Consider the following reactions, which is an important atmospheric reaction. As the reaction progresses, sulfur trioxide is formed and oxygen and sulfur dioxide are consumed. The pressure of the system is the sum of the partial pressures of the components of the gas, and each of these partial pressures differs from the initial values. A useful concept for this purpose is the extent of reaction , which will be given the symbol x for this experiment. In reality the reaction occurs on the molecular scale, with individual molecules of oxygen, sulfur dioxide, and sulfur trioxide being involved.
Chemists prefer to think in terms of moles of material, however.
The sense of the change can be reversed. The extent of reaction allows the change in amount of one species to be related to the change in the amount of a different species. The most straightforward way to illustrate this relationship is through the use of a table such as that shown below. The equilibrium expression, however, requires partial pressures or molar concentrations. In this example the extent of reaction is expressed in terms of partial pressure.