Rates of Reaction 3

When investigating rates of reaction, it is necessary to make a series of measurements over a period of time, for example, how much hydrogen is produced during the reaction of an acid with zinc. The experiment should be repeated several times and, after discarding any anomalous results, the readings averaged and plotted on a scatter graph with time along the horizontal axis. The line of best fit will usually be a curve, with the steepest gradient at the start indicating the fastest rate of reaction. Where the curve is horizontal, is shows that the reaction had finished. The conditions of the experiment can then be changed and the whole process repeated. Plotting the results on the same graph, using different colors, gives a quick and easy visual interpretation, from which you can write your conclusion.

When working out rates of reaction mathematically, as higher tier candidates are more likely to do, dividing the amount of reactant used (or product formed) by the time taken gives a valid rate. This is effectively the same as working out the gradient of a graph. If 15 cm³ of carbon dioxide were released in the first 20 seconds of a reaction, the rate would be 0.75 cm³/s. Later in the reaction, it may take 45 seconds to produce 10 cm³ of carbon dioxide, in which case the rate would have dropped to 0.22 cm³/s. This slowing down of the rate occurs because the concentration of one or both of the reactants changes during the reaction. Professional scientists usually refer to the initial rate of reaction, in other words the rate at the very start.

How would a decrease in temperature affect the rate of a reaction? What about surface area? Have a go at this quiz and test your knowledge of the factors which affect rates of reaction, such as temperature, surface area or pressure.