Catalysts

Catalysts are substances that speed up chemical reactions without being used up themselves. Understanding how catalysts work, their role in industrial processes, and their biological equivalents (enzymes) is an important part of the AQA GCSE Chemistry specification. This lesson covers the definition and properties of catalysts, how they affect activation energy, their industrial importance, and the distinction between catalysts and enzymes.

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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 at the end of the reaction.

Key properties of a catalyst:

Property	Explanation
Increases rate	The reaction happens faster with the catalyst present
Not used up	The catalyst can be recovered at the end of the reaction, unchanged in mass and chemical composition
Provides an alternative pathway	The catalyst offers a different route from reactants to products with a lower activation energy
Specific	Different reactions require different catalysts; a catalyst for one reaction may not work for another
Small amounts needed	Only a small amount of catalyst is needed because it is recycled during the reaction

Exam Tip: The AQA mark scheme specifically requires you to state that a catalyst "provides an alternative reaction pathway with a lower activation energy." Memorise this exact phrase and use it whenever you are asked to explain how a catalyst works.

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How Catalysts Work

A catalyst works by providing an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This means:

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

graph TD
    A[Without Catalyst] --> B[High activation energy barrier]
    B --> C[Few particles have enough energy]
    C --> D[Slow rate of reaction]
    E[With Catalyst] --> F[Lower activation energy barrier]
    F --> G[More particles have enough energy]
    G --> H[Faster rate of reaction]

What a Catalyst Does NOT Do

It is equally important to understand what a catalyst does not do:

Common Misconception	Reality
"A catalyst gives particles more energy"	No — it lowers the energy barrier, not increases particle energy
"A catalyst is used up in the reaction"	No — it is recovered chemically unchanged
"A catalyst increases the yield of product"	No — it only increases the rate, not the amount of product
"A catalyst changes the products formed"	No — the same products are formed, just more quickly
"A catalyst affects the position of equilibrium"	No — it speeds up both forward and reverse reactions equally [H]

Exam Tip: A catalyst does NOT increase the total amount of product formed. It only makes the reaction reach completion faster. If a question asks about yield, the catalyst does not change it. This is a very common trap in exam questions.

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Catalysts and Energy Profile Diagrams

On an energy profile diagram, the effect of a catalyst is shown as a lower energy pathway from reactants to products.

Feature	Without Catalyst	With Catalyst
Reactant energy level	Same	Same
Product energy level	Same	Same
Activation energy	Higher	Lower
Overall energy change	Same	Same
Peak of curve	Higher	Lower

The key point is that the catalyst lowers only the activation energy. The overall energy change (the difference between reactant and product energy levels) remains the same.

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Industrial Catalysts

Catalysts are extremely important in the chemical industry. They reduce costs, save energy and speed up production.

Industrial Process	Catalyst Used	Reaction
Haber process	Iron	N2 + 3H2 --> 2NH3 (making ammonia)
Contact process	Vanadium(V) oxide (V2O5)	Making sulfuric acid
Catalytic cracking	Aluminium oxide or zeolite	Breaking down long-chain hydrocarbons
Catalytic converter	Platinum, palladium, rhodium	Converting CO and NOx in car exhausts to CO2 and N2
Margarine production	Nickel	Hydrogenation of vegetable oils
Ostwald process	Platinum/rhodium	Making nitric acid from ammonia

Economic and Environmental Benefits

Benefit	Explanation
Lower energy costs	Catalysts allow reactions to occur at lower temperatures and pressures, saving fuel
Faster production	Products are made more quickly, increasing output and profit
Reduced waste	Some catalysts make reactions more selective, producing fewer unwanted by-products
Environmental	Catalytic converters reduce harmful emissions from vehicles

Exam Tip: If asked about the importance of catalysts in industry, always mention three points: (1) they reduce energy costs because lower temperatures can be used, (2) they speed up the rate of production, and (3) they are not used up so they do not need to be continuously replaced.

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Enzymes as Biological Catalysts

Enzymes are biological catalysts — they are proteins that speed up chemical reactions in living organisms. Enzymes are vital for life because without them, the reactions in our bodies would be far too slow to sustain life.

Feature	Chemical Catalyst	Enzyme
Nature	Usually metals or metal compounds	Proteins
Temperature range	Often high temperatures (100-500 C)	Body temperature (around 37 C in humans)
Specificity	May catalyse several related reactions	Highly specific — each enzyme catalyses one reaction
Sensitivity	Robust; can withstand harsh conditions	Denatured at high temperatures or extreme pH
Reusability	Reusable	Reusable (unless denatured)

graph TD
    A[Catalysts] --> B[Chemical Catalysts]
    A --> C[Biological Catalysts - Enzymes]
    B --> B1[Used in industry]
    B --> B2[Often metals or metal oxides]
    B --> B3[Work at high temperatures]
    C --> C1[Found in living organisms]
    C --> C2[Made of protein]
    C --> C3[Work at body temperature]
    C --> C4[Denatured by high temperature or extreme pH]

How Enzymes Work: Lock and Key Model

Each enzyme has an active site with a specific shape. Only substrates with a complementary shape can fit into the active site. This is called the lock and key model.