Collision Theory — IB Chemistry SL Revision Notes & Topical Questions

Section 1 of 10The Theory

Collision theory links the rate of a reaction to what happens when particles meet.

Collision requirement

• Collision requirement Particles must collide in order to react, so a reaction cannot take place unless reactant particles meet.

Sufficient energy

• Sufficient energy The colliding particles must have at least the activation energy, so that existing bonds can be broken.

Orientation

• Orientation The particles must be aligned so that the reactive parts of the molecules meet, which is why not all energetic collisions succeed.

Successful collisions

• Successful collisions Only collisions which meet both the energy and orientation requirements lead to products.

Section 2 of 10Rate and Collisions

1

The rate depends on the frequency of collisions, which is the number of collisions per second.

2

It also depends on the fraction of those collisions that are successful.

3

More frequent collisions give more successful collisions per second and so a higher rate.

4

A larger fraction with sufficient energy also increases the rate, even when collision frequency is unchanged.

Section 3 of 10Why Collisions Fail

Why Collisions Fail

Insufficient energy

• Insufficient energy Most collisions at room temperature involve particles with too little energy and the particles rebound unchanged.

Wrong orientation

• Wrong orientation Particles may collide with unreactive parts of the molecules facing each other.

Large molecules

• Large molecules For large molecules the chance of the correct orientation is lower, so the reaction is slower.

Rare successes

• Rare successes Only a small fraction of collisions are successful, even in fast reactions.

Section 4 of 10Applying the Theory

Applying the Theory

Concentration

• Concentration More particles per unit volume raise the collision frequency.

Temperature

• Temperature Faster particles collide more often and more of them reach the activation energy.

Surface area

• Surface area A larger exposed area gives more collisions between reactants in different phases.

Catalyst

• Catalyst Lowers the activation energy so more collisions are successful.

Change	Collision frequency	Fraction with E ≥ Eₐ
Higher concentration	Increases	No change
Higher temperature	Increases slightly	Increases greatly
Catalyst	No change	Increases

Section 5 of 10Exam Checklist & Tips

5.2.3 – Key Points for the Exam

• Collision theory Reactions occur when particles collide with sufficient energy and the correct orientation.
• Activation energy The minimum energy colliding particles need for a successful collision.
• Collision frequency The number of collisions per unit time.
• Successful collision A collision that results in a chemical reaction.

Common Mistakes

• Saying that all collisions lead to a reaction.
• Ignoring the need for correct orientation.
• Describing activation energy as the energy released by the reaction.
• Stating that a higher temperature increases the rate only because collisions are more frequent.

Section 6 of 10The Theory

Collision theory links the rate of a reaction to what happens when particles meet.

Collision requirement

• Collision requirement Particles must collide in order to react, so a reaction cannot take place unless reactant particles meet.

Sufficient energy

• Sufficient energy The colliding particles must have at least the activation energy, so that existing bonds can be broken.

Orientation

• Orientation The particles must be aligned so that the reactive parts of the molecules meet, which is why not all energetic collisions succeed.

Successful collisions

• Successful collisions Only collisions which meet both the energy and orientation requirements lead to products.

Section 7 of 10Rate and Collisions

1

The rate depends on the frequency of collisions, which is the number of collisions per second.

2

It also depends on the fraction of those collisions that are successful.

3

More frequent collisions give more successful collisions per second and so a higher rate.

4

A larger fraction with sufficient energy also increases the rate, even when collision frequency is unchanged.

Section 8 of 10Why Collisions Fail

Why Collisions Fail

Insufficient energy

• Insufficient energy Most collisions at room temperature involve particles with too little energy and the particles rebound unchanged.

Wrong orientation

• Wrong orientation Particles may collide with unreactive parts of the molecules facing each other.

Large molecules

• Large molecules For large molecules the chance of the correct orientation is lower, so the reaction is slower.

Rare successes

• Rare successes Only a small fraction of collisions are successful, even in fast reactions.

Section 9 of 10Applying the Theory

Applying the Theory

Concentration

• Concentration More particles per unit volume raise the collision frequency.

Temperature

• Temperature Faster particles collide more often and more of them reach the activation energy.

Surface area

• Surface area A larger exposed area gives more collisions between reactants in different phases.

Catalyst

• Catalyst Lowers the activation energy so more collisions are successful.

Change	Collision frequency	Fraction with E ≥ Eₐ
Higher concentration	Increases	No change
Higher temperature	Increases slightly	Increases greatly
Catalyst	No change	Increases

Section 10 of 10Exam Checklist & Tips

5.2.3 – Key Points for the Exam

• Collision theory Reactions occur when particles collide with sufficient energy and the correct orientation.
• Activation energy The minimum energy colliding particles need for a successful collision.
• Collision frequency The number of collisions per unit time.
• Successful collision A collision that results in a chemical reaction.

Common Mistakes

• Saying that all collisions lead to a reaction.
• Ignoring the need for correct orientation.
• Describing activation energy as the energy released by the reaction.
• Stating that a higher temperature increases the rate only because collisions are more frequent.