Monitoring the rate of a reaction

The rate of a chemical reaction is a measure of how fast the reactants are being used up and how fast the products are being made.

Reactions in which a gas is produced can be used to monitor the rate.

For example, hydrogen gas is one of the products released when dilute hydrochloric acid reacts with zinc metal.

By collecting the hydrogen gas that is produced over water or in a syringe, rate graphs can be produced. The volume of gas produced and the time taken need to be recorded.

A stoppered conical flask contains a lump of zinc in dilute hydrochloric acid. Bubbles of hydrogen gas pass through the stopper, via tubing, into a gas syringe, where the volume produced is measured. In an alternative setup, the tubing from the flask leads into an upturned measuring cylinder full of water, which rests in a trough of water. The gas produced will displace water from the cylinder and its volume can be measured. — Figure caption,
Two different ways to measure the volume of a gas that is produced

The rate of the same reaction could be monitored by measuring the change in the mass of reactants as they react to form products.

If the reaction was set up on a balance as shown, the mass of the apparatus can be monitored and recorded at time intervals throughout the reaction. As hydrogen bubbles escape, the apparatus will lose mass.

A conical flask rests on a digital balance. The flask contains a lump of zinc in dilute hydrochloric acid. Bubbles of hydrogen gas are produced in the liquid and escape through the neck of the flask. The change in mass is measured on the balance's display.

Rate graphs

In chemistry, graphs can be used to follow the course of a reaction. A graph can tell us many things about a reaction.

The graph below shows two similar reactions.

Graph of total amount of product against time from start of reaction. The reaction is plotted twice, as line A and line B. Both lines level off at the same amount of product produced, but it takes less time for line A to achieve this. Line A represents a faster reaction and reflects higher temperature, higher concentration or small pieces. Line B represents a slower reaction and represents lower temperature, lower concentration or larger pieces.

The magenta line has a steeper gradient and represents conditions favouring a faster reaction than the green line. When the reaction is finished (the end-point) the graph goes flat as no more products are being produced.

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Rates of reactionMonitoring the rate of a reaction