The magnitude of K can be calculated if the initial concentrations of all components in solution AND the equilibrium concentration of only one component is known.
This method depends on the fact that the change in concentration of the one component due to the reaction occurring to reach equilibrium can be related to the change in all other components through the stoichiometry of the reaction.
Calculating equilibrium constants from given initial amounts and one equlibrium amount
Construct a table having
First enter given data
Initial amounts are assumed to be zero if not present before reaction.
Then calculate one change from the given data and deduce other changes from it
Calculating the equilibrium amounts of other substances
Once x is known, 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 concentration of the one component due to the reaction occurring to reach equilibrium can be related to the change in all other components through the stoichiometry of the reaction.
Consider a system where the intial concentration of Cr2O72– is known.
Reaction with water occurs according to the equation given.
The equilibrium concentraton of Cr2O72– is measured.
These data and the equilibrium expression can be used to calculate Kc for the reaction.
Why is there no [H2O] in the K expression? The H2O reactant is also the solvent in this reaction. Thus H2O is present in large excess and reaction to restore/reach equilibrium does not change the amount of water present to a significant extent. Therefore [H2O] is constant.
Reaction with water occurs according to the equation given.
The equilibrium concentraton of Cr2O72– is measured.
These data and the equilibrium expression can be used to calculate Kc for the reaction.
Cr2O72–(aq) + H2O  2H+(aq) + 2CrO42–(aq)K = [CrO42–]2[H+]2 [Cr2O72–] |
Why is there no [H2O] in the K expression? The H2O reactant is also the solvent in this reaction. Thus H2O is present in large excess and reaction to restore/reach equilibrium does not change the amount of water present to a significant extent. Therefore [H2O] is constant.
Calculating equilibrium constants from given initial amounts and one equlibrium amount
Construct a table having
four rows (reaction, initial, change, equilibrium).
a column for each component in the equilibrium constant expression.
a column for each component in the equilibrium constant expression.
First enter given data
Initial amounts are assumed to be zero if not present before reaction.
| Cr2O72–(aq) | 2H+(aq) | CrO42–(aq) | |||
| initial/ | given | given | given | ||
| change/ | -x | +2x | +x | ||
| equilibrium/ | given |
Then calculate one change from the given data and deduce other changes from it
The ratio of the changes is the ratio of coefficients in the balanced equation.
The sign of a change is negative for substances consumed in reaction to reach equilibrium.
The sign of a change is negative for substances consumed in reaction to reach equilibrium.
Calculating the equilibrium amounts of other substances
| equilibrium = change + initial |
Once x is known, the amount in moles of all substances present at equilibrium can be calculated from their initial amount and their known change.