Factors Affecting Rates of Reaction (DP IB Chemistry): Revision Note

Factors affecting rates of reaction

What are the factors affecting rate of reaction?

• The rate of reaction is dependent on any factor that changes the number of successful collisions

• Five such factors are:

  • Concentration

  • Pressure

  • Temperature

  • Surface area

  • The use of catalysts

Concentration

• The more concentrated a solution is, the greater the number of particles in a given volume of solvent

• An increase in concentration causes an increased collision frequency and therefore the frequency of successful collisions increases

• This leads to an increased rate of reaction

Effect of concentration on rate of reaction

Pressure

• An increase in pressure in reactions that involve gases has the same effect as an increased concentration of solutions

• When the pressure is increased, the particles have less space in which they can move

• This means that the number of successful collisions increases due to an increased collision frequency

• An increase in pressure, therefore, increases the rate of reaction

Effect of pressure on rate of reaction

Temperature

• Increasing the temperature of the reaction mixture increases the rate of reaction in the following two ways:

  • At higher temperatures, the particles are moving faster, so collide more frequently

    • A higher number of collisions in total mean a higher number of successful collisions

  • At higher temperatures, a higher proportion of the particles have the activation energy or more

    • This means that a higher proportion of collisions are successful

Effect of temperature on rate of reaction

• For more information about how to explain the effect of temperature on the rate of reaction, see our revision note on Maxwell-Boltzmann Distribution Curves

Surface area

• Only the particles on the surface of a solid will collide with particles of the other reactant

• If the surface area is increased, more particles will be on the surface and able to collide with particles of the other reactant

  • This means that there will be more collisions in total and therefore more successful collisions.

• Surface area can be increased by decreasing the size of the reactant, e.g. from large pieces to a fine powder

  • Large pieces have a much smaller surface area than powders, which have a very large surface area

Effect of surface area on rate of reaction

Catalysts

• A catalyst provides the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction

  • This means that more collisions will be successful

  • The rate of reaction therefore increases

• A catalyst does not itself undergo permanent chemical change i.e. it is chemically unchanged at the end of the reaction

• For more information on catalysts, see our revision notes on Energy Profiles With & Without Catalysts