Chemicals react together when their particles collide with each other. To collide means to bump into each other or to hit each other. Some reactions are very slow and others very fast, but there are five factors that affect the rate of a reaction:
- surface area
The collision theory states that the more collisions between particles there are, the faster the reaction. The particles must be moving very fast and have lots of kinetic energy for collisions to occur.
'Kinetic' means 'moving'; it is easy to remember because it is about particles moving and colliding.
Let's now see how the five factors above affect the number of collisions between particles. You will notice it will be all about the number of collisions.
If the concentration of one or more of the reactants increases, the particles become more crowded. The diagram below shows the particles of two chemicals. The box on the right contains more red particles representing one of the chemicals about to react. This means that the concentration of the 'red' chemical is now higher, but the particles are still in the same space. This increases the probability of collisions and the rate of the reaction.
When the temperature increases, the particles have more energy; as an increase in temperature means more thermal energy (heat) is present. More heat energy gives the particles more kinetic energy. More kinetic energy means more movement, so the probability of collisions increases and so does the rate of the reaction.
The graph below shows the rate of the same reaction in different temperatures. Note that the blue line showing the total amount of product as the reaction at a higher temperature progresses is much steeper than the purple line.
When a solid chemical is broken down into smaller pieces - or even a powder - there are more particles that can react, as the diagram below shows. We say that a chemical in a powder form has more surface area than the same mass of the same chemical in a solid block form. An increased surface area allows for more collisions and the rate of the reaction increases.
Pressure (in gases)
An increase in pressure speeds up a reaction. It has the same effect as an increase in concentration. The way you increase pressure on a gas is by squeezing it into a smaller volume, but the mass remains the same. This results in the same number of particles moving about in a smaller volume, which increases the number of collisions and the rate of the reaction.
A catalyst is a chemical that speeds up a reaction without being used in it. Catalysts are specific to reactions, so the catalyst for one reaction would not work for another.