AQA A Level Chemistry

Revision Notes

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:

Equilibria Equilibrium Expression, downloadable AS & A Level Chemistry revision notes

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

Kp Expressions, downloadable AS & A Level Chemistry revision notes

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)

Answer 1

Worked Example Answer 1, downloadable AS & A Level Chemistry revision notes

Answer 2

Worked Example Answer 2, downloadable AS & A Level Chemistry revision notes

 

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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