# Equilibrium in physical and chemical process

This page explains equilibrium constants expressed in terms of partial pressures of gases, Kp. It covers an explanation of the terms mole fraction and partial pressure, and looks at K_{p} for both homogeneous and heterogeneous reactions involving gases.The page assumes that you are already familiar with the concept of an equilibrium constant, and that you know about K_{c} - an equilibrium constant expressed in terms of concentrations

Before we can go any further, there are two terms relating to mixtures of gases that you need to be familiar with.

#### Mole fraction

If you have a mixture of gases (A, B, C, etc), then the mole fraction of gas A is worked out by dividing the number of moles of A by the total number of moles of gas.

The mole fraction of gas A is often given the symbol xA. The mole fraction of gas B would be xB - and so on.

X_{A} = ^{Number Of Moles of Gas A } / _{Total Number of Mole of Gas}

**For example,** in a mixture of 1 mole of nitrogen and 3 moles of hydrogen, there are a total of 4 moles of gas. The mole fraction of nitrogen is 1/4 (0.25) and of hydrogen is 3/4 (0.75).

#### Partial pressure

The partial pressure of one of the gases in a mixture is the pressure which it would exert if it alone occupied the whole container. The partial pressure of gas A is often given the symbol PA. The partial pressure of gas B would be PB - and so on. There are two important relationships involving partial pressures. The first is again fairly obvious. The total pressure of a mixture of gases is equal to the sum of the partial pressures.

**Total Pressure P = P _{A}+P_{B}+P_{C}+......**