The Partial pressure reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Partial pressure

The partial pressure of a gas in a mixture is a measure of thermodynamic activity of gas molecules.

The pressure a gas exerts is proportional to the temperature and the concentration of the gas.

Table of contents

1 John Dalton's law of partial pressures 2 Fluid phase vs. gas phase 2.1 Henry's law 3 Partial pressure vs. concentration

John Dalton's law of partial pressures

The pressure of a gas in a mixture is equal to the pressure it would exert if it occupied the same volume alone at the same temperature. This is because gas molecules are so far apart that they don't interfere with eachother.

A consequence of this is that the total pressure of a mixture at equilibrium is equal to the sum of partial pressures of the gases present. For example, given the reaction:

N₂ + 3H₂ ↔ 2NH₃

The total pressure is equal to the sum of the individual partial pressures of the gases in the mixture:

P = P_N2 + P_H2 + P_NH3

Where P is the total pressure of the mixture and P_x denotes the partial pressure of x.

If the volume of the container is decreased the total pressure of the gases increases. Because the reaction is reversible, the equilibrium position shifts to the side of the reaction with the least moless (in this case the product side, on the right). By Le Chatelier's Principle, this has the effect of increasing the fraction of the whole pressure available to the products, and decreasing the fraction available to the reactants (because there is more moles of reactant than product). The composition of the gases change so more ammonia is present. Similarly, changing the temperature of the system causes more reactants to be produced (because the reverse reaction is endothermic.)

The partial pressure of a gas is proportional to its mole fraction, which is a measure of concentration. This means that it is possible to work out the equilibrium constant for an equilibrium reaction involving a mixture of gases given the partial pressure of each and the chemical formula for the reaction. The equlibrium constant for gases is denoted K_P. For a reaction:

aA + bB ↔ cC + dD

So the equilibrium constant, K_P can be calculated with,

Although the composition of the gases change when the container is compressed, the equilibrium remains the same (assuming the temperature also remains constant).

Fluid phase vs. gas phase

Partial pressure in the fluid is equal to that in the gas with which the fluid is in equilibrium.

When a liquid is exposed to a gas, molecules of the gas will dissolve in the liquid.

Henry's law

Henry's law can be used to determine the partial pressure of a gas in a fluid.

Partial pressure vs. concentration

Gasses dissolve, diffuse, and react according to their partial pressures, not necessarily according to their concentrations.