## Section 2: Equilibrium Constant (Concentrations)

#### Concentration and Equilibrium

Now that we understand that the same amount of products and reactants are produced as have been used up in an equilibrium reaction, we can apply this information to a few more ideas. First, because we know that the amount of each substance is staying constant, we know that the number of moles are staying constant. We also know that the concentrations are staying the same (remember concentration = moles / L and moles and L are staying constant so the concentration must stay constant at equilibrium.)

#### Equilibrium Constant

Right now, we are going to explain what is called the equilibrium constant. We will not explain what it is used for until later in the unit, but trust me, it is important.

The history of the equilibrium constant began with Cata Guldberg and Peter Waase. They used variables to describe what normally happens in an equilibrium reaction. Their reaction is shown below. (Please note that <=====> is used in place of ------> to show that the reaction has reached equilibrium.)

j A + k B <=====> l C + m D

In english, the react is A and B react to form C and D and because it is at equilibrium C and D react at the same rate to produce A and B. The letters j, k, l, and m are the coefficients when the rection is balanced.

Now you are saying to yourself, that's nice, but where is the equilibrium constant in that whole mess. Well, the equilibrium constant is defined as:

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

K (the equilibrium constant) is equal to the concentration of C to the l power times the concentration of D to the m power divided by the quantity of the concentration of A to the j power times the concentrion of B to the k power. (We would like to note that the only substances that get put in to the equilibrium constant are aqueous solutions and gases.) For example, 2 H2 + O2 -----> 2 H2O, has an equilibrium constant of:

K = 1 / [H2]2 [O2]