Reaction Rates

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Reaction rates

  • The rate of a reaction is the rate of change of the concentration of a reactant or product with time
  • The rate of reaction is given by the gradient at any point on a graph

Effect of concentration

  • The rate of reaction increases as the concentration increases
  • This is because a successful collision has to take place between the particles of reactants for a chemical reaction to occur
  • If the concentration of one or more reactants increases, then the particles are closer together and so they collide more often
  • For gases, increasing the pressure increases the concentration and therefore increases the rare for the same reason

Activation Energy

  • For reactions to occur, the reacting particles must have at least a certain minimum amount of energy called the activation energy when they collide
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Reaction rates

  • This energy is needed to begin to break existing bonds in the reactants before new bonds in the product are formed
  • If the particles have insufficient energy, they simply bounce off one another unchanged

Activation energy is 'the minimum energy that particles must have in order to react'

Effect of temperature changes

  • Raising the temperature increases rate of reaction
  • As the temperature increases, so does the proportion of molecules that have energy greater than the activation energy. Consequently, when the molecules collide, it is more likely that this will successfully lead to a reaction. This is the main reason for the rate increasing with temperature
  • As the temperature increases, the molecules gain more kinetic energy so they are moving faster and therefore collide more often


A catalyst increases the rate of a reaction, but is not used up in the process. It does this by providing an alternative pathway of lower activation energy

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Reaction rates

  • The effect is that a higher proportion of molecules have an energy greater than the activation energy and hence successful collisions are more frequent
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