# The Equilibrium Constant Expression

### Equilibrium Constant

The equilibrium between reactants and products is described by an equilibrium constant. For the balanced reaction:
aA + bB cC + dD
The equilibrium constant, Keq is defined as:

```      [C]c [D]d
Keq = ---------
[A]a [B]b
```
where the [] brackets indicate the concentration of the chemical species.

For the example of water, H2O(l) H+(aq) + OH-(aq)
the equilibrium constant is:

```      [H+] [OH-]
Keq = ----------
[H2O]
```
The concentration of water in a water solution is constant and this expression simplifies to:
Kw = (55.56 M)*Keq = [H+] [OH-]
where Kw is called the dissociation constant of water and equals 1.00x10-14 at room temperature. The concentrations of [H+] and [OH-] therefore equal 1.00x10-7 M.

### Rules for Writing Keq Expressions

1. Products are always in the numerator.
2. Reactants are always in the denominator.
3. Express gas concentrations as partial pressure, P, and dissolved species in molar concentration, [].
4. The partial pressures or concentrations are raised to the power of the stoichiometric coefficient for the balanced reaction.
5. Leave out pure solids or liquids and any solvent.

Example:

Zn (s) + 2 H+(aq) Zn2+(aq) + H2 (g)

```       PH2 [Zn2+]
Keq = -----------
[H+]2
```
where PH2 is the partial pressure of H2, [Zn2+] and [H+] are the molar concentrations of Zn2+ and H+, respectively, and Zn (s) is left out of the Keq expression because it is a pure solid.

General Solution to Equilibria Problems
1. Introduction 2. Equilibrium Constant 3. Reaction Quotient 4. General Solution