Rate of Reaction

2026 Syllabus Objectives

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

  1. Explain and use the terms: rate of reaction, frequency of collisions, effective collisions, and non-effective collisions
  2. Explain qualitatively, in terms of frequency of effective collisions, the effect of concentration and pressure changes on the rate of a reaction
  3. Use experimental data to calculate the rate of a reaction

1. What is Rate of Reaction?

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The rate of reaction is the speed at which a chemical reaction happens. It measures how quickly reactants (the starting substances) get used up or how quickly products (the new substances) are formed.

Units: The rate of reaction is measured in mol dm⁻³ s⁻¹ (moles per cubic decimetre per second).

Formula:

Rate of reaction=change in concentration (mol dm−3)time (s)\text{Rate of reaction} = \frac{\text{change in concentration (mol dm}^{-3}\text{)}}{\text{time (s)}}

You can calculate the rate using either:

  • The decrease in concentration of reactants, OR
  • The increase in concentration of products

Both methods give you the same information about how fast the reaction is happening.

2. Collision Theory

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For a chemical reaction to happen, particles (atoms, molecules, or ions) must collide with each other. However, not all collisions lead to a reaction. This is explained by collision theory.

Frequency of Collisions

Frequency of collisions means the number of collisions that happen per unit time (for example, per second).

  • The more collisions that happen in a given time, the faster the reaction.
  • If you can increase the number of collisions, you increase the rate of reaction.

Effective Collisions vs Non-Effective Collisions

Not every collision between particles causes a reaction. For a collision to be successful, two conditions must be met:

Effective collision = A collision that results in a chemical reaction

For a collision to be effective:

  1. Correct orientation: The particles must hit each other in the right way (at the right angle).
  2. Sufficient energy: The particles must have enough energy to break the bonds in the reactants and form new bonds in the products.

Non-effective (ineffective) collision = A collision that does NOT result in a chemical reaction

This happens when:

  1. The particles collide in the wrong orientation, OR
  2. The particles don't have enough energy

When a collision is ineffective, the particles simply bounce off each other without reacting.

Activation Energy (Eₐ)

Activation energy (Eₐ) is the minimum energy that colliding particles must have for a collision to be effective and for a reaction to take place.

Think of it like a hurdle that particles must jump over:

  • If particles have energy equal to or greater than Eₐ, they can react → effective collision
  • If particles have energy less than Eₐ, they cannot react → non-effective collision

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