Collision theory

IB Chemistry HL· 35 min read

1. Core Postulates of Collision Theory★★☆☆☆⏱ 10 min

Collision theory is a simple kinetic model developed to explain reaction rates, originally for gas-phase reactions but applicable to almost all reaction systems. It makes testable predictions about how reaction conditions change rate, which match experimental observations.

📘 Definition

Collision Theory

A model that links reaction rate to the frequency and properties of collisions between reactant particles.

Example:

It correctly predicts that increasing temperature always increases reaction rate for non-catalyzed reactions.

For a reaction to occur, reactant particles must first collide with one another.
Only collisions with energy equal to or greater than the reaction's activation energy ( $E_{a}$ ) can produce a reaction.
Colliding particles must also approach each other with the correct orientation to break existing bonds and form new product bonds.

📐 Worked Example

State which collision between two HI molecules will result in a successful reaction, given the activation energy for $2 H I \to H_{2} + I_{2}$ is 235 kJ mol⁻¹: (a) 250 kJ mol⁻¹, correct orientation, (b) 200 kJ mol⁻¹, correct orientation, (c) 250 kJ mol⁻¹, incorrect orientation

1
Recall the three requirements for successful collision: collision occurs, energy ≥ $E_{a}$ , correct orientation.
2
Evaluate option (a): Energy (250 kJ mol⁻¹) > 235 kJ mol⁻¹, orientation is correct. This collision is successful.
3
Evaluate option (b): Energy (200 kJ mol⁻¹) < activation energy. Even with correct orientation, no reaction occurs.
4
Evaluate option (c): Energy meets the requirement, but orientation is incorrect. Bonds cannot break to form product, so collision is unsuccessful.
5
Conclusion: Only collision (a) is successful.

Exam tip:

Always mention both energy and orientation when explaining successful collisions — exam markers require both points for full marks.

2. Effect of Reaction Conditions on Rate★★★☆☆⏱ 15 min

Collision theory can explain how changing common reaction conditions alters reaction rate, which is a very common exam question.

📘 Definition

Successful Collision

A collision that meets all three requirements to form new product molecules.

📐 Worked Example

Use collision theory to explain why increasing concentration of a reactant increases reaction rate at constant temperature.

1
Increasing concentration means more reactant particles per unit volume of the reaction mixture.
2
More particles per unit volume leads to a higher frequency of collisions between reactant particles per second.
3
At constant temperature, the proportion of collisions with energy ≥ $E_{a}$ stays the same, so higher collision frequency gives more successful collisions per second.
4
More successful collisions per unit time equals a higher reaction rate.

Temperature: The dominant effect of increasing temperature is that a much greater proportion of particles have energy ≥ $E_{a}$ , leading to more successful collisions per second. There is also a small increase in collision frequency.
Surface area (solid reactants): Breaking a solid into smaller pieces increases the surface area of reactant exposed to other reactants, increasing collision frequency and rate.
Pressure (gases): Increasing pressure compresses gas into a smaller volume, which increases concentration, collision frequency and rate.

Exam tip:

When explaining temperature effects, emphasize the change in proportion of particles with energy ≥ $E_{a}$ , not just more collisions. The proportion change is the dominant effect.

3. Catalysts and Activation Energy★★★☆☆⏱ 10 min

Catalysts increase reaction rate without being consumed in the reaction, and their action is easily explained by collision theory principles.

📘 Definition

Activation Energy

$E_{a}$

The minimum energy that colliding reactant particles must have to form products in a successful collision.

📐 Worked Example

Use collision theory to explain why a catalyst increases reaction rate at the same temperature.

1
A catalyst does not change the energy of reactant particles, instead it provides an alternative reaction mechanism (pathway) for the reaction.
2
This alternative pathway has a lower activation energy than the uncatalyzed pathway:
3
$E_{a(\text{catalyzed})} < E_{a(\text{uncatalyzed})}$
4
At a given temperature, a much larger proportion of reactant particles have energy equal to or greater than the lower activation energy.
5
This leads to a higher frequency of successful collisions per second, so reaction rate increases, without changing temperature or total collision frequency.

4. Common Pitfalls

Wrong move:

Claiming that increasing temperature increases activation energy.

Why:

Activation energy is a fixed property of the reaction pathway, it does not change with temperature.

Correct move:

State that increasing temperature increases the proportion of particles with energy ≥ $E_{a}$ .

Wrong move:

Forgetting to mention orientation when explaining successful collisions.

Why:

Most IB exam mark schemes require both energy and orientation conditions for full marks.

Correct move:

Always explicitly state that successful collisions require both sufficient energy (≥ $E_{a}$ ) and correct orientation.

Wrong move:

Claiming that catalysts work by increasing the frequency of collisions.

Why:

Catalysts do not change the total number of collisions between reactant particles, only the pathway.

Correct move:

Explain that catalysts provide an alternative lower activation energy pathway, increasing the proportion of successful collisions.

Wrong move:

Stating that increasing concentration increases the proportion of particles with energy ≥ $E_{a}$ .

Why:

At constant temperature, the energy distribution of particles does not change when concentration increases.

Correct move:

Explain that higher concentration increases collision frequency, leading to more successful collisions per second.

5. Quick Reference Cheatsheet

Condition Change	Key Change per Collision Theory	Effect on Rate
Increase concentration	Higher collision frequency	Increase
Increase temperature	Higher proportion of collisions ≥ $E_{a}$	Increase
Increase surface area	Higher collision frequency	Increase
Increase pressure (gases)	Higher collision frequency	Increase
Add catalyst	Alternative lower $E_{a}$ pathway	Increase

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · Paper 1
Explain effect of temperature on rate
2023 · Paper 2
Explain catalyst action via collision theory
2021 · Paper 1
Identify successful collision conditions

Going deeper

What's Next

Collision theory is the core foundation for all further study of reaction kinetics in IB Chemistry HL. You will next build on this model to explore the Arrhenius equation, which quantifies the relationship between temperature, activation energy and the rate constant. Understanding collision theory is also critical for interpreting reaction profile diagrams and studying the mechanisms of organic reactions, and it is assessed in almost every kinetics exam question.