Edexcel Jun 2012 Paper 4 Q18 (with explained solutions) |

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Edexcel Jun 2012 Paper 4 Q18

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 18 7KLVTXHVWLRQLVDERXWWKHVROXELOLWRIVRPH*URXSKDOLGHV (a) Potassium fluoride is a soluble, white, crystalline solid used in etching glass. A Hess cycle can be used to calculate its enthalpy of solution, using data including enthalpies of hydration of ions. Define the term enthalpy of hydration of an ion. (2)(b) Consider the Hess cycle below. K+(g) + F(g) Hsol H KF(s) H2 N-PRO (i) Complete the cycle by filling in the empty box. (1) *P39308A02124* 21 Turn over<br />
LL $SSO+HVVV/DZWRREWDLQDQH[SUHVVLRQIRUHsol LQWHUPVRIH DQGH2 . Hsol = LLL *LYHWKHQDPHRIWKHHQHUJFKDQJHH. (1) (1)LY 5HIHUULQJWRSDJHRIWKHGDWDERRNOHWDQGRXUDQVZHUWR LL FDOFXODWHWKH standard enthalpy of solution of potassium fluoride. F 7KHVWDQGDUGHQWKDOSRIVROXWLRQRIVRGLXPFKORULGHLVN-PRO. L JRIVRGLXPFKORULGHZDVDGGHGWRFP3RIZDWHULQDEHDNHUDQGVWLUUHG ZLWKDWKHUPRPHWHUJUDGXDWHGLQLQWHUYDOVRI&'HVFULEHDQGH[SODLQ what would happen to the reading on the thermometer as the sodium chloride dissolves. No calculation is required. (2) (3)22 *P39308A02224*<br />
*(ii) Explain, in terms of entropy changes, why sodium chloride dissolves in water under standard conditions. No calculation is required. (4)*(d) Lithium iodide is generally much more soluble in organic solvents than lithium chloride. Explain this observation using values of lattice energies from your data ERRNOHWDQGRXUNQRZOHGJHRIWKHWUHQGLQLRQLFUDGLLGRZQ*URXS (4)(Total for Question 18 = 18 marks) TOTAL FOR SECTION C = 18 MARKS TOTAL FOR PAPER = 90 MARKS *P39308A02324* 23<br />

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Question Number 18(a) Correct Answer First mark Enthalpy change when 1 mol of gaseous ions (1) ALLOW energy change/heat change/energy evolved/released/ given out/exothermic Second mark Is dissolved/hydrated/solvated such that further dilution causes no further heat change OR Is dissolved to produce an infinitely dilute solution/in excess water (1) Reject Energy required or energy taken in Atoms or molecules (0) 1 mol of water ALLOW Is dissolved to produce a solution of 1.0 mol dm-3 Question Number 18(b)(i) K+(aq) (+) F(aq) Acceptable Answers Reject K+ F(aq) Reject Question Number 18(b)(ii) Hsol = H1 + H2 Acceptable Answers OR Hsol = H2H1 Reject Acceptable Answers Question Number 18(b)(iii) (Standard) Lattice(enthalpy/energy/H) LE/Lat - Lattice Mark Mark Mark Mark<br />
Mark Mark Reject -12 (max 1) Reject Any reference to temp increase /exothermic Just accuracy +/- 1 oC Question Number 18(b)(iv) First mark Acceptable Answers Selection of (-)817 rather than (-)807 (1) Second mark Hsol = 817805 = (+)12 (kJ mol-1) (1) Just (+)12 (kJ mol-1) (2) ALLOW TE for second mark e.g. for 807 gives (+) 2 (kJ mol-1) ALLOW TE from incorrect b(ii) Question Number 18(c)(i) EITHER Acceptable Answers No change/no measurable change in temperature OR (Very small) decrease in temperature (1) Thermometer not sensitive/precise enough/precision of thermometer is + or - 0.5 oC/graduations too large (1) Amount of energy taken in is small /Hsol is small/mass of sodium chloride is small/slightly endothermic (1)<br />
Mark Reject S sur is negative S sys is positive Question Number *18(c)(ii) (The reaction is endothermic so) Acceptable Answers Entropy(change) of surroundings decreases OR S sur is negative OR -H/T is negative (1) But entropy (change)of system increases (as there is an increase in disorder) OR S sys is positive (1) Increase in entropy of system outweighs/greater than decrease in entropy of surroundings / value for entropy change of system is greater than entropy change of surroundings (1) Total entropy (change) is positive (1) All marks are stand alone<br />
The difference between Born Haber and theoretical LE is greater for LiI than for LiCl (1) (845 and 848 =) 3 for LiCl whereas (738 and 759 =) 21 for LiI (1) Iodide ion is larger than chloride ion/lower charge density on iodide ion (1) The iodide ion is more likely (than the chloride ion) to be polarized (by lithium ion) (1) LiI likely to have more covalent character than LiCl (1) Mark Reject Reject values with + Iodine/Chlorine atoms or molecules Iodine/Chlorine atoms or molecules TOTAL FOR SECTION C = 18 MARKS Acceptable Answers Question Number *18(d) Any four from:<br />

Question 14 15 16 17 18

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