Factors Affecting Rates of Reaction (HL IB Chemistry)

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

The diagram shows a higher concentration of particles in (b) which means that there are more particles present in the same volume than (a) so the chances and frequency of collisions between reacting particles are increased causing an increased 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

The diagram shows a higher pressure in (b) which means that the same number of particles occupy a smaller volume, resulting in an increased collision frequency and therefore increased 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

An increase in temperature causes an increase in the kinetic energy of the particles. The number of collisions increases and the proportion of successful collisions increases 

• 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

An increase in surface area of a solid reactant allows more particles to come into contact with each other

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