The partial pressure of the various components in an equilibrium system can be calculated IF Kp is known as well as the initial pressures of all components are known.
Relating the change in one component in the equlibrium mixture to changes in other components
Construct a table that has a column for each component appearing in the equilibrium constant expression (reaction quotient). The table should have three rows (initial, change, equilibrium).
Consider the partially completed table for a system where a known amount of NO2 is put into vessel which is then closed, and reaction occurs to reach equilibrium.
Kp is given as well as the initial pressures of both components of the equilibrium.
The table shows that the change in both components can be expressed in terms of the unknown x.
Deducing expressions for the equilibrium concentrations in terms of x and the known initial pressures
Calculating the change
The magnitude of x can be calculated by substituting the expressions into the reaction quotient expression, setting it equal to the known Kp and solving for x. The method used for solving for x depends on the form of the equation in x.
This method depends on the fact that the changes in the amount in moles of the all components due to the reaction occurring to reach equilibrium can be related to one another through the balanced equation for the reaction occurring to reach equilibrium.
| Q = | p(N2O4) |
| p(NO2)2 |
Construct a table that has a column for each component appearing in the equilibrium constant expression (reaction quotient). The table should have three rows (initial, change, equilibrium).
| p(NO2) |  | p(N2O4) | |
| initial/kPa | given | given | |
| change/kPa | -2x | +x | |
| equilibrium/kPa | given - 2x | given + x |
Kp is given as well as the initial pressures of both components of the equilibrium.
The table shows that the change in both components can be expressed in terms of the unknown x.
The ratio of the changes is the ratio of the coefficients in the balanced equation.
The sign of the change is negative for substances that are consumed in the reaction to reach equilibrium and positive for substances that are produced.
The sign of the change is negative for substances that are consumed in the reaction to reach equilibrium and positive for substances that are produced.
| equilibrium = change + initial |
| Kp = | given + x |
| (given - 2x)2 |
The magnitude of x can be calculated by substituting the expressions into the reaction quotient expression, setting it equal to the known Kp and solving for x. The method used for solving for x depends on the form of the equation in x.