Topic 2 of 5

Calorimetry

Convert the surroundings temperature change into energy per mole of reaction.

A-Level 9476 (2026-2027)

The thermometer measures the surroundings, not the reaction directly

Find q for what warms or cools, reverse its sign, then divide by the reacting amount.

q = mcΔT, where m is the mass being warmed, c is its specific heat capacity and ΔT is final minus initial temperature. With m in g and c in J g^-1 K^-1, q is in J. A temperature interval of 1 K equals an interval of 1 °C. For an insulated arrangement, q_reaction = -q_surroundings.

Worked example

Neutralisation measured in an insulated cup

50.0 cm³ of 1.00 mol dm^-3 HCl is mixed with 50.0 cm³ of 1.00 mol dm^-3 NaOH. The temperature rises by 6.80 K. Assume density 1.00 g cm^-3, c = 4.18 J g^-1 K^-1 and negligible cup heat capacity.

Total solution mass is 100.0 g, not 50.0 g.
q(solution) = 100.0 × 4.18 × 6.80 = +2842.4 J, so q(reaction) = -2.8424 kJ.
Both acid and base supply 0.0500 mol; 0.0500 mol water forms.
Divide by the water amount: delta H = -2.8424/0.0500.

Answer

ΔH_neut = -56.8 kJ mol^-1 to three significant figures.

If the calorimeter heat capacity C is supplied, include its heat gain: q_surroundings = (mc + C)ΔT. In a combustion experiment, use the mass of water warmed and the amount of fuel actually burnt. In a dissolution experiment, use the appropriate solution mass and heat capacity, and divide by the amount of solute dissolved.

What a smaller temperature rise might mean
Effect	Consequence for an exothermic result
Heat lost to the room or absorbed by an ignored vessel	Measured heat release is too small in magnitude; the calculated enthalpy is less negative.
Incomplete combustion	Less energy is released than for complete combustion to the stated products.
Fuel evaporates without burning	Apparent fuel consumption is too large; energy per measured mole is too small in magnitude.
Reactant concentrations or final temperature are mismeasured	Determine the direction from the actual calculation; there is no universal error direction.

A temperature-time record before and after mixing can support extrapolation to estimate the temperature at mixing, reducing a systematic heat-loss error. Repeated trials reveal scatter but do not by themselves remove the same heat loss from every trial. Always identify the limiting reagent before dividing q by an amount.