Q25.
(a) (Q = mcΔT)
= 50 × 4.18 × 27.3
If incorrect (eg mass = 0.22 or 50.22 g) CE = 0 / 2
1
= 5706 J (accept 5700 and 5710)
Accept 5.7 kJ with correct unit. Ignore sign.
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(b) Mr of 2-methylpropan-2-ol = 74(.0)
For incorrect Mr, lose M1 but mark on.
1
Moles = mass / Mr
= 0.22 / 74(.0)
= 0.00297 moles
1
ΔH = –5706 / (0.002970 × 1000)
= –1921 (kJ mol–1)
If 0.22 is used in part (a), answer = –8.45 kJ mol–1 scores 3
(Allow –1920, –1919)
If uses the value given (5580 J), answer = –1879 kJ mol–1 scores 3
Answer without working scores M3 only.
Do not penalise precision.
Lack of negative sign loses M3
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(c) ΔH = ΣΔH products – ΣΔH reactants
OR a correct cycle
Correct answer with no working scores 1 mark only.
1
ΔH = −(−360) + (4 × −393) + (5 × −286)
M2 also implies M1 scored.
1
ΔH = –2642 (kJ mol–1) This answer only.
Allow 1 mark out of 3 for correct value with incorrect sign.
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(d) (–2422 – part (b)) × 100 / –2422
Ignore negative sign.
Expect answers in region of 20.7
If error carried forward, 0.22 allow 99.7
If 5580 J used earlier, then allow 22.4
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(e) Reduce the distance between the flame and the beaker / put a sleeve around the flame to protect from drafts / add a lid / use a copper calorimeter rather than a pyrex beaker / use a food calorimeter
Any reference to insulating material around the beaker must be on top.
Accept calibrate the equipment using an alcohol of known enthalpy of combustion.
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(f) Incomplete combustion
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