Partial Pressure (AQA A-Level Chemistry): Revision Notes
5.3.2 Partial Pressure
What is Partial Pressure?
The partial pressure of a gas in a mixture is the pressure that each individual gas contributes to the total pressure of the gas mixture. It depends on the mole fraction of that gas in the mixture and the total pressure.
Deriving Partial Pressure from Mole Fraction and Total Pressure
Mole Fraction ():
The mole fraction of a gas in a mixture is the ratio of the moles of to the total moles of all gases in the mixture:
Partial Pressure ():
The partial pressure of gas , denoted , is calculated by multiplying its mole fraction by the total pressure of the gas mixture:
This relationship can be applied to each component gas in the mixture. The sum of the partial pressures of all gases in the mixture equals the total pressure.
Example: Calculating Partial Pressures Suppose a mixture contains gases , , and with mole fractions , , and , and the total pressure is 100 kPa.
To find the partial pressures:
Calculating Partial Pressures at Equilibrium Using I.C.E Tables
In equilibrium problems, an I.C.E (Initial, Change, Equilibrium) table is often useful to organise the moles of each species throughout the reaction stages.
Example Calculation with I.C.E Table For the reaction:
Suppose 1.00 mol of and 1.00 mol of are initially present, and at equilibrium, there is 0.824 mol of
| Species | Initial Moles | Change in Moles | Equilibrium Moles |
|---|---|---|---|
| 1.00 | -0.412 | 1.00 • 0.412 = 0.588 | |
| 1.00 | -0.412 | 1.00 • 0.412 = 0.588 | |
| 0.00 | +0.824 | 0.00 + 0.824 = 0.824 |
Explanation of Each Column
- Initial Moles: Moles of each species before the reaction starts.
- Change in Moles: The change in moles due to the reaction proceeding to equilibrium. Since 0.824 mol of is formed, half of this amount (0.412 mol) of and reacts due to their 1:1 stoicheiometry with
- Equilibrium Moles: Calculated by adding the change to the initial amounts.