Explaining Rates Using Collision Theory (CIE IGCSE Chemistry)
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Chemistry
Explaining Rates Using Collision Theory
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Temperature
- Increasing the temperature will increase the rate of reaction
- 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
Concentration
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- 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
- Increasing the surface area of a solid will increase the 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
Catalysts
- Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in 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
- 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
Exam Tip
When answering questions on the effect of concentration on the rate of reaction, you should mention that there are more particles per unit volume (usually cm3) and this causes an increase in the rate of collisions.
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