Change Rates (OCR GCSE Chemistry A (Gateway))

Changing Rates

• 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 Increased Concentration or Pressure 

Graph showing the effect of the concentration of a solution or gas pressure on the rate of reaction

Explanation:

• 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, the rate of reaction will increase

The Effect of Increasing Temperature 

Graph showing the effect of temperature on the rate of reaction

Explanation:

• 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

The Effect of Increasing Surface Area

Graph showing the effect of increased surface area on a reaction rate

Explanation: 

• 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

Collision Theory

• Collision theory states that chemical reactions occur only when the reactant particles collide with sufficient energy to react

• The minimum amount of energy needed is called the activation energy, which is different for each reaction

• Particles that collide with insufficient energy have unsuccessful collisions and just bounce off each other

• The rate of a reaction is therefore also dependent on the energy of collisions as well as the number of collisions

• To increase the rate of a reaction then the number of successful collisions needs to be increased

Diagram showing a successful and an unsuccessful collision

• Increasing the number of successful collisions means that a greater proportion of reactant particles collide to form product molecules.

• We can use collision theory to explain why increasing concentration, temperature, and surface area increase the rate of a reaction

Concentration of a Solution/ Pressure of a Gas

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 number of collisions between reacting particles is increased causing an increased rate of reaction

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

Temperature

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

Explanation:

• Increase in the temperature, the rate of reaction will increase

• This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction

• The effect of temperature on collisions is not so straight forward 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 degree (Kelvin) increase in temperature the rate of reaction approximately doubles

Surface Area

• For any solid, only the particles at the surface can take part in collisions 

• Bigger lumps of solid have a smaller surface area to volume ratio than smaller lumps or powders 

Diagram showing that surface area increase as particle size decreases. A 2 cm³ cube has a surface area of 24 cm² and the same cube cut up into 8 cubes has a surface area of 48 cm²

• The total surface area available for collisions is therefore greater for smaller lumps and powders resulting in a faster rate of reaction

• 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