Activation Energy
What is 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
Energy profile of an exothermic reaction, showing Ea
The diagram shows that the reactants are higher in energy than the products in the exothermic reaction
Energy profile of an endothermic reaction, showing Ea
The diagram shows that the reactants are lower in energy than the products in the endothermic reaction
- 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 successful the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction
Examiner Tip
How to find the activation energy of a reverse reaction
- You may be required to show or calculate the activation energy for a reverse reaction using a labelled energy profile like those above. The activation energy for the reverse reaction is found by:
For an exothermic reaction = ∆H + Ea (forward)
For an endothermic reaction = Ea (forward) - ∆H
- Calculations of the value of the activation energy from experimental data are not required at Standard Level but are required in Higher Level Chemistry
- For more information on how to calculate activation energy, see our revision notes on Determining Activation Energy & the Arrhenius Factor
