Catalysts

Catalysts are substances that play a crucial role in chemistry, both in the laboratory and in industrial processes. In this lesson you will learn the definition of a catalyst, how catalysts work in terms of activation energy, key examples of catalysts, their economic and environmental importance, and how they differ from the reactants and products in a reaction. This is an important part of the Edexcel GCSE Chemistry (1CH0) specification.

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What Is a Catalyst?

A catalyst is a substance that increases the rate of a chemical reaction without being chemically changed or used up in the process.

Key points about catalysts:

• A catalyst is not consumed during the reaction — it is still present at the end and can be reused.
• A catalyst does not appear in the overall chemical equation for the reaction (it is not a reactant or product).
• A catalyst works by providing an alternative reaction pathway that has a lower activation energy (Eₐ).
• A catalyst does not change the products of the reaction.
• A catalyst does not change the total amount of product formed — it only makes the reaction faster.
• A catalyst is usually specific to a particular reaction — different reactions require different catalysts.

Exam tip: The definition of a catalyst is one of the most commonly tested definitions in GCSE Chemistry. Make sure you include all three key parts: (1) increases the rate, (2) not chemically changed, (3) not used up. Missing any of these parts will lose you marks.

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How Do Catalysts Work?

A catalyst provides an alternative reaction pathway with a lower activation energy. This means:

1. The energy barrier that particles must overcome to react is reduced.
2. A greater proportion of colliding particles have energy equal to or greater than this new, lower activation energy.
3. Therefore more collisions are successful.
4. The rate of reaction increases.

The following diagram shows reaction profiles with and without a catalyst:

graph TD
    subgraph Without Catalyst
    A1[Reactants] -->|High activation energy Eₐ| B1[Transition State — High Energy]
    B1 --> C1[Products]
    end
    subgraph With Catalyst
    A2[Reactants] -->|Lower activation energy Eₐ| B2[Transition State — Lower Energy]
    B2 --> C2[Products]
    end
    style B1 fill:#ff9999,stroke:#cc0000
    style B2 fill:#66cc66,stroke:#228B22

Key Features of the Diagram

• The reactants start at the same energy level in both pathways.
• The products end at the same energy level in both pathways.
• The overall energy change (ΔH) is the same with or without a catalyst.
• The only difference is the height of the energy barrier (the activation energy) — it is lower with a catalyst.

Exam tip: A very common exam mistake is to say that a catalyst "gives the particles more energy." This is wrong. A catalyst does not change the energy of the particles. It lowers the activation energy so that more of the existing particles already have enough energy to react.

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Examples of Catalysts

Laboratory Catalysts

Reaction	Catalyst	Notes
Decomposition of hydrogen peroxide (2H₂O₂ → 2H₂O + O₂)	Manganese dioxide (MnO₂)	MnO₂ can be filtered out after the reaction and reused
Decomposition of hydrogen peroxide (biological)	Catalase (an enzyme found in liver and blood)	Biological catalyst — works at body temperature

Industrial Catalysts

Industrial Process	Reaction	Catalyst	Conditions
Haber process	N₂ + 3H₂ ⇌ 2NH₃	Iron	450 °C, 200 atm
Contact process	2SO₂ + O₂ → 2SO₃	Vanadium pentoxide (V₂O₅)	450 °C, 1–2 atm
Catalytic converter (car exhaust)	2CO + 2NO → 2CO₂ + N₂	Platinum, palladium, rhodium	Honeycomb structure for high surface area
Hydrogenation of vegetable oils	C=C + H₂ → C–C	Nickel	~60 °C

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Biological Catalysts: Enzymes

Enzymes are biological catalysts — they are proteins that catalyse specific biochemical reactions in living organisms.

Key features of enzymes:

• They are highly specific — each enzyme catalyses only one type of reaction (lock and key model).
• They work best at an optimum temperature (usually around 37 °C for human enzymes).
• They work best at an optimum pH (varies depending on the enzyme).
• They can be denatured if the temperature or pH moves too far from the optimum — the active site changes shape and the enzyme no longer works.

Feature	Chemical catalysts	Enzymes (biological catalysts)
Specificity	Often work for several similar reactions	Highly specific to one reaction
Temperature	Often need high temperatures	Work at low temperatures (body temperature)
Reusability	Reusable	Reusable (unless denatured)
Sensitivity	Robust	Sensitive to temperature and pH

Exam tip: You may be asked to compare chemical catalysts and enzymes. The key difference is that enzymes are highly specific and work at low temperatures, while industrial catalysts often need high temperatures and can catalyse a broader range of reactions.

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Economic and Environmental Importance of Catalysts

Catalysts are vitally important in industry for several reasons:

Economic Benefits

• Lower temperatures and pressures can be used, reducing energy costs.
• Reactions happen faster, so more product is made in less time — increasing efficiency and profits.
• Catalysts are not used up, so they do not need to be replaced frequently.
• They allow processes to operate under milder conditions, reducing the cost of high-temperature and high-pressure equipment.

Environmental Benefits

• Lower temperatures mean less fuel is burned to heat reactors, reducing CO₂ emissions and the use of fossil fuels.
• Catalytic converters in cars convert harmful exhaust gases (CO, NO) into less harmful substances (CO₂, N₂), reducing air pollution.
• More efficient reactions produce less waste.

Benefit	Explanation
Reduced energy costs	Lower activation energy means reactions proceed at lower temperatures
Increased production rate	Faster reactions = more product per hour
Reduced environmental impact	Less fuel burned, fewer emissions, less waste
Catalyst longevity	Not consumed — long-lasting, cost-effective