Rate of Reaction

The rate of reaction tells us how fast a reaction is happening. It depends on certain factors, like concentration of the reactants, temperature and whether a catalyst is present. On a graph, reaction rate is equal to the gradient - the steeper the line, the faster the rate of reaction.

Calculating rates of reaction

Worked example

Calculating reaction rates by drawing tangents

You might also be asked to work out the reaction rate from a graph showing the change in amount of reactant or product against time. If we have a straight line on our graph then we just need to calculate the gradient of that line by dividing the change on the y-axis by the change on the x-axis. However, if our graph is a curved line, this means that the rate is different at various points along the curve. In this case, we need to draw a tangent to the curve at a certain time point. To draw a tangent, you simply take your ruler and draw a line which touches the curve and has the same gradient as the line at that time point. You can then calculate the gradient of the tangent by working out the change in y over the change in x.

Collision theory

Just placing two different molecules together doesn’t necessarily mean a reaction will happen. In order for particles to react, they need to collide with enough energy. The minimum amount of energy needed for particles to react is called the activation energy.

If particles collide with enough energy they will react to form products - we call this a ‘successful collision’. The more successful collisions there are in a given time, the faster the rate of reaction. There are a number of factors which affect the number of collisions and therefore reaction rate.

Activation energy

There is an ‘energy barrier’ that molecules need to overcome in order to react. This barrier, or threshold, is known as the activation energy. A reaction with a high activation energy will be less likely to happen than one with low activation energy. Adding a catalyst speeds up the rate of reaction by lowering the activation energy. It does this by providing an alternative reaction pathway - a bit like showing you a short-cut from your house to the supermarket when usually you’d go the long way around. We can show how adding a catalyst lowers the activation energy on a reaction profile like the one below:

Factors which affect reaction rate

There are two ways in which the factors below may increase reaction rate:
1. By increasing the frequency of collisions (molecules bump into one another more often)
2. By giving the molecules more energy - this means that a higher number of molecules will have more energy than the activation energy so there will be more successful collisions

• Increasing the concentration of a solution or increasing the pressure of a gas means that the reacting particles are closer together (there are more particles in the same volume). The frequency of collisions between reactant particles increases, increasing reaction rate.

• Increasing the surface area : volume ratio of a reacting solid means that more reacting particles are exposed at the surface. The frequency of collisions between reactant particles increases, increasing reaction rate.

• Increasing the temperature of the reactants causes them to move more quickly and increases the energy of the particles. The frequency of collisions increases and the proportion of successful collisions increases (since more molecules will now have energy that is greater than the activation energy). Therefore reaction rate increases.

• Adding a catalyst increases the rate of reaction by providing an alternative pathway with a lower activation energy. Since the ‘energy barrier’ for the reaction to happen has been reduced, a larger number of particles will have sufficient energy to react and there will be more successful collisions. Catalysts are chemically unchanged at the end of the reaction and can be reused.

Next Page: Reversible Reactions