Catalysts and Reversible Reactions

This lesson brings together two important topics from the AQA GCSE Combined Science Trilogy (8464) specification: catalysts and reversible reactions. You need to understand how catalysts work, their importance in industry, and how reversible reactions differ from irreversible reactions.

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Catalysts

What Is a Catalyst?

A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. It can be recovered chemically unchanged at the end of the reaction.

How Do Catalysts Work?

Catalysts provide an alternative reaction pathway with a lower activation energy.

$\text{Activation energy (with catalyst)} < \text{Activation energy (without catalyst)}$Activation energy (with catalyst)<Activation energy (without catalyst)

Because the activation energy is lower, a greater proportion of colliding particles have enough energy for a successful collision. This means more reactions happen per second — the rate increases.

Key Point: A catalyst does NOT change the overall energy change (ΔH) of a reaction. The energy of the reactants and products remains the same. Only the activation energy is lowered.

Energy Profile Diagram

Feature	Without Catalyst	With Catalyst
Activation energy	Higher	Lower
Overall energy change	Same	Same
Reactant energy	Same	Same
Product energy	Same	Same

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Properties of Catalysts

Property	Detail
Not used up	Can be recovered unchanged at the end
Specific	Different reactions need different catalysts
Small amounts needed	A small amount can catalyse a large amount of reaction
Can be poisoned	Impurities can coat the surface and reduce effectiveness

Examples of Catalysts

Reaction	Catalyst
Decomposition of hydrogen peroxide	Manganese dioxide (MnO₂)
Haber process (making ammonia)	Iron
Contact process (making sulfuric acid)	Vanadium pentoxide (V₂O₅)

Biological Catalysts: Enzymes

Enzymes are biological catalysts — proteins that speed up reactions in living organisms. They are highly specific (each enzyme catalyses one reaction) and work best at an optimum temperature and pH.

Importance of Catalysts in Industry

• Reduce energy costs by allowing reactions at lower temperatures.
• Increase rate without needing extreme conditions.
• Reduce environmental impact by lowering energy consumption.

Exam Tip: If asked why catalysts are important, mention both economic benefits (lower costs) and environmental benefits (less energy wasted, lower carbon emissions).

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Reversible Reactions

What Is a Reversible Reaction?

A reversible reaction is a reaction that can proceed in both the forward and backward directions. The products can react to re-form the reactants.

The symbol for a reversible reaction is: ⇌

$\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}$A+B⇌C+D

Energy Changes in Reversible Reactions

If the forward reaction is exothermic (releases energy), the reverse reaction is endothermic (absorbs energy) by exactly the same amount — and vice versa.

$\text{If forward: } \Delta H = -x \text{ kJ} \quad \text{then reverse: } \Delta H = +x \text{ kJ}$If forward: ΔH=−x kJthen reverse: ΔH=+x kJ

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Example: Hydrated Copper Sulfate

$\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \rightleftharpoons \text{CuSO}_4 + 5\text{H}_2\text{O}$CuSO4​⋅5H2​O⇌CuSO4​+5H2​O

Direction	Observation	Energy Change
Forward (heating)	Blue crystals → white powder + steam	Endothermic
Reverse (adding water)	White powder → blue solid; gets hot	Exothermic

Example: Ammonium Chloride

$\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}$NH4​Cl(s)⇌NH3​(g)+HCl(g)

When heated, ammonium chloride decomposes into ammonia and hydrogen chloride gases. When the gases cool, they recombine to form white ammonium chloride.

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Equilibrium (Combined Science Foundation)

In a closed system (where no substances can enter or leave), a reversible reaction can reach equilibrium. At equilibrium:

• The rate of the forward reaction equals the rate of the reverse reaction.
• The concentrations of reactants and products remain constant (but are not necessarily equal).

Key Definition: A closed system is one where no substances can enter or leave, though energy can still be transferred.

graph LR
    A["Reactants"] -- "Forward reaction" --> B["Products"]
    B -- "Reverse reaction" --> A
    C["At equilibrium: forward rate = reverse rate"]

Common Mistake: "Equilibrium means the reaction has stopped." This is wrong. At equilibrium, both the forward and reverse reactions are still occurring — they are just happening at the same rate, so there is no overall change.

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Summary

• A catalyst increases rate by providing an alternative pathway with lower activation energy
• Catalysts are not used up and are specific to particular reactions
• Catalysts are economically and environmentally important in industry
• Reversible reactions can go forwards and backwards (symbol: ⇌)
• If the forward reaction is exothermic, the reverse is endothermic by the same amount
• In a closed system, reversible reactions reach equilibrium where forward rate = reverse rate

Exam Tip: AQA commonly asks you to compare catalysed and uncatalysed reactions on an energy profile diagram. Practise sketching these and labelling activation energy with and without the catalyst.

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Worked Examples and Extended Explanation

Worked Example 1: Catalyst Energy Profile

Question: An exothermic reaction has an activation energy of 90 kJ/mol and an overall energy change of −60 kJ/mol. When a catalyst is added, the activation energy falls to 35 kJ/mol.

(a) State the overall energy change when the catalyst is used. (b) Explain, using collision theory, why the catalyst increases the rate.

Solution: (a) The overall energy change is unchanged: −60 kJ/mol. A catalyst only alters the activation energy, not the energies of reactants and products.

(b) Lowering the activation energy means a greater proportion of colliding particles now have energy ≥ Eₐ. More collisions are successful per second, so the rate of reaction increases.

Worked Example 2: Interpreting a Reversible-Reaction Question (Trilogy-level)

Question: The reaction between hydrogen and iodine is reversible: H₂(g) + I₂(g) ⇌ 2HI(g). The forward reaction is exothermic.

(a) What is meant by a dynamic equilibrium? (b) State the energy change of the reverse reaction.