Rates of reaction
What you need to know
Reflections and Exam tips
Rates of reaction
The rate of reaction measures the speed of a reaction. There are two ways to measure, such as either measuring how long it takes to use a certain amount of reactant or how much product is formed in a certain amount of time.
Collision theory
Reactions can only take place when particles collide, this is called collision theory. Collision theory also states that it is not enough for particles just to collide, but they have to collide with sufficient energy to form new substances.
The minimum amount of energy needed for the particles to react is called the ACTIVATION ENERGY, and is different for each reaction.
The rate of a reaction depends on two things:
- the frequency of collisions between particles
- the energy with which particles collide.
Reactions with high values of activation energy (Ea) take place more slowly than those with small activation energy values.
Factors affecting rates of reaction
Temperature - particles have more kinetic energy. This means they move faster and are more likely to collide with other particles with more energy, and so the number of successful collisions increases.
Surface area - If the solid is split into several pieces, the surface area increases. The smaller the pieces, the larger the surface area and the more collisions and a greater chance of reaction.
Concentration - higher concentration, there are more particles in the same amount of space. The particles are more likely to collide and therefore more likely to react.
Pressure - The gas particles become closer together, increasing the frequency of collisions and the likelihood of a reaction.
Catalyst - Catalysts are substances that change the rate of a reaction without being used up in the reaction. Catalysts provide an alternative pathway with a lower activation energy.
Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils).
Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).
Catalysts reduce the need for high temperatures, saving fuel and reducing pollution.
Understand that the rate of reaction changes depending on how far through the reaction is
Understand the term ‘activation energy’ and be able to use it correctly in explaining rate of reaction
Be able to explain using particle theory why the rate of reaction changes as a reaction proceeds
Be able to take measurements from a graph and calculate the rate of reaction at a particular point
Be able to explain why temperature effects the rate of reaction, using particle theory
Be able to explain why pressure effects the rate of reaction, using particle theory
Be able to explain why concentration effects the rate of reaction, using particle theory
Be able to explain why surface area effects the rate of reaction, using particle theory