Measuring Rates of Reactions

In order to study the rates of reactions, we need to be able to measure those rates. This raises the question of how we can tell how fast a reaction is. Rates can be measured by measuring any property that is changing as a result of the reaction. Some examples are:

Change in pressure—any reaction that is producing a gas will cause an increase in pressure. Rate can be determined by measuring rate of that increase (mm Hg/sec). Similarly, any reaction that uses up a gas will cause a measurable decrease in pressure.

Change in mass—a reaction where solids or liquids are producing a gas will cause an apparent loss of mass. If the reaction was run in an open container on a balance, rate could be determined by measuring the rate of “loss” of mass (g/sec).

Change in temperature—a reaction that produces (or takes in heat) will cause an increase (or decrease) of temperature in the surroundings. In this case, a reaction that releases heat could be run in a container that is surrounded by water. The rate could then be determined by measuring the rate of temperature change in the surrounding water ( ^oC/sec).

Change in color—a reaction that produces a color (or causes one to fade) can be used because the amount of light that passes through such a solution will decrease (or increase) over time. Thus, the rate could be determined by measuring the change in absorbance (absorbance/sec)