Collision Theory and Activation Energy

Section 1 of 7Collision theory

Particles must collide

• Particles must collide A chemical reaction can only happen when reactant particles collide with each other.
• Most collisions do not result in a reaction.

Collisions must be successful

• Collisions must be successful A collision is successful only if the particles hit with at least a minimum amount of energy.
• This minimum energy is called the activation energy.

Rate and successful collisions

• Rate and successful collisions The rate depends on how many successful collisions occur per second.
• More frequent collisions, or a larger proportion with enough energy, both raise the rate.

Section 2 of 7Activation energy on a reaction profile

1

Draw the reactants at their energy level on the left of the diagram.

2

Draw a curve rising to a peak: the height from the reactants to the peak is the activation energy.

3

Bonds in the reactants must be partly broken to reach this peak, which is why energy is needed even in an exothermic reaction.

4

Draw the curve falling to the products; a lower product level means an exothermic reaction, a higher one an endothermic reaction.

Section 3 of 7How each factor changes the rate

Change	Effect on particles	Effect on rate
Higher concentration of a solution	More particles in the same volume, so collisions are more frequent	Increases
Higher pressure of a gas	Particles closer together, so collisions are more frequent	Increases
Larger surface area of a solid	More exposed particles meet the other reactant more often	Increases
Higher temperature	Particles move faster: more frequent collisions and more of them have at least the activation energy	Increases

Section 4 of 7Why temperature has the biggest effect

1Heat is supplied

• Heat is supplied The average kinetic energy of the particles increases.

2Faster movement

• Faster movement The particles travel faster, so they collide more often.

3Higher energy collisions

• Higher energy collisions A greater proportion of collisions has energy equal to or above the activation energy.

4Rate rises sharply

• Rate rises sharply Many more successful collisions happen each second;
  • a rise of about 10 °C roughly doubles the rate for many reactions.

Section 5 of 7Writing an exam explanation

Three-part answer

Name the change

• Name the change State what was altered, for example 'the concentration was increased'.

Describe the particles

• Describe the particles Say there are more particles in the same volume, or that they move faster.

Link to collisions

• Link to collisions Finish with 'more frequent (successful) collisions per second, so the rate increases'.

Section 6 of 7Key terms

Term	Meaning
Collision theory	The idea that reactions occur when particles collide with enough energy.
Activation energy	The minimum energy particles need to react when they collide.
Successful collision	A collision in which the particles react because they have at least the activation energy.
Frequency of collisions	How many collisions take place each second.
Reaction profile	An energy diagram showing reactants, products and the activation energy peak.

Section 7 of 7Exam technique

1

Saying that raising the temperature makes particles 'collide' without adding that more collisions have enough energy to react.

2

Believing that every collision causes a reaction; only those reaching the activation energy do.

3

Showing the activation energy as the difference between reactants and products rather than reactants and the peak.

4

Stating that increasing concentration raises the particles' energy; it only increases how often they collide.