Lesson 5 of 5

Catalytic cycles, surfaces and enzymes

Follow regeneration of the catalyst in all four specified examples.

A-Level 9476 (2026-2027)

I can outline homogeneous and heterogeneous catalytic action.Syllabus
Syllabus 9476, 8(j). Outline homogeneous and heterogeneous catalytic action.
I can describe enzymes and their sensitivities.Syllabus
Syllabus 9476, 8(k). Describe enzymes and their sensitivities.

A homogeneous catalyst reacts through a regenerated intermediate

Catalyst and reactants occupy the same phase in the two specified cycles.

Atmospheric nitrogen oxides: a simplified catalytic oxidation cycle
Step	Equation and role
Oxidise sulfur dioxide	SO₂(g) + NO₂(g) → SO₃(g) + NO(g)
Regenerate nitrogen dioxide	2NO(g) + O₂(g) → 2NO₂(g)
Overall after doubling the first step	2SO₂(g) + O₂(g) → 2SO₃(g)

NO and NO₂ change back and forth, so they cancel out of the overall equation. This gas-phase scheme shows homogeneous catalysis of SO₂ oxidation in the air. Real atmospheric sulfur chemistry also involves other oxidants and reactions in water droplets. Sulfur oxides that form and dissolve in water add to acid rain.

Iron catalysis of iodide oxidation by peroxodisulfate
Step	Balanced equation
Peroxodisulfate oxidises iron(II)	S₂O₈^2-(aq) + 2Fe²⁺(aq) → 2SO₄^2-(aq) + 2Fe³⁺(aq)
Iron(III) oxidises iodide and regenerates iron(II)	2Fe³⁺(aq) + 2I^-(aq) → 2Fe²⁺(aq) + I₂(aq)
Overall	S₂O₈^2-(aq) + 2I^-(aq) → 2SO₄^2-(aq) + I₂(aq)

Without a catalyst, two negative ions, I^- and S₂O₈^2-, must collide. They repel each other, so this route is slow. Iron ions offer a different pathway. Each step now involves a positive iron ion and a negative ion, which attract. This route has a lower activation energy. Fe²⁺ is oxidised to Fe³⁺ and then made again, so only a small amount is needed.

Check your understanding

Fe²⁺(aq) catalyses the reaction between I^- and S₂O₈^2-. Why is the catalysed route faster?

Fe²⁺ is a solid surface that adsorbs the ions, so it is a heterogeneous catalyst.
Each catalysed step joins oppositely charged ions, so each step has a lower Ea.
Fe²⁺ gives the reaction more energy, so more collisions have energy above Ea.

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Each catalysed step joins oppositely charged ions, so each step has a lower Ea.

Uncatalysed, two negative ions must collide and they repel. With Fe²⁺, S₂O₈^2- reacts with Fe²⁺, then Fe³⁺ reacts with I^-: attractions, not repulsions. Fe²⁺ is remade, and it is homogeneous because all species are aqueous.

A solid surface brings adsorbed reactants into a more reactive arrangement

Adsorption, surface reaction and desorption must all remain possible.

Haber-process iron catalyst

Adsorb the gases
N₂ and H₂ stick to active sites on the solid iron. This adsorption weakens their bonds and can split the molecules into atoms.
React on the surface
The adsorbed nitrogen and hydrogen atoms combine in a series of steps, each with a lower barrier.
Desorb ammonia
NH₃ leaves the surface, which frees the sites for more reactant. Overall: N₂ + 3H₂ ⇌ 2NH₃.

The solid catalyst is in a different phase from the reacting gases. A larger surface area gives more active sites. A poison binds strongly to the sites and blocks them. Reactants must bind strongly enough to weaken their bonds, but products must not bind so strongly that they cannot leave. A catalyst gives a useful rate at a lower temperature. It does not, by itself, raise the equilibrium yield.

In a car catalytic converter, exhaust gases pass over a solid surface of metals such as platinum and rhodium. The gases adsorb, react on the surface and then leave. Carbon monoxide acts as a reducing agent and removes nitrogen oxides, for example 2NO(g) + 2CO(g) → N₂(g) + 2CO₂(g). Other reactions on the surface oxidise CO and unburnt hydrocarbons. The converter must be hot to work well, so it is less effective just after a cold start.

Exam-style question

Outline how solid iron speeds up the reaction between nitrogen and hydrogen in the Haber process. [3]

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Enzymes are highly specific protein catalysts

Their active sites select substrates and depend on temperature and pH.

The lock-and-key model

Substrate fits the active site
The substrate has a shape and chemical groups that match the active site of the enzyme.
An enzyme-substrate complex forms
The active site gives a pathway with a lower barrier for one particular reaction.
Products leave
The enzyme is free to work again: E + S ⇌ ES → E + P.

Enzymes are specific in two ways: which substrate binds, and which reaction is catalysed. A substrate that does not fit the active site is not converted efficiently. The lock-and-key model explains this selectivity. It is a simple model; real proteins are not perfectly rigid.

Why enzyme rate depends on its surroundings
Condition	Effect on activity
Temperature rises within a suitable range	More frequent energetic encounters increase the catalytic rate.
Temperature becomes too high	The active-site structure can be disrupted, reducing effective binding and catalysis; activity falls despite faster molecular motion.
pH moves away from the suitable range	Changes in ionisation of active-site groups and substrate can alter interactions and structure, reducing activity.
Different enzyme	Its useful temperature and pH ranges can differ; do not assume all enzymes have the same optimum.

Check your understanding

Why can cooling slow an enzyme reaction without the lasting effect of strong heating?

Cooling denatures the enzyme gently, and the protein unfolds back when it is warmed.
Cooling makes the enzyme bind its substrate more tightly, so the products leave slowly.
Cooling slows collisions reversibly; strong heating can destroy the active site's shape.

Show the answer

Cooling slows collisions reversibly; strong heating can destroy the active site's shape.

At low temperature fewer collisions have enough energy, but the enzyme keeps its shape and recovers on warming. Severe heating denatures the protein, so activity may not return.