Particle Theory

Section 1 of 7Collision theory

Reactions need collisions

• Reactions need collisions A reaction can only happen when reactant particles collide with each other.
• A successful collision rearranges the atoms to form products, while an unsuccessful one makes the particles bounce apart unchanged.

Rate of reaction

• Rate of reaction The more frequent the successful collisions, the faster the reaction.
• Each factor that raises the rate does so by increasing the number of successful collisions per second.

Section 2 of 7Explaining each factor

Factor increased	What changes	Result
Concentration	More particles in each unit volume	More collisions per second, so a faster rate
Pressure of a gas	The same particles squeezed into a smaller volume	More collisions per second, so a faster rate
Temperature	Particles have more kinetic energy and move faster	More frequent, and more successful, collisions
Surface area of a solid	More of the solid exposed to the other reactant	More collisions per second, so a faster rate

Section 3 of 7Proportionality and temperature

Doubling concentration

• Doubling concentration Doubling the number of particles in a given volume doubles the collisions per second.
• Doubling the surface area likewise doubles the collision rate.

Temperature is different

• Temperature is different A small rise in temperature gives a large rise in rate.
• For aqueous and gaseous systems, a rough guide is that every 10 °C rise approximately doubles the rate of reaction.

Section 4 of 7Activation energy (HT only)

1Energy needed

• Energy needed Particles must collide with at least a minimum energy, called the activation energy, to react.

2Too little energy

• Too little energy Collisions below this energy are unsuccessful and the particles just bounce off.

3Enough energy

• Enough energy Collisions with energy equal to or above it are successful and products form.

4High activation energy

• High activation energy If the activation energy is high, fewer collisions succeed and the reaction is slower.

Section 5 of 7Which factors change energy (HT only)

Rate depends on successful collisions per second

Concentration, pressure and surface area

• Concentration, pressure and surface area These only change the number of collisions, not the energy of each collision.

Temperature

• Temperature It is the only one that directly changes the energy of collisions, because thermal energy becomes kinetic energy.
• It increases both the frequency of collisions and the proportion with energy above the activation energy.

Exam wording

• Exam wording Say 'more frequent successful collisions' rather than just 'more collisions'.

Section 6 of 7Key terms

Term	Meaning
Collision theory	The idea that reactions happen when particles collide with enough energy.
Successful collision	A collision that results in the reactants forming products.
Activation energy	The minimum energy particles need when they collide in order to react.
Kinetic energy	The energy a particle has because it is moving.
Frequency of collisions	The number of collisions per second.

Section 7 of 7Exam technique

1

Saying particles collide harder when concentration increases - only the number of collisions changes, not their energy.

2

Explaining the effect of temperature only by faster particles; also mention the greater proportion of collisions that are successful.

3

Writing 'more particles' instead of 'more particles per unit volume' for concentration and pressure.

4

Thinking every collision causes a reaction - many collisions are unsuccessful.