Effect of Temperature on Reaction Rates and the Concept of Activation Energy

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Define activation energy, EA, as the minimum energy required for a collision to be effective
  2. Sketch and use the Boltzmann distribution to explain the significance of activation energy
  3. Explain qualitatively, in terms both of the Boltzmann distribution and of frequency of effective collisions, the effect of temperature change on the rate of a reaction

1. Activation Energy (Ea)

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What is Activation Energy?

Activation energy (Ea) is the minimum amount of energy that particles must have when they collide for a reaction to occur.

Think of it like this: imagine you're trying to push a heavy box over a hill. You need to apply a certain minimum amount of force to get it over the top. Activation energy is similar — it's the minimum energy "push" needed to make reactant particles turn into product particles.

Why Do We Need Activation Energy?

Not every collision between particles causes a reaction. For a collision to be effective (meaning it actually causes a chemical reaction), two things must happen:

  1. The particles must collide with the correct orientation — they need to hit each other in the right way
  2. The particles must have energy equal to or greater than the activation energy (Ea) — they must collide hard enough

If particles collide with the right orientation but don't have enough energy (less than Ea), the collision is ineffective and no reaction happens. The particles just bounce off each other.

Activation Energy in Reaction Pathways

We can show activation energy on a graph called a reaction pathway diagram (also called an energy profile).

For an exothermic reaction (a reaction that releases energy):

  • The reactants start at a higher energy level than the products
  • The particles must gain enough energy to reach the peak (called the transition state)
  • The activation energy (Ea) is the vertical distance from the reactant energy level up to the peak
  • After reaching the peak, the particles lose energy as they form products
  • The overall energy change (ΔH) is negative because energy is released

For an endothermic reaction (a reaction that absorbs energy):

  • The reactants start at a lower energy level than the products
  • The particles must gain energy to reach the transition state
  • The activation energy (Ea) is the vertical distance from the reactant energy level up to the peak
  • This Ea is relatively larger than in exothermic reactions because the reactants are starting from a lower energy position
  • The overall energy change (ΔH) is positive because energy is absorbed

Key point: Even though the particles collide in the correct orientation, if they don't possess the minimum energy (Ea), the reaction will not take place.

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