Activation Energy and Catalysts

Activation energy is the energy barrier that must be overcome for a chemical reaction to occur. Catalysts are substances that lower this barrier, making reactions faster and more efficient. This lesson covers these concepts in detail as required by the AQA GCSE Chemistry specification, linking them to reaction profiles and collision theory.

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What Is Activation Energy?

Activation energy (Ea) is the minimum amount of energy that colliding particles must have in order to react. Even if particles collide, they will only react if they have enough energy to break the existing bonds in the reactants.

Think of activation energy as an energy hill that reactant particles must climb over before they can form products. If they do not have enough energy, they simply bounce off each other without reacting.

Key Point	Detail
Definition	Minimum energy required for a reaction to occur
Unit	kJ/mol (kilojoules per mole)
On a reaction profile	Shown as the height from the reactants to the peak of the energy barrier
Applies to	Both exothermic AND endothermic reactions

Exam Tip: All reactions — including exothermic ones — require activation energy to start. This is why a match is needed to light a fuel. The match provides the initial activation energy; once started, the exothermic reaction sustains itself.

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Collision Theory and Activation Energy

Collision theory states that for a reaction to occur, particles must:

1. Collide with each other.
2. Have sufficient energy (at least the activation energy).
3. Collide with the correct orientation.

If any of these conditions is not met, the collision is unsuccessful and no reaction occurs.

graph TD
    A["Particles Collide"] --> B{"Sufficient Energy?"}
    B -->|"Yes (energy >= Ea)"| C{"Correct Orientation?"}
    B -->|"No (energy < Ea)"| D["Unsuccessful Collision - No Reaction"]
    C -->|"Yes"| E["Successful Collision - Reaction Occurs"]
    C -->|"No"| D

Factors That Increase the Rate of Successful Collisions

Factor	How It Works
Increasing temperature	Particles move faster, collide more frequently, AND more particles have energy greater than or equal to Ea
Increasing concentration	More particles in the same volume, so collisions are more frequent
Increasing pressure (gases)	Same effect as concentration — particles are closer together
Increasing surface area	More of the solid is exposed, so more collisions can occur
Using a catalyst	Lowers the activation energy, so more particles have sufficient energy to react

Exam Tip: Temperature is special because it affects BOTH the frequency of collisions AND the proportion of particles with sufficient energy. When explaining the effect of temperature, you MUST mention both effects for full marks.

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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. It can be recovered unchanged after the reaction is complete.

How Do Catalysts Work?

Catalysts work by providing an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This means that more particles have enough energy to react at any given temperature, so the rate of reaction increases.

Feature	Without Catalyst	With Catalyst
Activation energy	Higher	Lower
Rate of reaction	Slower	Faster
Overall energy change	Same	Same (unchanged)
Products formed	Same	Same (unchanged)
Catalyst at the end	N/A	Unchanged — can be reused

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The Effect of a Catalyst on a Reaction Profile

On a reaction profile diagram, a catalyst is shown as a second curve with a lower peak than the original uncatalysed curve.

Key points:

• The reactants start at the same energy level.
• The products end at the same energy level.
• The activation energy is lower (the peak is lower).
• The overall energy change is exactly the same.

graph TD
    subgraph "Reaction Profile with Catalyst"
        A["Reactants"] -->|"Uncatalysed pathway (high Ea)"| B["High Peak"]
        A -->|"Catalysed pathway (low Ea)"| C["Lower Peak"]
        B --> D["Products"]
        C --> D
    end

Exam Tip: When drawing the catalysed pathway on a reaction profile, make sure your curve starts and ends at EXACTLY the same points as the original curve. Only the HEIGHT of the peak should change. The overall energy change must remain identical.

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Types of Catalysts

Type	Description	Example
Heterogeneous	Catalyst is in a different state (phase) from the reactants	Iron in the Haber process (solid catalyst, gaseous reactants)
Homogeneous	Catalyst is in the same state as the reactants	Sulfuric acid in esterification (liquid catalyst, liquid reactants)
Biological (enzymes)	Protein catalysts in living organisms	Amylase breaks down starch, lipase breaks down fats

Enzymes as Biological Catalysts

Enzymes are highly specific biological catalysts. Each enzyme catalyses one particular reaction or type of reaction. They work by lowering the activation energy, just like chemical catalysts.

Feature	Enzymes	Chemical Catalysts
Specificity	Very specific — each enzyme works on one substrate	Less specific — one catalyst may work for several reactions
Operating conditions	Narrow range of temperature and pH (optimum conditions)	Often work at high temperatures and pressures
Denaturation	Enzymes can be denatured by high temperature or extreme pH	Chemical catalysts are generally more robust
Speed	Extremely fast — can process millions of substrate molecules per second	Fast, but generally slower than enzymes

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Catalysts in Industry

Catalysts are essential in industrial chemistry because they:

• Reduce energy costs by lowering the temperature needed for the reaction.
• Increase the rate of production, saving time.
• Reduce environmental impact by lowering energy consumption and therefore CO2 emissions.