When writing the equilibrium expression for a reaction, remember that the products are on the top of the expression and the reactants are on the bottom. Any coefficients in the balanced reaction become the exponents in the equilibrium constant.

A general formula and corresponding equilibrium expression are given here:

Our equation follows the same format. Note, however, that pure solids and liquids are not included in the equilibrium expression, but pure gases (such as hydrogen) are. This is because the pressure of pure gases can affect equilibrium, but manipulating pure solids or liquids cannot.

When writing the equilibrium expression for a reaction, remember that the products are on the top of the expression and the reactants are on the bottom. Any coefficients in the balanced reaction become the exponents in the equilibrium constant.

A general formula and corresponding equilibrium expression are given here:

Our equation has only one product, but follows the same format.

Note that pure solids and liquids are not included in the equilibrium expression, but pure gases (such as nitrogen) are. This is because the pressure of pure gases can affect equilibrium, but manipulating pure solids or liquids cannot.

The general formula for the equilibrium constant of a reaction is:

For gaseous reactions, however, partial pressure can be substituted for concentration. For the given reaction, this means our equilibrium constant will be equal to the partial pressure of the products divided by the partial pressure of the reactants.

The equilibrium constant gives the ratio of product concentration to reactant concentration that results in equal forward and reverse rates of reaction. Essentially, the equilibrium constant defines the parameters of equilibrium for a given reaction. Altering the concentration of reactants or products will change the state of the reaction, driving it to produce more reactants or products in order to accommodate the change (Le Chatelier's principle), but will not alter the equilibrium parameters. Adding an enzyme will increase the rate of reaction by decreasing the activation energy, but will still not affect the equilibrium ratios.

The main factor that is capable of altering the equilibrium constant is change in temperature, which alters the amount of energy available to the system and affects the balance of equilibrium.

When creating the solubility product expression for a salt, keep in mind that salts are solids, and are therefore not included in the expression. Also, the coefficients in the balanced reaction become exponents in the expression.

For a general reaction, the solubility product constant would be given as follows:

Aluminum hydroxide will dissociate based on the following balanced reaction:

As a result, the solubility product expression is:

The correct answer gives the accurate definition of both the equilibrium constant and the reaction quotient. 

This is one of the properties of the law of mass action. The equilbrium constant for a reaction written in reverse is 1/K.

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.

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: