Edexcel Jun 2011 Paper 1 Q18 (with explained solutions) |

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Edexcel Jun 2011 Paper 1 Q18

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18 Copper(II) sulfate exists as blue hydrated crystals and white anhydrous crystals. The enthalpy changes of solution for these two substances may be represented by the following simplified equations: CuSO4.5H2O(s) + aq (cid:111) CuSO4(aq) (cid:507)H1 = +11.5 kJ mol1 PMT blue CuSO4(s) + aq (cid:111) CuSO4(aq) white (cid:507)H2 = 66.1 kJ mol1 (a) (i) Fill in the box and add labelled arrows to complete the Hess cycle to enable you to calculate (cid:507)Hreaction. CuSO4.5H2O(s) (cid:127)(cid:127)(cid:127)(cid:111) CuSO4(s) + 5H2O(l) (cid:507)Hreaction (cid:3) (3) (2) (1) (ii) Calculate a value for the enthalpy change (cid:507)Hreaction. (b) Suggest why it is not possible to directly measure the enthalpy change for the conversion of the blue hydrated copper(II) sulfate crystals into the white anhydrous crystals.14 *P38433A01424*<br />
*(c) (i) CuSO4.5H2O(s) + aq (cid:111) CuSO4(aq) (cid:507)H1 = +11.5 kJ mol1 Describe briefly the experimental procedure that you would use to obtain the data necessary to calculate (cid:507)H1, given a known mass of hydrated copper(II) sulfate crystals, CuSO4.5H2O(s). You should state the apparatus that you would use and any measurements that you would make. You are not required to calculate the amounts of substances or to explain how you would use the data obtained. PMT (4)(ii) The value for the enthalpy change from (c)(i) obtained by experiments in a school laboratory is likely to be significantly different from a data book value. List three possible reasons for this which do not relate to the quality of the apparatus or chemicals used or possible mistakes in carrying out the procedure. (3) 123(Total for Question 18 = 13 marks) *P38433A01524* 15 Turn over<br />

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Question Number 18 (a) (i) Reject Mark Acceptable Answers Product in box: CuSO4(aq) (1) Either Mark the arrows and then the labels: Two downward arrows (1) labelled with symbols or values with or without units (1) OR Mark each arrow and label separately Downward arrow & H1 or value (1) Downward arrow & H2 or value (1) Allow reversed arrows with reversed signs on H Ignore any other labels on the arrows. Ignore 5H2O in bottom product CuSO4.5H2O(s) CuSO4(s) + 5H2O(l) Hreaction H1 / +11.5 H2 /-66.1 CuSO4(aq) 6CH0101 1106<br />
Question Number 18 (a)(ii) Question Number 18 (b) Reject Mark Acceptable Answers Award higher mark from: Route 1 Mark the calculation based on their cycle TE from (a)(i) ignoring incorrect bottom product Route 2 Mark a calculation which is independent of the cycle (cid:85)Hreaction = (cid:85)H1 - (cid:85)H2 stated or implied = +11.5( 66.1) (1) = (+) 77.6 (kJ mol1) (1) Correct answer alone scores (2) 77.6 (kJ mol1) alone or from a correct addition scores (1) Mark Acceptable Answers Reject Temperature of solid / crystals cannot be measured Dehydration reaction cannot be controlled OR temperature change (of dehydration reaction) cannot be measured OR CuSO4.5H2O would need heating (so temperature change cannot be measured) OR impossible to add exact amount of water (to obtain value by reverse process) OR cannot mix solid with water to obtain perfect crystals 6CH0101 1106<br />
Question Number *18 (c)(i) Question Number 18 (c)(ii) Mark Mark Acceptable Answers Reject First & second marks stand alone 1. Pipette/burette / measuring cylinder / balance to transfer (a known amount of) (water) (1) 2. to (expanded) polystyrene cup / calorimeter / any insulated container allow coffee / plastic cup (1) Third & fourth marks only awarded if correct chemicals and procedure used 3. add solid and stir (allow mix or shake) mixture (1) 4. measure initial and final temperature allow temperature change (1) Just mass / volume measured Temperature increase unless exothermic penalised in (b) Acceptable Answers Reject Any three from:heat transfer (from surroundings) (allow loss or gain)approximation in (specific) heat capacity of solutionneglecting (specific) heat capacity of calorimeter/apparatus (allow energy absorbed by the apparatus) reaction / dissolving may be incomplete/slow Errors in calculation including adding mass of solid to mass of water loss of reagents / water incomplete combustion Just difficult to measure temperature change is very small (and difficult to measure)Density of solution is taken as the same as waterconditions not standard (allow) 6CH0101 1106<br />

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