Enthalpy change from calorimetry
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.
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.
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.
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.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.