Activation Energy & Boltzmann Distribution Curves (CIE A Level Chemistry)

• For a reaction to take place, the reactant particles need to overcome a minimum amount of energy
• This energy is called the activation energy (E_a)
• 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 E_a in endothermic reactions in endothermic reactions is relatively larger than in exothermic reaction

Exothermic reaction pathway diagram

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

Endothermic reaction pathway diagram

The reactants are lower in energy than the products in an 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 E_a 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 E_a of that reaction

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

A Boltzmann distribution curve

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

How temperature affects a Boltzmann distribution curve

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