Explaining Rates Using Collision Theory (Cambridge O Level Chemistry)

Rates of reaction factors

• There are several factors that can affect the rate of a reaction. These are:
  • Concentration of the reactants in solution or the pressure of reacting gases
  • Temperature at which the reaction is carried out
  • Surface area of solid reactants
  • The use of a catalyst
• Changes in these factors directly influence the rate of a reaction
• It is of economic interest to have a higher rate of reaction as this implies a higher rate of production and hence a more efficient and sustainable process

The effect of temperature

• Compared to a reaction at a low temperature, the graph line for the same reaction but at a higher temperature has a steeper gradient at the start and becomes horizontal sooner
• This shows that with increased temperature, the rate of reaction will increase

Explanation:

• This is because the particles will have more kinetic energy than the required activation energy
• Therefore there will be more frequent collisions and a higher proportion of particles have energy greater than the activation energy
• This causes more successful collisions per second, increasing the rate of reaction
• The effect of temperature on collisions is not so straightforward as concentration or surface area; a small increase in temperature causes a large increase in rate
• For aqueous and gaseous systems, a rough rule of thumb is that for every 10 ^oC increase in temperature, the rate of reaction approximately doubles
  
  

Diagram showing the increased kinetic energy that particles have at higher temperatures

 The effect of increased concentration or pressure

• Compared to a reaction with a reactant at a low concentration, the graph line for the same reaction but at a higher concentration/pressure has a steeper gradient at the start and becomes horizontal sooner
• This shows that with increased concentration of a solution or increased pressure of a gas, the rate of reaction will increase 

Explanation:

• Increasing the concentration of a solution will increase the rate of reaction 
• This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second, increasing the rate of reaction
• For a gaseous reaction, increasing the pressure has the same effect as the same number of particles will occupy a smaller space, increasing the concentration
• If you double the number of particles you will double the number of collisions per second
• The number of collisions is proportional to the number of particles present
  
  

Diagram showing the decrease in space between particles at higher concentrations

Surface Area

• Compared to a reaction with lumps of reactant, the graph line for the same reaction but with powdered reactant has a steeper gradient at the start and becomes horizontal sooner
• This shows that with increased surface area of the solid, the rate of reaction will increase

Explanation:

• This is because more surface area of the particles will be exposed to the other reactant, producing a higher number of collisions per second
• If you double the surface area you will double the number of collisions per second
  
  

An increase in surface area means more collisions per second

Catalysts

• Catalysts are substances which speed up the rate of a reaction but are unchanged at the end of the reaction
• The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of the equation

• Compared to a reaction without a catalyst, the graph line for the same reaction but with a catalyst has a steeper gradient at the start and becomes horizontal sooner
• This shows that with a catalyst, the rate of reaction will increase

Explanation:

• Different processes require different types of catalysts but they all work on the same principle of providing a different pathway for the reaction to occur that has a lower activation energy
• This means a higher proportion of the reactant particles have energy greater than the activation energy and will result in more successful collisions per second
• An important industrial example is iron, which is used to catalyse the Haber Process for the production of ammonia
• Iron beads are used to increase the surface area available for catalysis
• Enzymes are biological catalysts, they work best at specific temperature and pH ranges
• Normally only small amounts of catalysts are needed to have an effect on a reaction
  
  

A catalyst lowers the activation energy of a reaction by providing an alternative reaction pathway