Rates & Equilibrium

IGCSE Edexcel Chemistry
3.9–3.18 Rates of reaction, collision theory, catalysts, and reversible reactions
Key Concepts: Reaction rate depends on the frequency and energy of successful collisions. Factors including temperature, concentration, surface area, and catalysts affect the rate. Reversible reactions reach dynamic equilibrium in a closed system.

Section A — Measuring Reaction Rate

1. Define rate of reaction and state two ways of measuring it. [4]
2. A student collects gas from a reaction. The volume collected every 10 seconds is shown below. Calculate the average rate of reaction between 0 and 30 seconds. [2]
Time (s)01020304050
Volume of gas (cm³)01222303640

Section B — Factors Affecting Rate

3. State four factors that affect the rate of a reaction. [4]
4. Explain, using collision theory, why increasing temperature increases the rate of reaction. [3]
5. Marble chips react with hydrochloric acid. Explain, using collision theory, why powdered marble reacts faster than large marble chips. [3]
6. Explain why increasing the concentration of a reactant increases the rate of reaction. [2]

Section C — Catalysts

7. Define catalyst. [2]
8. Explain how a catalyst increases the rate of reaction. Refer to activation energy. [3]
9. Name the catalyst used in the Haber process and the Contact process. [2]

Section D — Reversible Reactions and Equilibrium

10. Define a reversible reaction. Use the symbol ⇌ in your answer. [2]
11. Describe the reversible reaction of hydrated copper(II) sulfate. Write the equation. [3]
12. Define dynamic equilibrium. State two conditions required for a system to reach equilibrium. [4]
13. In the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Explain what happens to the equilibrium position if: [4]

a) Temperature is increased

b) Pressure is increased

Total marks: 38

Mark Scheme

1. Rate = change in amount of reactant or product per unit time [2]; any two: measure volume of gas produced per unit time; measure decrease in mass per unit time; measure change in colour/turbidity; measure change in concentration [2] [4]
2. Volume collected in 30 s = 30 cm³; rate = 30/30 = 1.0 cm³ s⁻¹ [2]
3. Temperature; concentration (of dissolved reactants or pressure of gases); surface area (of solid); catalyst [4]
4. Increasing temperature increases the kinetic energy of particles [1]; particles move faster so collide more frequently [1]; a greater proportion of particles have energy ≥ activation energy → more successful collisions per second [1] [3]
5. Powdered marble has a greater surface area than large chips [1]; more acid particles can collide with the marble surface at any one time [1]; more frequent successful collisions → faster rate [1] [3]
6. Higher concentration means more reactant particles in the same volume [1]; more frequent collisions with other reactant particles → greater rate [1] [2]
7. A substance that speeds up a chemical reaction [1] without being used up / without changing itself chemically [1] [2]
8. A catalyst provides an alternative reaction pathway [1]; with lower activation energy [1]; more particles have energy ≥ Ea, so more successful collisions occur per second [1] [3]
9. Haber process: iron catalyst; Contact process: vanadium(V) oxide (V₂O₅) [2]
10. A reaction in which the products can react to reform the original reactants [1]; represented using ⇌ symbol [1] [2]
11. CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O [1]; heating drives the forward reaction (dehydration) [1]; adding water to the anhydrous solid reverses it [1] [3]
12. Dynamic equilibrium: the forward and reverse reactions proceed at equal rates [1]; concentrations of reactants and products remain constant [1]; conditions: closed system [1]; constant temperature [1] [4]
13. a) Increasing temperature favours the endothermic (reverse) reaction [1]; equilibrium shifts to the left, less NH₃ is produced [1]; b) Increasing pressure shifts equilibrium to the side with fewer moles of gas [1]; there are 4 moles on the left and 2 on the right → equilibrium shifts right, more NH₃ produced [1] [4]