particles formed not stated<br />
F325 Mark Scheme January 2013 Question 3 (a) (ii) For 1st two marking points (Charge and Size), Answer IGNORE atomic and atoms and assume that Mg or Na refer to ions, e.g. ALLOW Mg has a smaller (atomic) radius ----------------------- Charge Magnesium ion/Mg2+ has greater charge OR Mg2+ has greater charge density (cid:57) ---------------------- Size Magnesium ion OR Mg2+ is smaller (cid:57) ------------------------ Attraction Note: Correct particles required for this mark i.e. DO NOT ALLOW Mg; Mg atoms; Na; Na atoms Mg2+ has a stronger attraction/ force/ bonding to H2O /O (cid:57) Marks Guidance Note: Charge density can be used to credit the charge mark but not size mark --------------------------- ORA Sodium ion/Na+ has smaller charge OR Na+ has smaller charge density --------------------------- ORA: Sodium ion OR Na+ is larger IGNORE smaller charge density (charge mark above) IGNORE idea of close packing of ions ----------------------------- Note: Response must refer to attraction/bonding with H2O or this must be implied from the whole response ALLOW Mg2+ has a stronger iondipole attractions ORA: Na+ has weaker attraction/bonding to H2O DO NOT ALLOW a response implying that ionic bonds (between ions) OR covalent bonds OR hydrogen bonds are formed<br />
F325 Question (iii) 3 (a) (a) Answer Mg2+(g) + 2OH(g) Mg2+(aq) + 2OH(g) Mark Scheme January 2013 (cid:57) (cid:57) Marks Guidance Correct species AND state symbols required for both marks Mark each marking point independently ALLOW response on lower line: Mg2+(g) + 2OH(aq) (i.e. OH hydrated before Mg2+) (iv) FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 2694 (kJ mol1) award 2 marks -------------------------------------------------------------------- Lattice enthalpy (Mg(OH)2) = [1926 + (2 x460)](152) OR2846 + 152 (cid:57) = 2694 (cid:57) (kJ mol1) 10 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: 2234: use of OH rather than 2 x OH (+)2694: signs all reversed 2998: sign wrong for 152 (+)1158: sign wrong for 1926 854: sign wrong for 2 x 460 (+)2998: sign wrong for 2846 IF ALL 3 relevant values from the information at the start of Q3 have NOT been used, award zero marks unless one number has a transcription error, where 1 mark can be awarded ECF<br />
F325 Question (i) 3 (b) (b) (ii) Mark Scheme January 2013 Marks Answer H positive (Intermolecular) bonds/forces are being broken (cid:57) S Increase in disorder/ randomness/ number of arrangements (of particles/molecules/energy) (cid:57) Comparison of S (QWC) In a gas, molecules/particles are much more disordered/ random (than in liquids and solids) (cid:57) (cid:120) (cid:120) (cid:120) = 273 (K) S = (cid:54)S(products)(cid:54)S(reactants) = 70.048.0 OR 22(.0) OR 0.022 (kJ K1 mol1) (cid:57)(cid:3) T = 6.01 0.022 OR G = 6.01273 x 0.022 (cid:57) G = 0 OR 0 = HTS stated anywhere (cid:57) Guidance ALLOW hydrogen bonds DO NOT ALLOW breaking of ionic OR covalent bonds IGNORE a response comparing bonds made and bonds broken (boiling involves just breaking bonds) ALLOW liquids are more disordered than solids OR gases are more disordered than liquids ALLOW in a gas, molecules are much further apart (than in liquids and solids) IGNORE S is much greater (in question) FULL ANNOTATIONS MUST BE USED --------------------------------------------------------------------------- NO UNITS required ALLOW 273.18 (K) OR 273.2 (K) ASSUME units are K unless told otherwise ALLOW G = 6.016.006 = +4 x 103 ALLOW 4 x 103 ~ 0 ALLOW 4 x 103 is very close to zero Total 16 11<br />

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