Q15.
(a) Not possible to prevent some dissolving
ALLOW It is soluble / dissolves / other hydrates may form / suggestions related to difficulty of measuring T (change) of a solid
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(b) (ΔhydH =) –155 – (–39)
OR labelled cycle
Minimum needed for ‘labelled cycle’
–116 (kJ mol−1)
1/2 for (+)116 or for –29 or for seeing –116 that has then be processed further
(c) This question is marked using levels of response. Refer to the Mark Scheme Instructions for examiners for guidance on how to mark this question
Level 3 (5 – 6 marks)
All stages are covered and the explanation of each stage is correct and virtually complete.
Stage 2 must include use of a graphical method for Level 3 (i.e. ‘highest T reached’ method is max Level 2)
Answer communicates the whole explanation, including reference to enthalpy, coherently and shows a logical progression through all three stages.Answer is full and detailed and is supported by an appropriate range of relevant points such as those given below:
For the answer to be coherent there must be some indication of how the graph is used to find ΔT
Level 2 (3 – 4 marks)
All stages are covered (NB ‘covered’ means min 2 from each of stage 1 and 3) but the explanation of each stage may be incomplete or may contain inaccuracies
OR two stages covered and the explanations are generally correct and virtually complete
Answer is coherent and shows some progression through all three stages. Some steps in each stage may be out of order and incomplete
Level 1 (1 – 2 marks)
Two stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies
OR only one stage is covered but the explanation is generally correct and virtually complete
Answer shows some progression between two stages
Level 0 (0 marks)
Insufficient correct Chemistry to warrant a mark
Indicative Chemistry Content
Stage 1 Method
(1a) Measures water with named appropriate apparatus
(1b) Suitable volume/mass / volume/mass in range 10 – 200 cm3/g
(1c) Into insulated container / polystyrene cup (NOT just ‘lid’)
(1d) Add known mass of MgCl2(s)
(1e) Use of ‘before and after’ weighing method. NOT ‘added with washings’
Stage 2 Measurements (could mark from diagram)
(2a) Record initial temperature (min 2 measurements)
(2b) Record T at regular timed intervals for 5+ mins / until trend seen
(2c) Plot T vs time
Stage 3 Use of Results (3a and 3b could come from diagram)
(3a) Extrapolate lines to when solid added (to find initial and final T)
(3b) Tfinal – Tinitial = ΔT / idea of finding ΔT from graph at point of addition
(3c) q = mcΔT
(3d) amount = mass/Mr (0.80/95.3 = 8.39 × 10−3 mol)
(3e) ΔHsoln = –q/8.39 × 10−3 or in words
This could all be described in words without showing actual calculations but describing stages
If method based on ‘combustion’ Max Level 1
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