# 5.3.1 Kp Expressions

### Kp Expressions

• We have seen previously that equilibrium reactions can be quantified by reference to an equilibrium expression and equilibrium constant
• The equilibrium expression links the equilibrium constantKc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
• So, for a given reaction:

aA + bB ⇌ cC + dD

Kc is defined as follows: Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium

• Solids are ignored in equilibrium expressions
• The Kc of a reaction is constant and only changes if the temperature of the reaction changes

#### 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

#### Worked Example

Write a Kp expression for the following equilibria and deduce the units of Kp :

1. N(g) + 3H(g) ⇌ 2NH(g)
2. 2SO(g) + O(g) ⇌ 2SO(g)  #### Exam Tip

You are only required to know how to write Kp expressions for homogeneous systems, that is, where all the substances present are gases.

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