Burning 0.0200 mol of a liquid fuel raised the temperature of 100 g of water by 25.0 C. The specific heat capacity of water is 4.18 J/g/C. Calculate the enthalpy change of combustion of the fuel, in kJ/mol.
[4 marks]
Total for this question: 4
All calculations
Enthalpy change from calorimetry
Physical chemistry
AQA 7405
ORIGINAL
q = m c dT then dH = -q / n (n = moles of the substance that reacted)
q = m c dT gives the energy; divide by the moles reacted and flip the sign for exothermic.
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Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Where the marks get lost
- Using the mass of the fuel instead of the mass of the WATER in q = m c dT.
- Forgetting to divide by the moles of fuel to get a per-mole value.
- Leaving the answer positive — combustion is exothermic, so dH is negative.
Exam tip: q = m c dT uses the water's mass and temperature change. The sign is the last step: exothermic (temperature rose) means dH is negative.
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