Lesson 3 of 12 · 11 min

Homogeneous and heterogeneous equilibria; Kp

NCERT § "Homogeneous Equilibria"; § "Heterogeneous Equilibria"

Engineers at an ammonia plant don't read mol L⁻¹ off their gauges; they read bar. Can the same equilibrium be written in pressures?

The lesson in notes

In short

In a homogeneous equilibrium all species are in the same phase, such as all gases or all in one solution.

For gases, partial pressures can replace concentrations, giving Kp; the two are linked by Kp = Kc(RT)^Δn.

Δn = (moles of gaseous products) − (moles of gaseous reactants) from the balanced equation.

When pressure is in bar and concentration in mol L⁻¹, R = 0.0831 bar L mol⁻¹ K⁻¹.

If Δn = 0 (e.g. H₂ + I₂ ⇌ 2HI), Kp = Kc; for PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), Δn = +1 and Kp = Kc(RT).

In a heterogeneous equilibrium more than one phase is present, such as water in equilibrium with its vapour or CaCO₃(s) ⇌ CaO(s) + CO₂(g).

The concentrations of pure solids and pure liquids are constant and are left out of the K expression; for the decomposition of CaCO₃, Kp = p(CO₂).

Even though pure solids and liquids do not appear in K, they must be present for the heterogeneous equilibrium to exist.

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