# 1.8.3 Activation Energy & Boltzmann Distribution Curves

### Activation Energy

• For a reaction to take place, the reactant particles need to overcome a minimum amount of energy
• This energy is called the activation energy (Ea)
• In exothermic reactions the reactants are higher in energy than the products
• In endothermic reactions the reactants are lower in energy than the products
• Therefore, the Ea in endothermic reactions in endothermic reactions is relatively larger than in exothermic reaction

The diagram shows that the reactants are higher in energy than the products in the exothermic reaction, so the energy needed for the reactants to go over the energy barrier is relatively small

The diagram shows that the reactants are lower in energy than the products in the endothermic reaction, so the energy needed for the reactants to go over the energy barrier is relatively large

• Even though particles collide with each other in the same orientation, if they don’t possess a minimum energy that corresponds to the Ea of that reaction, the reaction will not take place
• Therefore, for a collision to be effective the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction

#### Exam Tip

The activation energy is the energy needed to ‘activate’ the reactant particles in order for them to collide effectively and cause a chemical reaction.

### Boltzmann Distribution Curves

#### Boltzmann distribution curve

• The Boltzmann distribution curve is a graph that shows the distribution of energies at a certain temperature
• In a sample of a substance, a few particles will have very low energy, a few particles will have very high energy, and many particles will have energy in between

The Boltzmann distribution curve shows the distribution of the energies and the activation energy

• The graph shows that only a small proportion of molecules in the sample have enough energy for an effective collision and for a chemical reaction to take place

#### Changes in temperature

• When the temperature of a reaction mixture is increased, the particles gain more kinetic energy
• This causes the particles to move around faster resulting in more frequent collisions
• Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction
• With higher temperatures, the Boltzmann distribution curve flattens and the peak shifts to the right

The Boltzmann distribution curve at T oC and when the temperature is increased by 10 oC

• Therefore, an increase in temperature causes an increased rate of reaction due to:
• There being more effective collisions as the particles have more kinetic energy, making them move around faster
• A greater proportion of the molecules having kinetic energy greater than the activation energy

#### Exam Tip

The increase in proportion of molecules having kinetic energy greater than the activation has a greater effect on the rate of reaction than the increase in effective collisions

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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