A-Level Chemistry May/June 2024 Q2(c)(i): Write an expression for the equilibrium constant, Kp, for reaction 1. State the units.…

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  For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g):

  The expression for Kp is the ratio of the partial pressures of the products to the reactants, with each raised to the power of its stoichiometric coefficient.

  $K_p = \frac{(p_{NH_3})^2}{(p_{N_2})(p_{H_2})^3}$

  To find the units, substitute the unit of pressure (atm) into the expression:

  Units = $\frac{(atm)^2}{(atm)(atm)^3} = \frac{atm^2}{atm^4} = atm^{-2}$

  Kp = $\frac{(p_{NH_3})^2}{(p_{N_2})(p_{H_2})^3}$

  units = $atm^{-2}$

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  How the marks are awarded

  • M1 — Awarded for the correct Kp expression, showing the partial pressure of the product (ammonia) squared in the numerator, and the partial pressures of the reactants (nitrogen and hydrogen) in the denominator, with hydrogen's partial pressure raised to the power of 3.
  • M2 — Awarded for the correct units of Kp. This is derived by cancelling the pressure units from the Kp expression: (atm)² / (atm × atm³) = atm⁻².

  Common mistakes

  • Inverting the Kp expression by placing reactants in the numerator and products in the denominator.
  • Forgetting to raise the partial pressures to the power of their stoichiometric coefficients, e.g., writing (pH₂) instead of (pH₂)³.
  • Using square brackets [ ] for concentration, which would be the expression for Kc, not Kp.
  • Incorrectly calculating the final index for the units, for example, stating atm² or atm⁴ instead of atm⁻².

  Examiner tip: For any equilibrium constant expression, remember the rule 'products over reactants', with each species raised to the power of its stoichiometric coefficient from the balanced equation.

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