Key Concepts: Exothermic reactions release heat energy; endothermic reactions absorb it. Energy profile diagrams show the relative energies of reactants, products, and the transition state. Heat energy changes can be calculated using Q = mcΔT.
Section A — Exothermic and Endothermic Reactions
1. Define exothermic reaction and endothermic reaction. [2]
2. State whether each reaction is exothermic or endothermic. [4]
| Reaction | Exothermic or Endothermic? |
|---|
| Combustion of methane | |
| Thermal decomposition of calcium carbonate | |
| Neutralisation of HCl with NaOH | |
| Dissolving ammonium nitrate in water | |
3. A reaction causes the temperature of a solution to fall from 22°C to 17°C. Is the reaction exothermic or endothermic? Explain your answer. [2]
Section B — Energy Profile Diagrams
4. Define activation energy. [2]
5. Sketch an energy profile diagram for an exothermic reaction. Label: reactants, products, activation energy (Ea), and the overall energy change (ΔH). [4]
6. State the sign of ΔH for: (a) an exothermic reaction, (b) an endothermic reaction. [2]
7. A catalyst is added to a reaction. Describe what happens to the activation energy and explain why the reaction rate increases. [3]
Section C — Q = mcΔT Calculations
8. State what each symbol in the equation Q = mcΔT represents and give the units. [4]
| Symbol | Meaning | Units |
|---|
| Q | | |
| m | | |
| c | | |
| ΔT | | |
9. 100 g of water warms by 12°C. Calculate Q. (c = 4.2 J g⁻¹ °C⁻¹) [2]
10. A reaction cools 50 g of water by 5°C. Calculate the heat energy absorbed by the reaction. State whether the reaction is exothermic or endothermic. [3]
11. 2.0 g of magnesium reacts with excess acid. The temperature of 100 g of solution rises by 5.7°C. Calculate Q and then determine ΔH per mole of Mg. (Ar Mg = 24, c = 4.2 J g⁻¹ °C⁻¹) [4]
Total marks: 32
Mark Scheme
1. Exothermic: releases heat energy to the surroundings; temperature of surroundings increases [1]; Endothermic: absorbs heat energy from the surroundings; temperature of surroundings decreases [1] [2]
2. Combustion of methane: exothermic; Thermal decomposition of CaCO₃: endothermic; Neutralisation of HCl+NaOH: exothermic; Dissolving ammonium nitrate: endothermic [4]
3. Endothermic [1]; the reaction absorbed heat from the solution, causing the temperature to fall [1] [2]
4. The minimum energy that colliding particles must have [1] for a reaction to occur / for bonds to begin breaking [1] [2]
5. Reactants higher than products on energy axis; curve peaks above reactants (transition state); Ea arrow from reactants to peak; ΔH arrow from reactants to products level (negative for exothermic) [4]
6. a) Negative (−ΔH); b) Positive (+ΔH) [2]
7. A catalyst provides an alternative reaction pathway with lower activation energy [1]; more particles have energy ≥ Ea [1]; more successful collisions per unit time → faster rate [1] [3]
8. Q: heat energy / J or kJ; m: mass of solution / g; c: specific heat capacity / J g⁻¹ °C⁻¹; ΔT: temperature change / °C [4]
9. Q = 100 × 4.2 × 12 = 5040 J [2]
10. Q = 50 × 4.2 × 5 = 1050 J [2]; the solution lost heat so the reaction absorbed it → endothermic [1] [3]
11. Q = 100 × 4.2 × 5.7 = 2394 J = 2.394 kJ [1]; moles Mg = 2.0/24 = 0.0833 mol [1]; ΔH per mole = 2.394/0.0833 = −28.7 kJ mol⁻¹ (negative = exothermic) [2] [4]