Kc and its units for the Haber process

Worked example

Find the value and units of Kc for the Haber process, N₂ + 3H₂ ⇌ 2NH₃, given equilibrium concentrations [N₂] 1.10 mol dm⁻³, [H₂] 1.98 mol dm⁻³, [NH₃] 0.333 mol dm⁻³.

1. 1. Kc from equilibrium concentrations: solve for Kc = [NH₃]² / ([N₂][H₂]³)

   $K_\mathrm{c} = \prod_i [\mathrm{X}_i]^{\nu_i}$

   = 0.01299 mol⁻² dm⁶

Answer: Kc = 0.0130 mol⁻² dm⁶

Method

Kc = [NH₃]² ÷ ([N₂][H₂]³). Units: (mol dm⁻³)² ÷ (mol dm⁻³)⁴ = (mol dm⁻³)⁻² = mol⁻² dm⁶. In general the units are (mol dm⁻³)^Δn, with Δn = moles of gas on the right − on the left.

Common mistakes

• Products go on top: Kc = [NH₃]² ÷ ([N₂][H₂]³).
• Each concentration is raised to the power of its coefficient: [H₂]³ and [NH₃]².

Formulas used

• Kc from equilibrium concentrations

  $K_\mathrm{c} = \dfrac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}$

More equilibrium questions

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