Activation Energy and Catalysts

This lesson explores activation energy in greater depth and examines how catalysts affect the energy requirements of reactions. Understanding the link between activation energy, catalysts, and reaction profiles is essential for the Energy Changes topic in AQA GCSE Combined Science Trilogy (8464).

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What Is Activation Energy?

Activation energy ($E_a$Ea​) is the minimum amount of energy that reacting particles must have when they collide in order for a reaction to occur. It is the energy needed to start breaking bonds in the reactants.

$E_a = \text{Energy at the peak of the reaction profile} - \text{Energy of the reactants}$Ea​=Energy at the peak of the reaction profile−Energy of the reactants

Even in highly exothermic reactions, an initial energy input is needed. This is why fuels do not spontaneously combust at room temperature — the activation energy barrier must be overcome first (e.g., by a spark or a match).

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Activation Energy and Collision Theory

For a reaction to occur, particles must:

1. Collide with each other.
2. Have sufficient energy (at least equal to $E_a$Ea​).
3. Collide with the correct orientation.

graph TD
    A["Particles Collide"] --> B{"Do they have \nenergy ≥ Ea?"}
    B -->|Yes| C{"Correct \norientation?"}
    B -->|No| D["No reaction — \nparticles bounce off"]
    C -->|Yes| E["SUCCESSFUL \nCOLLISION — \nreaction occurs"]
    C -->|No| D

If the activation energy is high, fewer particles have enough energy to react at a given temperature, so the reaction rate is slow.

If the activation energy is low, more particles have enough energy to react, so the reaction rate is fast.

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Effect of Temperature on Activation Energy

Increasing the temperature does not change the activation energy itself, but it gives more particles enough energy to exceed $E_a$Ea​:

Temperature	Proportion of Particles with Energy ≥ $E_a$Ea​	Effect on Rate
Low	Small proportion	Slow rate
High	Large proportion	Fast rate

Exam Tip: Do not say that increasing temperature "lowers the activation energy." It does NOT change $E_a$Ea​ — it increases the proportion of particles with energy greater than or equal to $E_a$Ea​.

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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 in the process. It is chemically unchanged at the end of the reaction.

A catalyst works by providing an alternative reaction pathway with a lower activation energy ($E_a$Ea​).

Key Properties of Catalysts

Property	Detail
Not used up	Can be recovered unchanged after the reaction
Only a small amount needed	A catalyst is not a reactant — it is not consumed
Specific	Most catalysts work for only one particular reaction or type of reaction
Does not change $\Delta H$ΔH	The overall energy change remains the same
Does not change products	The same products are formed, just faster

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Catalysts on a Reaction Profile

On a reaction profile, the effect of a catalyst is shown by drawing a second, lower curve (lower peak) alongside the original curve:

graph TD
    subgraph "Reaction Profile with Catalyst"
        A["Reactants"] --> B["Original Peak \n(without catalyst)"]
        B --> C["Products"]
        A --> D["Lower Peak \n(with catalyst)"]
        D --> C
    end
    E["Original Ea \n(higher)"]
    F["New Ea with catalyst \n(lower)"]
    G["ΔH unchanged"]

Feature	Without Catalyst	With Catalyst
Activation energy	Higher	Lower
Peak height	Taller	Shorter
$\Delta H$ΔH	Same	Same
Products	Same	Same
Rate of reaction	Slower	Faster

Exam Tip: A very common exam question asks: "How does a catalyst increase the rate of reaction?" The perfect answer is: "A catalyst provides an alternative reaction pathway with a lower activation energy, so more particles have sufficient energy to react on collision."

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Examples of Catalysts

Reaction	Catalyst Used
Haber process ($\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$N2​+3H2​→2NH3​)	Iron
Decomposition of hydrogen peroxide	Manganese dioxide ($\text{MnO}_2$MnO2​)
Contact process (making sulfuric acid)	Vanadium pentoxide ($\text{V}_2\text{O}_5$V2​O5​)
Catalytic converters (in car exhausts)	Platinum, palladium, rhodium
Biological reactions	Enzymes (biological catalysts)

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Enzymes as Biological Catalysts

Enzymes are proteins that act as biological catalysts in living organisms. They speed up reactions that would otherwise be too slow for life.

Feature	Enzymes	Industrial Catalysts
Made of	Protein	Usually metals or metal oxides
Specificity	Very specific — usually one substrate	Less specific
Conditions	Work best at moderate temperatures (around 37°C in humans)	Often require high temperatures and pressures
Denatured by	High temperatures or extreme pH	High temperatures (eventually)

Exam Tip: You may be asked to compare enzymes with industrial catalysts. Enzymes are biological, highly specific, and work at low temperatures. Industrial catalysts are often metals and can withstand much harsher conditions.

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Why Catalysts Are Important in Industry

Catalysts are used in industry because they:

1. Reduce energy costs — lower $E_a$Ea​ means reactions can proceed at lower temperatures.
2. Increase rate — products are made faster, improving efficiency.
3. Reduce environmental impact — lower temperatures mean less fossil fuel burned for heating.
4. Are not used up — they can be reused many times.

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Common Mistakes

Mistake	How to Avoid It
"A catalyst changes the energy change of a reaction"	Catalysts do NOT change $\Delta H$ΔH — only $E_a$Ea​
"A catalyst is used up in the reaction"	Catalysts are NOT consumed — they are unchanged at the end
"Temperature lowers the activation energy"	Temperature does NOT change $E_a$Ea​ — only a catalyst does that
Drawing the catalyst curve above the original	The catalyst curve must be LOWER (lower peak)
"A catalyst increases the energy of particles"	No — it provides an alternative pathway with lower $E_a$Ea​

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Summary