Write K for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

K = [NH₃]² / ([N₂][H₂]³)

Can you show a worked example?

Write K for CaCO₃(s) ⇌ CaO(s) + CO₂(g). K = [CO₂] (or Kp = P_CO₂).

Writing equilibrium expressions

Products over reactants, raised to coefficients — leave out solids and pure liquids.

The common mistake

Putting solids and pure liquids like CaCO₃(s) or H₂O(l) into the K expression.

Instead: Write products over reactants, each raised to its coefficient, but only include gases (aq concentrations or partial pressures). Solids and pure liquids have constant activity, so they're left out.

Worked example with Guide me

Write K for CaCO₃(s) ⇌ CaO(s) + CO₂(g).

Sugar · Which species are solids?
Student · CaCO₃ and CaO.
Sugar · What's left to include?
Student · Only CO₂, a gas.
Sugar · So what is K?
Student · K = [CO₂].

Answer: K = [CO₂] (or Kp = P_CO₂).

Try 3 practice questions

1. Write K for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
Your answer
2. Write K for AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq).
Your answer
3. Write K for 2H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq).
Your answer

Quick answers

What is the most common mistake with writing equilibrium expressions?: Putting solids and pure liquids like CaCO₃(s) or H₂O(l) into the K expression.
How do I get writing equilibrium expressions right?: Write products over reactants, each raised to its coefficient, but only include gases (aq concentrations or partial pressures). Solids and pure liquids have constant activity, so they're left out.
Can you show a worked example?: Write K for CaCO₃(s) ⇌ CaO(s) + CO₂(g). K = [CO₂] (or Kp = P_CO₂).

Related concepts

Save this as a miss — practice it before your test

Sugar brings it back for review until it sticks. Free, no card.

Save this as a miss — practice it before your test