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The Equilibrium Constant, Kc (AQA A Level Chemistry)

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Stewart Hird

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The Equilibrium Constant, Kc

Equilibrium expression & constant

  • The equilibrium constant expression is an expression that links the equilibrium constant, K, 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

  • The corresponding equilibrium constant expression is written as:

bold italic K bold space bold equals bold space fraction numerator stretchy left square bracket C stretchy right square bracket to the power of bold c stretchy left square bracket D stretchy right square bracket to the power of bold d over denominator stretchy left square bracket A stretchy right square bracket to the power of bold a stretchy left square bracket B stretchy right square bracket to the power of bold b end fraction

  • Where:

    • [A] and [B] = equilibrium reactant concentrations (mol dm-3)

    • [C] and [D] = equilibrium product concentrations (mol dm-3)

    • a, b, c and d = number of moles of corresponding reactants and products

  • Solids are ignored in equilibrium constant expressions

  • The equilibrium constant, K, of a reaction is specific to a given equation

Worked Example

Deducing equilibrium expressions

Deduce the equilibrium constant expression for the following reactions

  1. Ag+ (aq) + Fe2+ (aq) ⇌ Ag (s) + Fe3+ (aq)

  2. N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

  3. 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

Answer 1:

   K space equals fraction numerator open square brackets Fe to the power of 3 plus end exponent open parentheses aq close parentheses close square brackets over denominator open square brackets Fe to the power of 2 plus end exponent space open parentheses aq close parentheses close square brackets space open square brackets Ag to the power of plus space open parentheses aq close parentheses close square brackets end fraction

  • [Ag (s)] is not included in the equilibrium constant expression as it is a solid

Answer 2:

   K space equals fraction numerator open square brackets NH subscript 3 space open parentheses straight g close parentheses close square brackets over denominator stretchy left square bracket straight N subscript 2 space open parentheses straight g close parentheses stretchy right square bracket space stretchy left square bracket straight H subscript 2 space open parentheses straight g close parentheses stretchy right square bracket cubed end fraction

Answer 3: 

   K space equals fraction numerator stretchy left square bracket SO subscript 3 space open parentheses straight g close parentheses stretchy right square bracket squared over denominator stretchy left square bracket SO subscript 2 space stretchy left parenthesis straight g stretchy right parenthesis stretchy right square bracket squared space stretchy left square bracket straight O subscript 2 space stretchy left parenthesis straight g stretchy right parenthesis stretchy right square bracket end fraction

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    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    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.