In this section equilibrium pressures are calculated from known
initial pressures of all components and the
known equilibrium constant for the reaction.
The general method used in solving such problems is outlined below:
- Write the expression for Q that applies to the given system.
- Construct a table having rows labelled initial, change and final (equilibrium) with a column for each component appearing in Q.
- Enter given data. For components having no "initial pressure" mentioned in the problem, enter 0.
- Take x as the change in concentration of one substance. Express the changes in the other substances appearing in Q in terms of x.
- Obtain expressions for equilibrium pressures in terms of x.
- Divide each equilibrium pressure expression by the standard state pressure (100 kPa if in kPa or 1 atm if in atmospheres) and substitute into Qp.
The solution for x depends on the form of the resulting equation. In this section you will see three examples of different forms. The maths can get quite complicated. Don't worry about this. It is the chemistry in steps 1-6 that should be emphasized.