Energetics
Physical Chemistry · 3 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 7% of your exam marks.
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
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 7 °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 × 7
- M3: Q = 3586.8 J ≈ 3.59 kJ
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.