Kp and its units for the Haber process

Worked example

Find Kp, including units, for N₂ + 3H₂ ⇌ 2NH₃ when the equilibrium partial pressures are N₂ 4760 kPa, H₂ 12700 kPa and NH₃ 1820 kPa.

1. 1. Kp from equilibrium partial pressures: solve for Kp = p(NH₃)² / (p(N₂) p(H₂)³)

   $K_\mathrm{p} = \prod_i p_{\mathrm{X}_i}^{\,\nu_i}$

   = 3.397 × 10⁻¹⁰ kPa⁻²

Answer: Kp = 3.40 × 10⁻¹⁰ kPa⁻²

Method

Kp = p(NH₃)² ÷ (p(N₂) p(H₂)³). Units: kPa² ÷ kPa⁴ = kPa⁻². The number depends on the pressure unit: in Pa it would be 10⁶ times smaller.

Common mistakes

• Products go on top: Kp = p(NH₃)² ÷ (p(N₂) × p(H₂)³).

Formulas used

• Kp from equilibrium partial pressures

  $K_\mathrm{p} = \dfrac{p_\mathrm{C}^{\,c}\,p_\mathrm{D}^{\,d}}{p_\mathrm{A}^{\,a}\,p_\mathrm{B}^{\,b}}$

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