Calculating Energy Changes — Energetics | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable

Low

Stable7%

Calorimetry calculations and energy profile diagrams appear in nearly every series.

The heat-energy formula

Q = m c ΔT

Q = heat energy transferred, in joules (J)
m = mass of the substance being heated, in grams (g) — usually the mass of the solution, or the mass of water in a combustion experiment
c = , the energy needed to raise the temperature of 1 g of substance by 1 °C. For water, c = 4.18 J g⁻¹ °C⁻¹
ΔT = change in temperature, in °C (the same numerical value as the change in kelvin)

Sign convention

If the temperature rises, the reaction is exothermic and ΔH is negative
If the temperature falls, the reaction is endothermic and ΔH is positive
After computing Q with positive numbers, attach the correct sign at the end

Energy per gram and molar enthalpy of combustion

For a fuel, the answers most often wanted are:
- Energy released per gram: Q ÷ (mass of fuel burnt)
- Energy released per mole (the ): ΔH = Q ÷ (moles of fuel)
These two values make different fuels easy to compare

Worked example

Calculating heat energy from a neutralisation experiment

A student mixes 100 cm³ of sodium hydroxide solution with 22 cm³ of hydrochloric acid, and the temperature rises by 35 °C. Calculate the heat energy transferred to the solution (c = 4.2 J g⁻¹ °C⁻¹).

Solution:

M1: mass of solution = 100 + 22 = 122 g (add both volumes together; density ≈ 1 g cm⁻³)
M2: Q = m × c × ΔT = 122 × 4.2 × 35
M3: Q = 17 934 J ≈ 17.9 kJ

Exam tip

Sign convention and unit conversion for calorimetry calculations

What comes up: after finding Q in joules, the question asks for ΔH in kJ mol⁻¹ and requires the correct sign.

Write: divide Q by 1000 to convert J to kJ, then divide by the number of moles of the limiting reactant. Apply a negative sign if the reaction is exothermic (temperature rose); a positive sign if endothermic (temperature fell).

Watch out: in a combustion experiment the mass to use in Q = mcΔT is the mass of water being heated, not the mass of fuel. The sign is a separate mark; forgetting it costs a mark even if the numerical value is correct.