Chemistry 112

Quantitative Use of Equilibrium Constants

If an equilibrium constant is known for a reaction, then the equilibrium concentrations of all components in the reaction can be calculated if the initial conditions are known.

The strategy to do this is as follows:

  1. Write the properly balanced chemical reaction. This gives the stoichiometry and the species involved in the mass action expression.

  2. Write out the mass action expression for the equilibrium constant.

  3. Set up a Table of Concentrations for all the components that show up in the mass action expression. Use the reaction stoichiometry to express all unknown concentrations in terms of a single variable.

  4. Substitute the expressions for the equilibrium concentrations into the mass action expression.

  5. Solve the equation created in step 4 for the variable.

  6. Substitute the value of the variable into the expressions for the equilibrium concentrations.



Example

The decomposition of bromine monochloride to bromine and chlorine has an equilibrium constant, Kc, of 0.14 at 350 K. If the initial concentration of bromine monochloride is 0.062 M, what are the equilibrium concentrations of all components?

Example

Carbon monoxide reacts with water at 1000 oC to give carbon dioxide and hydrogen with Kc = 0.58. A reaction was started with the following composition: carbon monoxide, 1.00 M; hydrogen 1.20 M; water, 0.50 M, and carbon dioxide, 0.20 M. What are the equilibrium concentrations of all components?