5.1.2 (d) Expressions for Kc & Kp for homo- & hetero-geneous equilibria

Syllabus

(d) expressions for K_c and K_p for homogeneous and heterogeneous equilibria

{Note: liquid and solid concentrations are constant and are omitted in heterogeneous Kc and Kp expressions.}

Any equilibrium is a balance between products and reactants.

But it's very rarely a 50:50 balance.

Equilibrium constants are a way of showing how far on either side the equilibrium lies.

You already learned about Kc in (3.2.3 (f,g) The Equilibrium Constant, Kc) but for gaseous equilibria another equilibrium constant can be used, K_p.

For the equilibrium: N_2(g) + 3H_2(g) ⇌ 2NH_3(g) we would write K_c = [NH₃]² /[N₂] x [H₂]³

In the same way we could write K_p = p²(NH₃₎) /p(N₂) x p³(H₂)

where p(N₂) = Partial Pressure of Nitrogen etc.

Unlike K_c, K_p is affected by Pressure.

If the equilibrium contains substances that are not gases then only the gases show up in the Kp.

eg.

CO_2(g) + H₂O_(l) ⇌ H₂CO_3(aq)

K_p = 1 /p(CO₂)

There is a relationship between the two equilibrium constants.

Fortunately, it's not on the syllabus.

So you don't have to learn it or use it.

You can just admire its beauty!