A-Level Chemistry OCR Notes

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Partial Pressure

Partial pressure is the pressure each gas in a mixture would exert on its own. The partial pressure of a gas A is denoted by p (A) or PA .
The total pressure of a gas mixture is the sum of all partial pressures from each gas.
The amount of pressure a gas exerts in a fixed volume depends on how many particles there are – more particles means greater pressure.

p (A) = mole fraction of gas A x total pressure
p (A) partial pressure of gas A (kPa)
mole fraction of gas A (mol)
total pressure (kPa)

Equilibrium Constant, Kc

The equilibrium constant, Kc , indicates where the equilibrium lies – it is the ratio of the concentration of products and reactants in a reversible reaction
The concentration, in mol dm ^-3 , of a species X involved in the expression for Kc is represented by [X]
For the general reaction: aA + bB ⇌ dD + eE

The units of the equilibrium constant will vary depending on how many species take part in the reaction and the stoichiometry
The magnitude of Kc indicates the extent of the reaction:
- Kc = 1: equilibrium lies halfway between the reactants and the products
- Kc > 1: equilibrium lies further to the right and the products are favoured
- Kc < 1: equilibrium lies further to the left and the reactants are favoured

Equilibrium Constant, Kp

Kp is an equilibrium constant associated with equilibrium reactions, only involving gases. It is written in terms of partial pressures rather than concentrations.
For the equilibrium reaction:

The units of Kp are variable and depend on the specific reaction that is under consideration
Temperature can affect the position of equilibrium
- Increasing the temperature will cause the position of equilibrium to shift in the endothermic direction. Increasing Kp
- Decreasing the temperature will cause the position of equilibrium to shift in the exothermic direction. Decreasing Kp
Pressure can affect the position of equilibrium
- Increasing the pressure will shift the position of equilibrium to the side with less moles of gas
- Decreasing the pressure will shift the position of equilibrium to the side with more moles of gas
Changing pressure will not affect the value of Kp ; instead, the position of equilibrium will shift to keep Kp constant.
Catalysts do not affect the value of Kp. Instead, it affects how quickly equilibrium is reached.

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