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. Wording difference: 0620 limits this outcome to exactly those two named pairs. 5070’s wording instead says “including” these two pairs, meaning other hydrated/anhydrous compound pairs are not ruled out and could in principle be examined for a 5070 candidate, even though copper(II) sulfate and cobalt(II) chloride remain the standard teaching examples for both syllabuses.

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.

Effect of concentration on equilibrium position. Increasing the concentration of a reactant shifts the equilibrium position towards the products, as the system responds by converting more of the reactant to restore balance; increasing the concentration of a product shifts it back towards the reactants for the same reason. Removing a product as it forms (rather than letting it build up) therefore also pulls the equilibrium further towards the products, since the system continually “replaces” what is removed.

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.

Why the Contact process uses a much lower pressure than the Haber process. The Contact process reaction, 2SO2(g) + O2(g) ⇌ 2SO3(g), already gives a high percentage conversion to product (well over 95%) at ordinary atmospheric-ish pressures, because it has fewer gas molecules on the product side already favouring the product without much extra push. Raising the pressure further would only add substantial extra cost (stronger, more expensive plant and pipework) for a very small extra gain in yield, so 200 kPa (only about 2 atm) is used — just enough above atmospheric pressure to keep the gases flowing efficiently through the plant, rather than a genuinely “high” pressure. This is the opposite trade-off from the Haber process, where a much higher pressure (200 atm) is needed to achieve an acceptable yield in the first place, and safety/cost then limits how much higher it can practically go.

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
• Exothermic and Endothermic Reactions — activation energy and reaction pathway diagrams in full
• 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.