Collision Theory

This lesson covers collision theory, the concept of activation energy, and what makes a collision successful as required by the Edexcel GCSE Combined Science specification (1SC0). Collision theory explains why changing conditions (concentration, temperature, surface area, catalysts) affects the rate of reaction.

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What Is Collision Theory?

Collision theory states that for a chemical reaction to occur:

1. Reacting particles must collide with each other.
2. The collision must have sufficient energy — at least equal to the activation energy.
3. The particles must collide with the correct orientation (they must hit in the right way).

If either condition is not met, the collision is unsuccessful and the particles simply bounce apart unchanged.

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Activation Energy

The activation energy (Eₐ) is the minimum energy that colliding particles need in order to react. Every reaction has its own activation energy.

Term	Definition
Activation energy (Eₐ)	The minimum energy required for a collision to be successful and lead to a reaction
Successful collision	A collision in which the particles have energy ≥ Eₐ and the correct orientation — products are formed
Unsuccessful collision	A collision where the particles do not have enough energy or the correct orientation — no reaction occurs

graph TD
    A["Particles collide"] --> B{"Energy ≥ Eₐ<br/>AND correct<br/>orientation?"}
    B -->|"Yes"| C["Successful collision<br/>→ Products formed"]
    B -->|"No"| D["Unsuccessful collision<br/>→ Particles bounce apart"]

    style A fill:#2980b9,color:#fff
    style B fill:#f39c12,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#c0392b,color:#fff

Exam Tip: Many students forget to mention activation energy when explaining collision theory. The key phrase is: "particles must collide with energy equal to or greater than the activation energy."

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Energy Distribution of Particles

In any sample of gas or solution, particles have a range of energies. Some move slowly (low energy) and some move quickly (high energy). The distribution of energies can be shown on a Maxwell–Boltzmann distribution curve.

Key Points

• Most particles have an energy somewhere in the middle of the range.
• Only a small proportion of particles have energy equal to or above the activation energy at any given temperature.
• It is these particles that are capable of successful collisions.
• Changing temperature or adding a catalyst changes the proportion of particles with energy ≥ Eₐ.

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How Collision Theory Explains Rate Changes

Collision theory provides the explanation for every factor that affects rate:

Factor Changed	Effect on Collisions	Effect on Rate
Increase concentration	More particles per unit volume → more frequent collisions	Rate increases
Increase temperature	Particles move faster → collisions are more frequent AND a greater proportion have energy ≥ Eₐ	Rate increases
Increase surface area	More surface exposed → more collisions per unit time	Rate increases
Add a catalyst	Provides an alternative pathway with a lower Eₐ → more particles have enough energy to react	Rate increases
Increase pressure (gases)	Particles closer together → more frequent collisions	Rate increases

Exam Tip: When explaining why increasing temperature increases rate, you must give both reasons: (1) particles collide more frequently and (2) a greater proportion of collisions have energy ≥ Eₐ. Giving only one reason will not earn full marks.

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Frequency of Collisions vs Energy of Collisions

It is important to distinguish between the frequency (how often) collisions occur and the energy of those collisions.

Concept	Meaning
Collision frequency	The number of collisions per second. Increases when concentration, pressure or temperature increase.
Collision energy	The kinetic energy the particles have when they collide. Only increases with temperature.

Increasing concentration or surface area increases the frequency of collisions but does not change the energy of each collision. Increasing temperature increases both the frequency and the proportion with sufficient energy.

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Orientation of Collisions

Some reactions require particles to collide in a specific orientation. If the particles hit each other the wrong way round, the reaction will not proceed even if the energy is sufficient.

graph LR
    subgraph "Correct orientation"
        A1["Particle A<br/>(reactive site →)"] -->|"collision"| B1["← Particle B<br/>(reactive site)"]
        B1 --> C1["Reaction occurs ✓"]
    end
    subgraph "Incorrect orientation"
        A2["Particle A<br/>(reactive site →)"] -->|"collision"| B2["Particle B →<br/>(back face)"]
        B2 --> C2["No reaction ✗"]
    end

    style C1 fill:#27ae60,color:#fff
    style C2 fill:#c0392b,color:#fff

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Applying Collision Theory: Worked Example

Question: Explain why increasing the concentration of hydrochloric acid increases the rate of its reaction with magnesium ribbon.

Model Answer:

1. Increasing the concentration means there are more HCl particles per unit volume of solution.
2. This leads to more frequent collisions between HCl particles and the magnesium.
3. Therefore there are more successful collisions per second, so the rate of reaction increases.

Exam Tip: Structure your answers as: more particles → more frequent collisions → more successful collisions per second → rate increases. This chain of reasoning scores full marks.

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Summary

• Collision theory states that particles must collide with sufficient energy (≥ Eₐ) and the correct orientation for a reaction to occur.
• The activation energy is the minimum energy needed for a successful collision.
• Increasing concentration, temperature, surface area, pressure (for gases) or adding a catalyst all increase the rate by increasing the frequency or energy of collisions (or lowering Eₐ).
• Temperature is the only factor that increases both the frequency of collisions and the proportion of particles with energy ≥ Eₐ.

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Worked Example: Explaining Successful vs Unsuccessful Collisions

Question (6 marks): A student mixes magnesium powder with dilute sulfuric acid. Only a small fraction of collisions between Mg atoms at the surface and H⁺ ions are successful. Explain why, using collision theory. Refer to activation energy.

Model answer — full marks breakdown: