Measuring the heat of reaction
The heat of reaction is the heat change when the number of moles shown in the equation react. In the laboratory we measure the temperature change θ of a known mass of solution and calculate the heat released or absorbed using the formula below. Water is assumed to have specific heat capacity c = 4.2 J g⁻¹ °C⁻¹ and density 1 g cm⁻³.
Key idea / 要点
Q = mcθ where Q = heat (J), m = mass of solution (g), c = specific heat capacity (4.2 J g⁻¹ °C⁻¹), θ = temperature change (°C). Then ΔH = −Q ÷ n per mole, in kJ mol⁻¹.
Types of heat of reaction
- Heat of neutralisation — heat released when an acid is neutralised by an alkali to form one mole of water.
- Heat of combustion — heat released when one mole of a fuel burns completely in oxygen.
- Heat of displacement — heat released when one mole of a metal is displaced from its salt by a more reactive metal.
- Heat of precipitation — heat change when one mole of a precipitate forms.
Worked example / 例题
50 cm³ of 1.0 mol dm⁻³ HCl is neutralised by 50 cm³ of 1.0 mol dm⁻³ NaOH. The temperature rises by 6.8 °C. Total mass m = 100 g. Q = mcθ = 100 × 4.2 × 6.8 = 2856 J = 2.856 kJ. Moles of water n = 0.050 mol. ΔH = −2.856 ÷ 0.050 = −57.1 kJ mol⁻¹. The negative sign confirms neutralisation is exothermic.
Why the value can vary
The heat of neutralisation of a strong acid with a strong alkali is close to −57 kJ mol⁻¹ because the reaction is simply H+(aq) + OH−(aq) → H2O(l). With a weak acid the value is smaller because some energy is used to ionise the weak acid fully.
Remember / 记住
- Use m = total mass of solution, not the acid alone.
- Heat gained by solution = heat released by reaction (ignoring losses).
- Divide by moles of the substance stated to get ΔH per mole.