Using stoichiometry to deduce equilibrium amounts
The magnitude of Kp can calculated from the initial pressures of all components are known AND the equilibrium pressure of only one component.
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).
The table shows that the change in both components can be expressed in terms of the unknown x.
Calculating the change
The magnitude of x can be calculated using the two known amounts in moles for one substance and the relationship shown.
Calculating the equilibrium amounts of other substances Thus the amount in moles of all substances present at equilibrium can be calculated from their initial amount and their known change.
This method depends on the fact that the change in the amount in moles of the one component due to the reaction occurring to reach equilibrium can be related to the change in all components through the stoichiometry of the reaction.
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.
The total pressure at equilibrium and the amount in moles of NO2 present are given.
| p(NO2) |  | p(N2O4) | |
| initial/mol | given | given data | |
| change/mol | -2x | +x | |
| equilibrium/mol | given | deduced |
The total pressure at equilibrium and the amount in moles of NO2 present are given.
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.
Calculating the change
The magnitude of x can be calculated using the two known amounts in moles for one substance and the relationship shown.
| change = equilibrium - initial |
Calculating the equilibrium amounts of other substances Thus the amount in moles of all substances present at equilibrium can be calculated from their initial amount and their known change.
| equilibrium = change + initial |