Explain why the equilibrium constant expression for the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) contains concentration terms raised to different powers, and state what happens to the numerical value of K if the equation is written in reverse as 2NH₃(g) ⇌ N₂(g) + 3H₂(g).
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The powers in the equilibrium constant expression are the stoichiometric coefficients from the balanced equation. For N₂(g)+3H₂(g)⇌2NH₃(g) the expression is K=[NH₃]²/([N₂][H₂]³). Products are in the numerator and reactants in the denominator. If the equation is reversed to 2NH₃(g)⇌N₂(g)+3H₂(g), the new constant K′ is the reciprocal of the original, K′=1/K, because the numerator and denominator are interchanged.
Examiner tips
- Write the balanced equation first, then state that each concentration is raised to its coefficient. Mention that products go in the numerator, reactants in the denominator. Explain that reversing the reaction swaps numerator and denominator, giving K′=1/K. Use the exact wording "reciprocal" to match the mark scheme.
Common mistakes
- Using the same power for all species instead of the stoichiometric coefficients.\nConfusing the direction of the reaction and writing K′=K instead of 1/K.\nForgetting to put products in the numerator and reactants in the denominator.
Mark scheme (4 marks)
- The equilibrium constant expression is K = [NH₃]² / ([N₂][H₂]³), where each concentration is raised to the power equal to its stoichiometric coefficient in the balanced equation.
- Products appear in the numerator and reactants in the denominator of the K expression.
- When the equation is reversed, the new equilibrium constant K' is the reciprocal of the original K (i.e. K' = 1/K).
- This is because reversing the equation swaps products and reactants, so the numerator and denominator of the K expression are interchanged.
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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