Energetics

IGCSE Edexcel Chemistry
3.1–3.4 Energetics and Calorimetry
Key Concepts: Reactions may be exothermic or endothermic. Energy changes can be measured using calorimetry: $Q = mc\Delta T$. Molar enthalpy change can be calculated using $\Delta H = Q/n$.

Section A: Exothermic and Endothermic

1. Define exothermic and endothermic reactions and give one example of each. [4]

Section B: Calorimetry

2. A reaction heats 100 g of water from 20°C to 28°C. Calculate the energy released. ($c = 4.2\ \text{J g}^{-1}\,°C^{-1}$) [3]
3. 0.25 mol of magnesium reacts and releases 12 kJ. Calculate the molar enthalpy change in kJ/mol. [2]

Section C: Practical Applications

4. Describe a calorimetry experiment to compare energy changes for neutralisation reactions. [4]
5. A fuel releases 50 kJ when 2.0 g burns completely. Calculate energy per gram. [2]

Total marks: 15

Mark Scheme

1. Exothermic: releases heat; e.g. combustion. Endothermic: absorbs heat; e.g. thermal decomposition (any one each) [4]
2. Q = mcΔT = 100 × 4.2 × 8 = 3360 J (3.36 kJ) [3]
3. ΔH = Q/n = 12/0.25 = 48 kJ mol⁻¹ (exothermic) [2]
4. Measure initial temperature, mix reactants in insulated cup, record max/min temperature, calculate ΔT, use Q=mcΔT and compare (any four) [4]
5. 50 kJ / 2.0 g = 25 kJ g⁻¹ [2]