Control of variables

We shall take the measurement of the order of reaction of magnesium metal with hydrochloric acid as our example. We decide to measure the rate using a gas volume method (we may change our minds later!).

We need to be careful to control the variables. There are a number of factors which affect rate of reaction. Some thought needs to be put in to ensure that only one variable is changing at a time. It is no good investigating the effect of concentration on rate of reaction if the temperature is changing erratically. This reaction is quite exothermic, and without proper control, the temperature can rise significantly during the reaction. For example, in an experiment using 0.5 M acid, the temperature rose from 22 ºC to 39 ºC. Using 1.0 M acid, the temperature change was from 23 ºC to 57 ºC. Any change in rate due to the doubling in concentration is going to be confused by the additional change in temperature. We can see the error that this temperature change introduces when we plot a concentration-time graph for this reaction with 0.5 M acid:

The half-life has decreased slightly. It is clearly not a zero order straight line graph, so we shall probably conclude that the reaction is first order and the slight decrease in half-lives is due to experimental error. Unfortunately, this reaction is second order. The increase in temperature during the reaction has increased the rate significantly. The second half-life would have actually been longer if the temperature had stayed constant. Poor control of variables has led us to the wrong answer!

We could try to control the temperature change by using a water bath. This may have some effect, but given the speed and exothermic nature of this reaction there is still a significant temperature change. By changing the experimental method to an initial rates method, in which a large excess of acid was used, I managed to limit the temperature change with all acid concentrations to 1 ºC or less. This is an acceptable error and reveals the correct order of reaction. Make sure you measure the before and after temperatures and make a note of them.

Agitation (shaking) of the reaction mixture and cleaning of the metal are other other possible variables you will have to consider.

Trial experiments can be done to ensure that temperature has been controlled and to establish a suitable rate of agitation.