Rates of Reaction and Reversible Reactions

This guide covers subtopics 6.2, Rate of reaction, and 6.3, Reversible reactions and equilibrium, for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. They’re grouped together because equilibrium — where the forward and reverse reactions are both still happening, at equal rates — only makes sense once you understand what controls a reaction’s rate in the first place.

Scope note. This resource stays within the 0620/5070 treatment: qualitative collision theory and a fixed pair of worked industrial examples (the Haber and Contact processes). It does not use A Level (9701) ideas such as rate equations, orders of reaction, the equilibrium constant Kc, or the Arrhenius equation.

Rate of reaction

What changes the rate, and why

CORE (0620) · REQUIRED (5070) — describe the effect on rate of changing concentration, pressure (of gases), surface area (of solids), temperature, and adding or removing a catalyst (including enzymes); describe practical methods for investigating rate, including change in mass of a reactant or product, and the formation of a gas; interpret data, including graphs, from rate experiments.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — describe collision theory in terms of the number of particles per unit volume, the frequency of collisions, the kinetic energy of particles, and activation energy, Ea; use collision theory to explain, not just describe, the effect of each factor above; evaluate practical methods for investigating rate.

Collision theory says a reaction only happens when particles collide with enough energy to react — at least the activation energy, Ea. Anything that increases either the frequency of collisions or the proportion of collisions with enough energy increases the rate:

• Higher concentration or pressure — more particles in the same volume, so collisions happen more often.
• Smaller pieces (greater surface area) — more particle surface exposed, so more collisions happen at the reacting surface.
• Higher temperature — particles move faster and collide more often, and a greater proportion of collisions now have at least Ea — this second effect is the dominant one, which is why a modest temperature rise speeds a reaction up far more than the collision-frequency argument alone would suggest.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — a catalyst decreases the activation energy, Ea, of a reaction, providing an alternative reaction pathway; it increases the rate of reaction and is chemically unchanged at the end of the reaction (Core, both qualifications).

Reversible reactions and equilibrium

The idea of reversibility

CORE (0620) · REQUIRED (5070) — state that some chemical reactions are reversible, shown by the symbol ⇌; describe how changing conditions can change the direction of a reversible reaction, limited to the effect of heat on hydrated compounds and the addition of water to anhydrous compounds — specifically hydrated and anhydrous copper(II) sulfate, and hydrated and anhydrous cobalt(II) chloride.

CuSO4·5H2O(s)  ⇌  CuSO4(s) + 5H2O(l)
    blue              white

Heating blue hydrated copper(II) sulfate drives off the water of crystallisation, leaving white anhydrous copper(II) sulfate; adding water reverses this back to blue. The colour change is what makes this pair (and hydrated/anhydrous cobalt(II) chloride, pink to blue) a standard test for the presence of water.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — state that a reversible reaction in a closed system is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products are no longer changing; given information, predict and explain how the position of equilibrium is affected by changing temperature, pressure, concentration, or using a catalyst.

Equilibrium is not a static state — both reactions are still happening, at equal and opposite rates, so nothing appears to change overall. A catalyst speeds up both the forward and reverse reactions equally, so it changes how fast equilibrium is reached but never where the equilibrium position ends up.

The Haber process

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — state the symbol equation for the production of ammonia, the sources of the raw materials, and the typical industrial conditions.

N2(g) + 3H2(g)  ⇌  2NH3(g)

Nitrogen is sourced from the air; hydrogen from methane. The typical industrial conditions are 450 °C, 20 000 kPa (200 atm), with an iron catalyst.

The Contact process

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — state the symbol equation for the conversion of sulfur dioxide to sulfur trioxide, the sources of the raw materials, and the typical industrial conditions.

2SO2(g) + O2(g)  ⇌  2SO3(g)

Sulfur dioxide comes from burning sulfur or roasting sulfide ores; oxygen from the air. The typical conditions are 450 °C, 200 kPa (2 atm), with a vanadium(V) oxide catalyst.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — explain, in terms of rate of reaction and position of equilibrium, why these conditions are used, including safety and economic considerations.

Both processes use conditions that are a compromise, not the theoretical optimum for yield. Higher pressure would push both equilibria further toward the product, but very high pressures are expensive and, for the Haber process, present safety risks — 200 atm is already a compromise rather than the highest pressure achievable. Higher temperature would speed up both reactions, reaching equilibrium faster, but for reactions where the forward reaction is exothermic, a higher temperature also shifts the equilibrium position away from the product. The catalyst is what resolves this tension: it lets the reaction reach equilibrium quickly at a moderate temperature, without needing the higher temperature that would otherwise reduce the yield.

Common mistakes

• Saying a catalyst “speeds up the reaction” without saying how. At Extended/ 5070 level, the explanation must reference activation energy and an alternative pathway.
• Describing temperature’s effect on rate using only “more collisions.” The proportion of collisions with enough energy to react matters more than the raw collision frequency — both points are needed for full marks.
• Treating equilibrium as “the reaction has stopped.” It hasn’t — both directions are still occurring, at equal rates.
• Forgetting that a catalyst does not shift equilibrium position. It changes the rate of reaching equilibrium, not where it ends up.
• Quoting the wrong catalyst or conditions for Haber vs Contact. Iron for Haber, vanadium(V) oxide for Contact — the pressures and temperatures also differ between the two.
• 0620 Core candidates attempting to explain conditions using equilibrium reasoning they are not required to know — or O Level candidates skipping the Haber/Contact detail because a source labelled it “Extended.”

Quick revision checklist

All candidates (0620 Core and all 5070): factors that change rate (described, not explained) · practical methods for measuring rate · interpreting rate graphs · reversibility shown by ⇌ · hydrated/anhydrous CuSO₄ and CoCl₂ as a reversibility example

0620 Extended and all 5070 candidates, additionally: collision theory, and using it to explain each rate factor · how a catalyst lowers activation energy · the definition of equilibrium · how temperature, pressure, concentration and catalysts affect equilibrium position · Haber process: equation, sources, conditions · Contact process: equation, sources, conditions · why those specific conditions are chosen

Related resources

• Redox Reactions — the Contact process is itself a redox reaction
• Identification of Ions and Gases — testing the gas given off in a rate experiment
• Formulae, Equations and the Mole — balancing the Haber and Contact process equations
• Chemical Equilibria: Kc, Kp and Le Chatelier’s Principle — the AS Level continuation of the reversible-reactions half of this topic (9701)
• Reaction Kinetics: Collision Theory and Catalysis — the AS Level continuation of the rate-of-reaction half of this topic (9701)
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.