Catalysis

CIE A-Level Chemistry· Unit 8: Reaction Kinetics· 25 min read

1. How Catalysts Work★★☆☆☆⏱ 8 min

📘 Definition

Catalyst

A substance that increases the rate of a chemical reaction without being consumed in the overall reaction, and does not change the reaction's net enthalpy change.

Example:

Manganese(IV) oxide catalyses the decomposition of aqueous hydrogen peroxide.

Catalysts do not change the original uncatalysed reaction pathway. Instead, they provide an alternative reaction mechanism with a lower activation energy ( $E_{a}$ ) than the uncatalysed reaction.

A lower activation energy means a much higher proportion of reactant particles have sufficient energy to react at a given temperature, so the rate of reaction increases.

📐 Worked Example

Explain the key features of a reaction profile for a catalysed vs uncatalysed exothermic reaction.

1
Draw axes: x-axis = reaction progress (reaction coordinate), y-axis = enthalpy ( $H$ )
2
Draw reactants at a higher enthalpy than products, consistent with an exothermic reaction
3
Draw the uncatalysed pathway: a single peak, with the energy difference between reactants and the peak equal to uncatalysed activation energy $E_{a}$
4
Draw the catalysed pathway with a lower maximum peak, with energy difference $E_{a}^{'} < E_{a}$
5
The overall enthalpy change $e x t Δ H$ between reactants and products is identical for both pathways

Exam tip:

Always link lower activation energy to higher proportion of successful collisions per second when explaining rate increase.

2. Homogeneous Catalysis★★★☆☆⏱ 7 min

📘 Definition

Homogeneous Catalysis

Catalysis where the catalyst is in the same physical state (phase) as all reactants.

Example:

Aqueous hydrogen ions catalysing the hydrolysis of methyl esters, where all reactants are aqueous.

Homogeneous catalysts typically react with reactants to form unstable intermediate compounds, which then break down to release products and regenerate the unchanged catalyst.

📐 Worked Example

Explain why chlorine radicals are homogeneous catalysts for ozone depletion.

1
All species (chlorine radicals, ozone, oxygen) are in the gas phase, so catalyst and reactants share the same phase
2
First step: Chlorine radicals react with ozone to form an intermediate ClO radical: $C l^{∙} + O_{3} \to C l O^{∙} + O_{2}$
3
Second step: Intermediate ClO reacts with free oxygen to regenerate chlorine radical: $C l O^{∙} + O^{∙} \to C l^{∙} + O_{2}$
4
The chlorine radical is unchanged at the end of the reaction, so it acts as a catalyst, and can destroy thousands of ozone molecules

3. Heterogeneous Catalysis★★★☆☆⏱ 7 min

📘 Definition

Heterogeneous Catalysis

Catalysis where the catalyst is in a different physical state (phase) from the reactants.

Example:

Solid vanadium(V) oxide catalysing the gaseous reaction of sulfur dioxide with oxygen in the Contact process.

Most heterogeneous catalysts are solids, with gaseous or liquid reactants. The reaction proceeds in four key steps:

Reactant molecules diffuse to the catalyst surface
Reactants adsorb (bind) to active sites on the catalyst
Bonds in reactants are weakened, and reaction occurs to form products
Products desorb from the surface, freeing active sites for new reactants

📐 Worked Example

Outline how solid iron acts as a heterogeneous catalyst for the Haber process.

1
Gaseous nitrogen and hydrogen diffuse to the surface of solid iron catalyst
2
N₂ and H₂ molecules adsorb onto active sites on the iron surface
3
Adsorption weakens the strong covalent bonds in N₂ and H₂, lowering activation energy for reaction
4
N and H atoms react to form ammonia (NH₃) molecules
5
Ammonia desorbs from the iron surface, leaving active sites free for new reactants

Iron catalyst: Haber process (ammonia production)
Vanadium(V) oxide ( $V_{2} O_{5}$ ): Contact process (sulfuric acid production)
Nickel: Hydrogenation of alkenes to make margarine

4. Enzymes: Biological Catalysts★★☆☆☆⏱ 3 min

Enzymes are globular protein molecules that act as biological catalysts. They are classified as heterogeneous catalysts because the reaction occurs at the active site, a specific region on the large solid protein molecule.

📘 Definition

Active Site

A unique 3D region on an enzyme where the substrate molecule binds and the reaction takes place. Enzymes are highly specific to their substrate due to the shape of the active site.

📐 Worked Example

Explain why enzymes lose activity at high temperatures.

1
Below the optimum temperature, enzyme activity increases with temperature as more particles have energy greater than activation energy
2
Above the optimum temperature, increased kinetic energy breaks the weak hydrogen and ionic bonds holding the enzyme's 3D shape
3
The shape of the active site is permanently altered (this is called denaturation), so substrate can no longer bind
4
The enzyme loses all catalytic activity

5. Common Pitfalls

Wrong move:

Stating that catalysts lower activation energy of the original uncatalysed reaction pathway

Why:

Catalysts do not alter the original pathway; they provide an entirely new reaction mechanism

Correct move:

Always state that catalysts provide an alternative reaction pathway with a lower activation energy

Wrong move:

Claiming catalysts increase the yield of product by shifting equilibrium

Why:

Catalysts speed up forward and reverse reactions equally, so equilibrium position does not change

Correct move:

State that catalysts only increase the rate of reaction, allowing equilibrium to be reached faster with no change to yield

Wrong move:

Classifying catalysis based on number of components instead of physical phase

Why:

Classification of homogeneous/heterogeneous depends only on phase of catalyst vs reactants

Correct move:

Check the physical state: same phase = homogeneous, different phase = heterogeneous

Wrong move:

Adding catalysts to the product side of balanced chemical equations

Why:

Catalysts are not consumed overall, so they are not reactants or products in the net reaction

Correct move:

Write catalysts above the reaction arrow, not as a reactant or product

Wrong move:

Calling enzymes homogeneous catalysts because they work in aqueous solution

Why:

The reaction occurs at the active site on the solid protein enzyme, so it is a different phase from dissolved substrate

Correct move:

Classify enzymes as heterogeneous biological catalysts

6. Quick Reference Cheatsheet

Property	Homogeneous Catalysis	Heterogeneous Catalysis
Catalyst phase	Same as reactants	Different from reactants
Core mechanism	Forms intermediate compounds	Adsorption on active sites
Common examples	Acid catalysis, ozone depletion by CFCs	Haber process (Fe), Contact process (V₂O₅)
Catalyst regeneration	Regenerated at end of reaction cycle	Regenerated after product desorption

7. Frequently Asked

Do catalysts affect the position of equilibrium or reaction yield?

No. Catalysts speed up both the forward and reverse reactions equally, so equilibrium is reached faster, but the equilibrium constant and final yield of products do not change.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 12
Compare homogeneous/heterogeneous catalysis
2023 · 22
Explain catalyst effect on reaction rate
2021 · 11
Name industrial catalyst examples

Going deeper

syllabusOfficial CIE 9701 AS SyllabusFull syllabus for reference

What's Next

Catalysis is a core foundational concept for CIE A-Level Chemistry, linking reaction kinetics to industrial process design, organic reaction mechanisms, and environmental chemistry. Understanding how catalysts lower activation energy also prepares you for quantitative work with the Arrhenius equation, which relates activation energy to rate constant. Catalysis is a common topic in structured exam questions that assess both conceptual understanding and application to real industrial processes. Building on this, you will next explore rate-determining steps and the Arrhenius equation, before encountering catalysis again in A-level industrial chemistry and organic synthesis topics.