Collision Theory and Activation Energy
For a chemical reaction to occur, reactant particles must collide with a certain minimum amount of energy, known as the activation energy (Ea). This concept explains why not all collisions are productive and is visualized as an energy barrier on reaction profile diagrams.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Reactions happen when particles collide with both the correct orientation and sufficient energy.
- A collision that leads to a reaction is called a 'successful' or 'effective' collision.
- The activation energy (Ea) is the minimum energy required for a collision to be successful.
- On an energy level diagram (reaction profile), the activation energy is the difference in energy between the reactants and the peak of the energy curve (the transition state).
- The magnitude of the activation energy determines the reaction rate: a high Ea corresponds to a slow reaction, while a low Ea corresponds to a fast reaction.
- Activation energy is always a positive value as it represents an energy barrier that must be overcome.
Definitions
- Activation Energy (Ea)
- The minimum amount of kinetic energy that colliding reactant particles must possess for a chemical reaction to occur.
- Energy Level Diagram
- A graph that shows the change in potential energy of a chemical system as it proceeds from reactants to products.
Worked example
The energy profile for a reaction shows that the reactants have an enthalpy of -10 kJ/mol and the products have an enthalpy of -40 kJ/mol. The highest point of the energy profile, representing the transition state, is at +35 kJ/mol. What is the activation energy for the forward reaction?
- 1
Identify the energy of the reactants:
-10 kJ/mol.
- 2
Identify the peak energy of the transition state:
+35 kJ/mol.
- 3
The activation energy (Ea) is the energy difference between the transition state and the reactants.
- 4
Calculate Ea = (Energy of transition state) - (Energy of reactants).
- 5 Ea = (+35 kJ/mol) - (-10 kJ/mol) = 35 + 10 = 45 kJ/mol
Answer: 45 kJ/mol
Common mistakes
- ×Confusing the activation energy (Ea) with the overall enthalpy change (ΔH). Ea is from reactants to the peak, while ΔH is from reactants to products.
- ×Incorrectly calculating Ea for the reverse reaction when asked for the forward reaction. The reverse Ea is measured from the products to the peak.
- ×Mistaking a negative enthalpy change (exothermic reaction) for a negative activation energy. Activation energy is always a positive energy barrier.
No-calculator tips
- ✓On an energy level diagram, always think of the activation energy as the height of the 'hill' you have to climb starting from the reactant's energy level.
- ✓Visually estimate the relative sizes of the activation energy 'hump' and the overall enthalpy change to avoid mixing them up. The 'hump' is Ea.
- ✓When dealing with negative numbers in energy levels, be careful with subtractions. Remember that subtracting a negative is the same as adding a positive.