Rates of Reactions

Introduction

The rate of a reaction can be found by measuring the rate of formation of products or the rate at which reactants are consumed. This can be observed through colour changes, changes in gas volume and pH changes

Collision Theory

Particles are colliding all the time, however, not all collisions induce a reaction. Particles must collide with enough energy and the correct orientation to form new products. The minimum energy a collision must possess for a reaction to occur is called the activation energy

Reaction Rates

In order to increase a reaction rate, the proportion of successful collisions can be increased by either increasing the frequency of collisions occurring, or increasing the proportion of collisions that have enough energy for the reaction to proceed

  • Surface area: when increased, there is a greater number of exposed particles, increasing the frequency of collisions
  • Concentration and pressure: when increased, there are more particles in a fixed volume, increasing the frequency of collisions
  • Temperature: when increased, there is more energy for particles to have collisions, increasing the probability of a successful collision as there is a higher proportion of particles with energy that is sufficient to overcome the activation energy

Effect of Temperature

Maxwell Boltzmann Distribution

At any temperature, the particles in a substance will have a range of kinetic energies. While most of the particles have similar kinetic energies, there will always be some particles with a high or low energy. This is shown on a Maxwell Boltzmann distribution

Maxwell Boltzmann Distribution

Catalysts

Catalysts reduce the activation energy required to facilitate a reaction without being consumed during the reaction. With a catalyst present and a lower activation energy, the colliding particles are more likely to have energies that exceed this lower barrier, causing the bonds in the reactants to be broken more frequently.

  • Heterogeneous catalysts: those which are in a different state to the reactants
  • Homogeneous catalysts: those which are in the same state as the reactants

Effect of Catalysts