Catalysts

This lesson covers catalysts, how they work using collision theory, the concept of activation energy, biological catalysts (enzymes) and the economic importance of catalysts as required by the Edexcel GCSE Combined Science specification (1SC0).

---

What Is a Catalyst?

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

Key Features

Feature	Explanation
Increases rate	The reaction proceeds faster with a catalyst present
Not used up	The catalyst is the same mass and chemical composition before and after the reaction
Provides an alternative pathway	The catalyst offers a different route for the reaction that has a lower activation energy
Specific	Most catalysts only work for particular reactions
Small amount needed	A catalyst is not a reactant — it is not included in the balanced equation

---

How Do Catalysts Work?

A catalyst works by providing an alternative reaction pathway with a lower activation energy (Eₐ). This means that a greater proportion of the reacting particles have enough energy to undergo successful collisions, so the rate increases.

graph TD
    A["Reactant particles<br/>collide"] --> B{"Without catalyst:<br/>need high Eₐ"}
    A --> C{"With catalyst:<br/>need lower Eₐ"}
    B --> D["Fewer particles<br/>have enough energy"]
    C --> E["More particles<br/>have enough energy"]
    D --> F["Fewer successful<br/>collisions/s → slower"]
    E --> G["More successful<br/>collisions/s → faster"]

    style F fill:#c0392b,color:#fff
    style G fill:#27ae60,color:#fff

Exam Tip: A catalyst does not give particles more energy. It provides an alternative pathway that requires less energy. The result is that more of the existing particles can react.

---

Activation Energy Diagram

The effect of a catalyst is shown clearly on a reaction profile diagram. The catalyst lowers the height of the activation energy barrier:

Feature	Without catalyst	With catalyst
Eₐ (activation energy)	Higher	Lower
Overall energy change (ΔH)	Unchanged	Unchanged
Products	Same	Same
Rate	Slower	Faster

The overall energy change of the reaction (exothermic or endothermic) is not affected by a catalyst — only the activation energy changes.

---

Examples of Catalysts

Reaction	Catalyst	Notes
Decomposition of hydrogen peroxide	Manganese(IV) oxide (MnO₂)	2H₂O₂ → 2H₂O + O₂
Haber process (making ammonia)	Iron	N₂ + 3H₂ ⇌ 2NH₃
Contact process (making sulfuric acid)	Vanadium(V) oxide (V₂O₅)	SO₂ + ½O₂ → SO₃
Catalytic converters in cars	Platinum, palladium, rhodium	Convert CO and NOₓ to CO₂ and N₂

---

Biological Catalysts: Enzymes

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

Key Properties of Enzymes

Property	Detail
Specificity	Each enzyme only catalyses one specific reaction (lock-and-key model)
Optimum temperature	Enzymes work best at a particular temperature (usually around 37 °C for human enzymes)
Optimum pH	Each enzyme has an optimum pH (e.g. pepsin works best at pH 2 in the stomach)
Denaturation	If the temperature is too high, the enzyme's shape changes permanently and it can no longer catalyse the reaction

graph LR
    subgraph "Lock and Key Model"
        S["Substrate<br/>(specific shape)"] --> E["Enzyme<br/>(active site matches)"]
        E --> P["Products formed<br/>Enzyme released unchanged"]
    end

    style S fill:#3498db,color:#fff
    style E fill:#27ae60,color:#fff
    style P fill:#8e44ad,color:#fff

Exam Tip: Enzymes are denatured (not killed) at high temperatures. They are proteins, not living things.

---

Economic Importance of Catalysts

Catalysts are extremely important in industry because they:

Benefit	Explanation
Reduce energy costs	A lower Eₐ means the reaction can proceed at a lower temperature, saving fuel and electricity
Increase rate of production	Faster reactions mean more product is made in a given time
Reduce waste	Some catalysts improve selectivity, producing fewer by-products
Are not used up	A small amount can catalyse a large amount of reaction over a long period

Example: The Haber Process

Without an iron catalyst, the Haber process would need much higher temperatures to proceed at a useful rate. Higher temperatures would shift the equilibrium to the left, reducing the yield of ammonia, and would cost more energy. The iron catalyst allows a reasonable rate at a moderate temperature (about 450 °C), balancing rate and yield while reducing costs.

Exam Tip: In exam questions about industrial catalysts, always mention that catalysts reduce costs by allowing reactions to proceed at lower temperatures, which saves energy/fuel.

---

Catalysts on Rate–Time Graphs

On a graph of product formed vs time:

Feature	With catalyst	Without catalyst
Initial gradient	Steeper	Less steep
Total product	Same	Same
Time to complete	Shorter	Longer

A catalyst makes the reaction finish sooner but does not change the total amount of product formed.

---

Summary

• A catalyst increases the rate of reaction without being used up.
• It provides an alternative reaction pathway with a lower activation energy.
• More particles have energy ≥ the lower Eₐ, so more collisions are successful.
• The catalyst does not change the overall energy change (ΔH) or the products.
• Enzymes are biological catalysts — they are specific, have optimum temperature and pH, and denature at high temperatures.
• Catalysts are economically important: they reduce energy costs, increase production rates and are not consumed.

---

Extended Worked Example: Catalyst on a Reaction Profile

Question (4 marks): Sketch a reaction profile for an exothermic reaction. On the same axes, show the effect of adding a catalyst. Label Eₐ (uncatalysed), Eₐ (catalysed), reactants, products and ΔH.

Features expected: