Kc & Kp Expressions
- As previously discussed, the equilibrium expression links the equilibrium constant, Kc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
- So, for the general reaction
aA + bB ⇌ cC + dD
Kc = 
Gaseous Equilibria
- This section covers how we manage gases in calculating the equilibrium constant and how an equilibrium yield is affected by the partial pressures of reactants and products
- Many industrial process involve reactions between gases so this application has important consequences for controlling reaction conditions
- In the generic example above, if all the substances are gases, we can show the equation with that state symbol
aA (g) + bB (g) ⇌ cC (g) + dD (g)
- We can write a different equilibrium expression in terms of the partial pressure of the gases
- This equilibrium constant is called Kp and is defined as follows
Equilibrium expression linking the partial pressures of reactants and products at equilibrium
- Solids and liquids are ignored in Kp equilibrium expressions
- The Kp of a reaction is constant and only changes if the temperature of the reaction changes
Examiner Tip
There are a variety of ways to represent the partial pressure terms in a Kp expression
The only key point is do not use square brackets as these represent concentration and, therefore, imply a KC expression
Worked example
Write a Kp expression for the following equilibria and deduce the units of Kp :
- N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
- 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
Answer 1
Answer 2
