For any equilibrium expression, solids and liquids are excluded because their concentrations are presumed to be constant during the reaction. The equilibrium expression is defined as the ratio of the concentration of products divided by the concentration of the reactants. Each reactant or product is raised to the power corresponding to its coefficient in the balanced chemical equation.

Using an arbitrary example:

Compare this to our question. Remember to exclude the solid magnesium!

The equilibrium constant, , is the ratio of the concentration of products raised to their coefficients, over the concentration of reactants raised to their coefficients. To find its value, we first need to balance the equation and then consider only the products and reactants that actually have concentrations (i.e. aqueous and gaseous species). Liquids and solids can be omitted from the calculation.

We note that chlorine only appears once on the right, so we add a 2 coefficient to balance the equation: 

Since is a solid (most hydroxides precipitate, unless they are paired with alkali metals, barium, or calcium) it will not be included in the equilibrium constant calculation.

The chloride ion concentration is raised to the second power because of the 2 coefficient that we added. So, putting products (species on the right side, other than the solid) on the top and reactants (species on the left) on bottom, we get: 

The equilibrium constant is given by the concentraton of products over the concentration of reactants. If it lies to the right, it means that it favors the forward reaction, and thus the reaction is "more complete" or closer to completion.

When writing the equilibrium expression for an insoluble salt, remember that pure solids and liquids are not included in the expression. Also, the coefficients for the compounds in the balanced reaction become the exponents for the compounds in the equilibrium expression.

Given a generalized chemical reaction, we can determine the equilibrium constant expression.

In our reaction, the reactant is a pure solid and is not included in the equilibrium calculation.

The only factor that changes the equilibrium constant is temperature. Changes in concentration of reactants or products by any means (whether addition, taking away solvent, or adding a chemical that will cause side reactions) will remove the system from equilibrium, but will not change the equilibrium constant.

The only factor that changes the equilibrium constant is temperature. Changes in concentration of reactants or products by any means (whether addition, taking away solvent, or adding a chemical that will cause side reactions) will remove the system from equilibrium, but will not change the equilibrium constant.

By placing these initial concentrations in the equilibrium expression, we can compare the reaction quotient, , to the equilibrium constant, . 

The reaction quotient for this reaction is:

Note that the reactant is a pure solid, and will not be included in this expression. By setting this equal to the equilibrium constant, we can see whether  or is larger in value.

We can predict how a solution will change based on the value of . When  is less than , the reaction will shift to the right. If  is greater than , the reaction will shift to the left. If  is equal to , the solution is at equilibrium.

Since the value for  is greater than the equilibrium constant in this instance, the reaction will shift to the left.

The reaction quotient, or Q, of the above reaction is equal to the products over the reactants. Q is calculated in the same manner as K_eq, but does not require that the reaction be at equilibrium.

Le Chatelier's principle states that changes in pressure are attributable to changes in volume. If we increase the volume, the reaction will shift toward the side that has more moles of gas. If we decrease the volume, the reaction will shift toward the side that has less moles of gas. Since the product side has only two moles of gas, compared to the reactant side with four moles, the reaction would shift toward the product side, and more NH₃ would form.

Chemical equilibrium is defined as a state when the rates of the forward and reverse reactions are equal. Different reactions will have different concentrations of products are reactants, and concentration can vary depending on the process. Chemical equilibrium, however, is always the point at which there is no bias towards creating products or reactants.

If the forward reaction rate and reverse reaction rate are equal, then there is no net change in concentration of the reactants or products. This makes them appear to be stable.