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The Equilibrium Law (HL IB Chemistry)

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The Equilibrium Law

  • Equilibrium law explains how the equilibrium constant, K, can be found from the stoichiometry of the reaction

The equilibrium constant equation

  • 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 reactant 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

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

Exam Tip

  • You must use square brackets in equilibrium constant expressions as they have a specific meaning, representing concentrations
  • In an exam answer, you would lose the mark if you used round brackets.
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Caroline

Author: Caroline

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.