A LevelChemistryAQARevision NotesAdvanced Physical ChemistryEquilibrium Constant (Kp) for Homogeneous SystemsKp Expressions

Kp Expressions (AQA A Level Chemistry): Revision Note

Exam code: 7405

Stewart Hird

Written by: Stewart Hird

Reviewed by: Philippa Platt

Updated on

Kp Expressions

  • 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 a given reaction:

aA + bB ⇌ cC + dD

K subscript straight c equals fraction numerator open square brackets straight C close square brackets to the power of straight c space open square brackets straight D close square brackets to the power of straight d over denominator open square brackets straight A close square brackets to the power of straight a space open square brackets straight B close square brackets to the power of straight b end fraction

Gaseous Equilibria

  • If all the substances in the general equation above are gases:

aA (g) + bB (g) ⇌ cC (g) + dD (g)

  • The equilibrium constant Kp is deduced from a gaseous reversible reaction equation

  • Kp is defined as:

Error converting from MathML to accessible text.

  • Where:

    • a, b, c and d are the respective number of moles of each reactant and product

    • paA (g) and pbB (g) are the equilibrium partial pressures of A and B, kPa

    • pcC (g) and pdD (g) are the equilibrium partial pressures of C and D, in kPa

  • Equilibrium partial pressure is calculated from  the:

    • The total number of moles

    • The mole fraction

    • The total pressure

  • 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 Tips and Tricks

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:

  1. N2 (g) + 3H2 (g) size 16px rightwards harpoon over leftwards harpoon 2NH3 (g)

  2. N2O4 (g) size 16px rightwards harpoon over leftwards harpoon 2NO2 (g)

  3. 2SO2 (g) + O2 (g) size 16px rightwards harpoon over leftwards harpoon 2SO3 (g)

Answers:

  1. N2 (g) + 3H2 (g) rightwards harpoon over leftwards harpoon 2NH3 (g)

    • Kp = fraction numerator p squared space NH subscript 3 over denominator p cubed space straight H subscript 2 space cross times space p space straight N subscript 2 end fraction

    • Units = fraction numerator kPa squared over denominator kPa cubed cross times kPa end fraction = kPa-2

  2. N2O4 (g) rightwards harpoon over leftwards harpoon 2NO2 (g)

    • Kp = fraction numerator p squared space N O subscript 2 over denominator p space straight N subscript 2 straight O subscript 4 end fraction

    • Units = kPa squared over kPa = kPa

  3. 2SO2 (g) + O2 (g) rightwards harpoon over leftwards harpoon 2SO3 (g)

    • Kp = fraction numerator p squared space SO subscript 3 over denominator p squared space SO subscript 2 space cross times space p space straight O subscript 2 end fraction

    • Units = fraction numerator kPa squared over denominator kPa squared cross times kPa end fraction = kPa-1

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener