AQA Jun 2017 Paper 1 Q3 (with explained solutions) |

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AQA Jun 2017 Paper 1 Q3

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0 3 A student planned and carried out an experiment to determine the enthalpy of reaction when magnesium metal displaces zinc from aqueous zinc sulfate. Mg(s) + Zn2+(aq)Mg2+(aq) + Zn(s) The student used this method:A measuring cylinder was used to transfer 50 cm3 of a 1.00 mol dm3 aqueous solution of zinc sulfate into a glass beaker.A thermometer was placed in the beaker.2.08 g of magnesium metal powder were added to the beaker.The mixture was stirred and the maximum temperature recorded. The student recorded a starting temperature of 23.9 oC and a maximum temperature of 61.2 oC. 0 3 . 1 Show by calculation which reactant was in excess. Use the data to calculate the experimental value for enthalpy of reaction in kJ mol1(Assume that the specific heat capacity of the solution is 4.18 J K1g1and the density of the solution is 1.00 g cm3). [6 marks] Reactant in excess Enthalpy of reaction kJ mol1 *06* IB/M/Jun17/7404/1<br />
0 3 . 2 Another student used the same method and obtained a value for the enthalpy of reaction of 142 kJ mol1 A data book value for the enthalpy of reaction is 310 kJ mol1 Suggest the most likely reason for the large difference between the students experimental value and the data book value. [1 mark] Question 3 continues on the next page *07* Turn overIB/M/Jun17/7404/1<br />
Suggest how the students method, and the analysis of the results, could be improved in order to determine a more accurate value for the enthalpy of reaction. Justify your suggestions. Do not refer to the precision of the measuring equipment. Do not change the amounts or the concentration of the chemicals. [6 marks] 13 IB/M/Jun17/7404/1 0 3 . 3 *08*<br />

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MARK SCHEMEAS-LEVEL CHEMISTRY7404/1JUNE 2017 Question 03.1 03.2 Marking Guidance Mark Additional Comments/Guidance M1 Amount ZnSO4 = 1.0 x50/1000 mol or Amount ZnSO4 =0.050 mol M2 Amount Mg = 2.08/24.3 mol or Amount Mg= 0.0856 mol (Hence Mg in excess) M3 Q=mcT M4 Q=50.0 x 4.18 x 37.3 or Q=7795.7J M5 (Energy released per mole) =7.796/0.05kJmol1 or 7796/0.05 Jmol1 M6 H=156 kJmol1 Heat loss (from the apparatus would mean the experimental value is smaller/lower/less exothermic than the data source) 1 1 1 1 1 1 Mark M1 and M2 independently M3 could be scored in M4 If an error in M4, lose M4 and M5 and only award M6 for correct use of their incorrect M4 and division by their correct limiting reagent M5 division by their limiting reagent 13 of 24<br />
MARK SCHEMEAS-LEVEL CHEMISTRY7404/1JUNE 2017 Question 03.3 Marking Guidance Mark Comments Marks awarded for this answer will be determined by the as well as the standard of the scientific response. Examiners should apply a best-fit approach to the marking. Indicative Chemistry Content Stage 1 Improved insulation argument is well structured with minimum repetition or accurate and clear expression of ideas with only minor Covers 3 Stages with matching justifications Answer is full and detailed and is supported by an appropriate range of relevant points such as those given below: - irrelevant points - errors in the use of technical terms, spelling and punctuation and grammar Covers 2 Stages with matching justification. OR covers 3 Stages with incomplete justification Answer has some omissions but is generally supported by some of the relevant points below: - - the argument shows some attempt at structure the ideas are expressed with reasonable clarity but with a few errors in the use of technical terms, spelling, punctuation and grammar Covers 1 Stage with matching justification. OR covers 2 Stages with incomplete justification Answer is largely incomplete. It may contain valid points which are not clearly linked to an argument structure. Unstructured answer. Errors in the use of technical terms, spelling, punctuation and grammar or lack of fluency Level 3 (56 marks) Level 2 (34 marks) Level 1 (12 marks) 1a Insulate the beaker or use a polystyrene cup or a lid 1b To reduce heat loss Stage 2 Improved temperature recording 2a Record the temperature for a suitable time before adding the metal 2b To establish an accurate initial temperature OR 2c Record temperature values at regular time intervals 2d To plot the temperature results against time on a graph Stage 3 Improved analysis of results 3a Extrapolate the cooling back to the point of addition 3b To establish a (theoretical) maximum temperature OR temperature change (e.g. at the 4th minute) OR adjust for the cooling /apply a cooling correction 3a and 3b could be seen on an extrapolated sketch graph (Note- IGNORE use of measuring equipment with greater precision) 14 of 24<br />

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