Partial Pressure
- Mole fractions indicate the fraction of a mixture occupied by a particular gas.
- 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)
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.