How is reaction rate related to concentration?

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Suppose that at any one time 1 in a million particles have enough energy to equal or exceed the activation energy. If you had 100 million particles, 100 of them would react. If you had 200 million particles in the same volume, 200 of them would now react. The rate of reaction has doubled by doubling the concentration.

Cases where changing the concentration doesn't affect the rate of the reaction

At first glance this seems very surprising!

Where a catalyst is already working as fast as it can

Suppose you are using a small amount of a solid catalyst in a reaction, and a high enough concentration of reactant in solution so that the catalyst surface was totally cluttered up with reacting particles.

Increasing the concentration of the solution even more can't have any effect because the catalyst is already working at its maximum capacity.

In certain multi-step reactions

This is the more important effect from an A' level point of view. Suppose you have a reaction which happens in a series of small steps. These steps are likely to have widely different rates - some fast, some slow.

For example, suppose two reactants A and B react together in these two stages:

The overall rate of the reaction is going to be governed by how fast A splits up to make X and Y. This is described as the rate determining step of the reaction.

If you increase the concentration of B, that will undoubtedly speed up the second step, but that makes hardly any difference to the overall rate. You can picture the second step as happening so fast already that as soon as any X is formed, it is immediately pounced on by B. That second reaction is already "waiting around" for the first one to happen.

As concentration of reactants increases, the rate of the reaction will increase. This is due to the increased numbers of reactant particles having more frequent collisions with each other. A greater frequency of effective collisions will increase the rate of a reaction.

Here is a video of an experiment which illustrates the change in the rate of a reaction when the concentration of reactants is changed.

How does temperature affect the rate of a chemical reaction? When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. This is collision theory. The two molecules will only react if they have enough energy. By heating the mixture, you will raise the energy levels of the molecules involved in the reaction. Increasing temperature means the molecules move faster. This is kinetic theory. If your reaction is between atoms rather than molecules you just substitute "atom" for "molecule" in your explanation.

How do catalysts affect the rate of a reaction? Catalysts speed up chemical reactions. Only very minute quantities of the catalyst are required to produce a dramatic change in the rate of the reaction. This is really because the reaction proceeds by a different pathway when the catalyst is present. Adding extra catalyst will make absolutely no difference. There is a whole page on this site devoted to catalysts.

How does concentration affect the rate of a reaction? Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is collision theory again. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react. If reaction is between a substance in solution and a solid, you just vary the concentration of the solution. The experiment is straightforward. If the reaction is between two solutions, you have a slight problem. Do you vary the concentration of one of the reactants or vary the concentration of both? You might find that the rate of reaction is limited by the concentration of the weaker solution, and increasing the concentration of the other makes no difference. What you need to do is fix the concentration of one of the reactants to excess. Now you can increase the concentration of the other solution to produce an increase in the rate of the reaction.

How does surface area affect a chemical reaction? If one of the reactants is a solid, the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.e. on the surface of the solid. So the larger the surface area of the solid, the faster the reaction will be. Smaller particles have a bigger surface area than larger particle for the same mass of solid. There is a simple way to visualize this. Take a loaf of bread and cut it into slices. Each time you cut a new slice, you get an extra surface onto which you can spread butter and jam. The thinner you cut the slices, the more slices you get and so the more butter and jam you can put on them. This is "Bread and Butter Theory". You should have come across the idea in your biology lessons. By chewing your food you increase the surface area so that digestion can go faster.

What affect does pressure have on the reaction between two gasses? You should already know that the atoms or molecules in a gas are very spread out. For the two chemicals to react, there must be collisions between their molecules. By increasing the pressure, you squeeze the molecules together so you will increase the frequency of collisions between them. This is collision theory again. In a diesel engine, compressing the gaseous mixture of air and diesel also increases the temperature enough to produce combustion. Increasing pressure also results in raising the temperature. It is not enough in a petrol engine to produce combustion, so petrol engines need a spark plug. When the petrol air mixture has been compressed, a spark from the plug ignites the mixture. In both cases the reaction (combustion) is very fast. This is because once the reaction has started, heat is produced and this will make it go even faster.

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