Edexcel Jun 2014 (R) Paper 4 Q8 (with explained solutions) |

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Edexcel Jun 2014 (R) Paper 4 Q8

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 8 This question is about calcium chloride, CaCl2. It can be formed by burning calcium in chlorine. Ca(s) + Cl2(g) (cid:111) CaCl2(s) (cid:507)S(cid:57) surroundings = +2670 J mol1 K1 You must include a sign and units in your answers to the calculations in this question. (a) (i) The standard molar entropy at 298 K for 1 mole of chlorine molecules, Cl2, is +165 J mol1 K1. Use this, and appropriate values from your Data Booklet, to calculate the standard entropy change, (cid:507)S (cid:57) system, for this reaction. *(ii) Explain fully why the sign for the standard entropy change of the system, (cid:507)S (cid:57) system, is as you would expect. (2) (2)(b) Calculate the total entropy change, (cid:507)S (cid:57) your answer to three significant figures. total, in J mol1 K1, for this reaction, giving (2) *P42988A0924* Turn over<br />
(2) (1) (2) (c) Use the standard entropy change of the surroundings, (cid:507)S(cid:57) standard enthalpy change, (cid:507)H (cid:57) , in kJ mol1, for the reaction at 298 K. surroundings, to calculate the (d) 0.0500 mol of calcium chloride, prepared by burning calcium in chlorine, is added to 51.8 cm3 of water. 50.0 cm3 of a 1.00 mol dm3 solution is formed, and the temperature rise, (cid:507)T, is 15.0C. (i) Calculate the energy transferred, in joules, for this process using: Energy transferred in joules = volume of solution formed (cid:117) 4.2 (cid:117) (cid:507)T (ii) Calculate the enthalpy change of solution, (cid:507)Hsolution, of calcium chloride in kJ mol1. 10 *P42988A01024*<br />
*(iii) The enthalpy change of hydration of Ca2+(g) is 1560 kJ mol1. Use this, your value from (d)(ii) and the experimental lattice energy from your Data Booklet, to calculate the standard enthalpy change of hydration of Cl(g). (3) Ca2+(g) + 2Cl(g) CaCl2(s) Ca2+(aq) + 2Cl(aq) AnswerkJ mol1 (iv) Draw diagrams to represent hydrated calcium ions and hydrated chloride ions. (v) Suggest why the addition of anhydrous calcium chloride to water results in an increase in temperature and a decrease in volume. (2) (2) Temperature increasesVolume decreases(Total for Question 8 = 18 marks) *P42988A01124* 11 Turn over<br />

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Section B Question Number 8(a)(i) Reject Mark Acceptable Answers +104.6[+41.4 +165] (1) = -101.8 J mol-1 K-1 Value, sign and unit (1) Ignore SF except one Internal TE allowed for recognisable numbers, for example: Ho OR Halving So [Cl2] (82.5-19.3) Correct answer with no working (2) +/no sign 101.8 J mol-1 K-1 (1) at calcium instead of So (178.2-238.6) 6CH0401R 1406<br />
Question Number 8 (a)(ii) Acceptable Answers Reject (The sign is negative because) Mark Any two from:Energy (Positive) Answer is as expected (A solid and) a gas reacting to form a solid. OR (Entropy decreases because) a gas reacting to form a solid.There are fewer ways of arranging particles in a solid than a gas or vice-versa. OR Decrease in disorder as solid more ordered than gas or vice versaTwo mol(es) of reactant forming one mole of product. (Ignore two molecules form one molecule) OR Number of mol(es)/molecules decreases OR Fewer/less mol(es) of products than reactants COMMENT If answer to (a)(i) is positive then answer should start Answer is not as expected because Then score as above (which can score full marks). 6CH0401R 1406<br />
Question Number 8 (b) system total = So surroundings + So Correct Answer So OR = +2670 + (-101.8) = (+)2568.2 Value 2568.2/2568 (1) = (+)2570 (J mol-1 K-1) 3SF This mark is conditional on correct value or correct TE value from (a)(i) (1) Accept TE from (a)(i) -238.6+2431.4+2430 -19.32650.7+2650 Correct answer (2570, etc) with or without working scores (2) Reject Mark 6CH0401R 1406<br />
Question Number 8 (c) Correct Answer1 So surroundings = - Ho 298 Ho = - So OR = -2670 x 298 surroundings x 298 (1) = -795.660 = -795.7 (kJ mol-1) (1) ALLOW = -795.7 x103 J mol-1 Note 1. -796 = -796.1964 (if 2570 used to calculate entropy change of surroundings first.) 2. Ho (= + So = +795.7 (kJ mol1) (1) But Ho = - So 298 Ignore SF except one surroundings (0) surroundings x 298) Reject Mark 6CH0401R 1406<br />
Question Number 8 (d)(i) 50 x 4.2 x 15.0 Correct Answer = -63 (kJ mol-1) /-63000 J mol-1 Allow TE answer (d)(i) / 0.05 Ignore SF except one Value (1) Sign (1) The mark for the negative sign is awarded for the calculation even if the value is wrong, providing any energy divided by moles or energy multiplied by 1/number of moles calculation has been done. = 3150 (J) Ignore sign ALLOW 3.15 kJ Ignore SF except one Question Number 8 (d)(ii) 3150/0.05 or 20 x 3150 Correct Answer Reject Mark Reject Mark 6CH0401R 1406<br />
Question Number *8 (d)(iii) Correct Answer The correct answer: -380.5/-381 kJ mol-1 Full marks with or without correct working. First mark Appreciation of Hesss Law either in words, numbers, symbols or on the diagram For example, Hsolution + Lattice energy = Hhydration Ca2+ + (2)Hhydration Cl (1) Second mark 2 Hhydration Cl = 225863(1560) = 761 ALLOW Any number or group of numbers minus (-1560) (1) Third mark Hhydration Cl =380.5/-381 (kJ mol1) Any number, wherever it has come from, divided by two can score this mark, provided that the sign is consistent. (1) Ignore SF except one Use of lattice energy2223 gives 363 scores (2) ALLOW TE from (d)(ii) Reject Mark 6CH0401R 1406<br />
Correct Answer Question Number 8 (d)(iv) ORH/ hydrogen (one or two hydrogens from each water molecule) towards chloride ion and O(cid:31)(cid:31)(cid:31)/ oxygen (one oxygen from each water molecule) towards calcium ion With negative charge either on chlorine or on the whole hydrated ion and with double positive charge either on calcium or on the whole hydrated ion. ALLOWOne/several water molecule(s) all correctly orientated.A minus sign with a ring around it for the Cl and a 2+ sign with a ring around it for the Ca2+Bonds shown by lines/broken lines/dotted lines/wedges Reject Mark Cl-.H2O H/ H/ H Cl / Cl (with no charge) 6CH0401R 1406<br />
Question Number 8(d)(v) Both marks may be awarded in either part. Correct Answer Reject Mark First mark (Temperature increases) because the reaction/process/dissolving/hydration of ions is exothermic. OR Strong(er) forces between the + H and Cl OR Strong(er) forces between theO and Mg2+ OR Strong(er) ion-dipole forces OR Formation of bonds releases energy OR Strong(er) bonds formed OR Enthalpy of hydration is greater than lattice energy (1) Second mark (Volume decreases so) shorter bonds between ion and water molecules ALLOW Water molecules more tightly arranged/pack better/occupy less space OR Water molecules more ordered/ clustered (around the ions). (1) The breaking of the lattice is exothermic. Ions more tightly arranged Ions more ordered Total 18 marks 6CH0401R 1406<br />

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