Chemical Equilibrium

Learn about the basics of chemical equilibrium, including how to write the expression for chemical equilibrium and the factors that affect it.

Chemical equilibrium is the condition which occurs when the concentration of reactants and products participating in a chemical reaction exhibit no net change over time. Chemical equilibrium may also be called a "steady state reaction." This does not mean the chemical reaction has necessarily stopped occurring, but that the consumption and formation of substances have reached a balanced condition. The quantities of reactants and products have achieved a constant ratio, but they are almost never equal. There may be much more product or much more reactant.

Dynamic equilibrium occurs when the chemical reaction continues to proceed, but a number of products and reactants remain constant. This is one type of chemical equilibrium.

The equilibrium expression for a chemical reaction may be expressed in terms of the concentration of the products and reactants. Only chemical species in the aqueous and gaseous phases are included in the equilibrium expression because the concentrations of liquids and solids does not change. For the chemical reaction:

jA + kB → lC + mD

The equilibrium expression is

K = ([C]^l[D]^m) / ([A]^j[B]^k)

K is the equilibrium constant
[A], [B], [C], [D] etc. are the molar concentrations of A, B, C, D etc.
j, k, l, m etc. are coefficients in a balanced chemical equation

First, consider a factor that does not affect equilibrium: pure substances. If a pure liquid or solid is involved in equilibrium, it is considered to have an equilibrium constant of 1 and is excluded from the equilibrium constant. For example, except in highly concentrated solutions, pure water is considered to have an activity of 1. Another example is solid carbon, which may be form by the reaction of two carbom monoxide molecules to form carbon dioxide and carbon.

Factors that do affect equilibrium include:

• Adding reactant or product or a change in concentration affects equilibrium. Adding reactant can drive equilibrium to the right in a chemical equation, where more product forms. Adding product can drive equilibrium to the left, as more reactant forms.
• Changing the temperature alters equilibrium. Increasing temperature always shifts chemical equilibrium in the direction of the endothermic reaction. Decreasing temperature always shifts equilibrium in the direction of the exothermic reaction.
• Changing the pressure affects equilibrium. For example, decreasing the volume of a gas system increases its pressure, which increases the concentration of both reactants and products. The net reaction will see to lower the concentration of gas molecules.

Le Chatelier's principle may be used to predict the shift in equilibrium resulting from applying a stress to the system. Le Chatelier's principle states that a change to a system in equilibrium will cause a predictable shift in equilibrium to counteract the change. For example, adding heat to a system favors the direction of the endothermic reaction because this will act to reduce the amount of heat.