Lesson 3 of 12 · 8 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.