Chapter 8: Unit 4. Equilibrium Constant

Equilibrium Constant

The Equilibrium Constant is defined as the ratio of the products (raised to their corresponding number of moles) to the reactants (raised to their corresponding number of moles).

An example of calculating the equilibrium constant is given below:

2 NO2 (g)⇔ N2O4 (g)

Keq = Equilibrium Constant = [ N2O4 (g) ] / [ NO2 (g) ]2

Where [ N2O4 (g) ] is the equilibrium concentration of N2O4 (g) and [ NO2 (g) ] is the equilibrium constant of [ NO2 (g) ].

Keq = Kc when the concentrations of reactants and products are used in mole/Liter
Keq = Kp when the pressures of the reactants and products are used in Pascal (unit of the pressure in SI system) or in atm (unit of pressure in the Metric system). In this case both the reactants and products should be gases.

If [ N2O4 (g) ] and [ NO2 (g) ] are known, then the equilibrium constant Keq can be calculated.

When setting up the formula of Keq , one has to discard the liquids and the solids among the
reactants and the products. Gaseous and aqueous reactants and products are the only ones
considered for Keq expression.

Example:

PCl5 (s) + H2O (g)⇔ 2HCl (g) + POCl3 (g)

Keq = { [ HCl (g) ]2 [ POCl3 (g) ] } / { [ H2O (g) ] }

PCl5 (s) is solid and hence it is discarded from Keq expression.

The video below illustrates the equilibrium constant calculations.

The State City of New York (SUNY) simulation illustrates how to set up the expression of Keq.

http://employees.oneonta.edu/viningwj/sims/equilibrium_expressions_t.html

Finish all seven questions given in the simulation.