Calculating the heat
The heat change in the reaction is calculated from the temperature change of the mixture.
Heat change
Q = m c θ
- m — the mass of the substance exchanging heat (the mass of the mixture)
- c — its specific heat capacity
- θ — the temperature change of the mixture (highest temperature − starting temperature)
If the two solutions start at different temperatures, take their average as the starting temperature.
The assumptions
- All the heat of the reaction goes into raising the temperature of the 100 cm3 of solution.
- The solutions are dilute, so the mixture has the specific heat capacity and the density of water.
A worked example: the temperature rises by 10 °C
Specific heat capacity of water = 4200 J kg-1 °C-1
Density of water = 1 g cm-3, so 100 cm3 has a mass of 100 g = 0.1 kg
Q = m c θ
Q = 0.1 kg × 4200 J kg-1 °C-1 × 10 °C
Q = 4200 J
That is the heat change when 50 cm3 of 2 mol dm-3 sodium hydroxide reacts with 50 cm3 of 2 mol dm-3 hydrochloric acid.