Quick Notes Rates of Reaction

  • How fast a reaction happens is called its rate of reaction and is measured with the units mol dm-3 s-1.
  • Rates of reaction can only be found experimentally by measuring a change in concentration of a reactant or product in a given amount of time.
  • Concentration of reactants, temperature of the reaction, the pressure of a (gaseous) system and the activation energy for a reaction all determine the rate of a reaction.
    • Kinetics is the study of reaction rates (how quickly reactions occur) and the factors that control the rate of a reaction.

Full Notes Rates of Reaction

In chemistry, kinetics is the study of how fast reactions happen. Anything that can alter the speed of a reaction has to be considered and at A-level this includes concentrations of reactants, the temperature and pressure (for gaseous systems) as well as accounting for the activation energy of a reaction.

It's only possible to find the rate of a reaction experimentally.

Think about a simple reaction:

 

A + B + C forms D + E products

 

This equation just tells us that one mole of A reacts with one mole of B and one mole of C, it tells us nothing about how fast the reaction can occur. If you add more data (for example, enthalpy changes) you may be able predict whether the reaction is fast or slow at a particular temperature, but only predict – you cannot actually know unless you carry out the reaction and measure how quickly it happens.

There are two ways we can measure how quickly a reaction is happening. We can measure how quickly reactants are used up, or we can measure how long it takes a certain amount of product to form.

In both cases, we are measuring a change in concentration in a given amount of time. This gives us the units mol dm-3 s-1.

Key Point: How fast a reaction happens is called its rate of reaction and is measured with the units mol dm-3 s-1.

There are various experimental methods for determining the rate of a reaction. However, all of them are just measuring how quickly a concentration changes in a given amount of time.