The equilibrium constant, Kc (every paper)

Write the expression, calculate a value or a missing concentration, and remember Kc changes ONLY with temperature.

Writing and using the Kc expression

For aA + bB ⇌ cC + dD: Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ) — products over reactants, each concentration raised to its coefficient.
You can also work BACKWARDS from a given Kc expression to the balanced equation — match each species' power to its coefficient.
To find a missing equilibrium concentration, substitute everything you know into the expression and solve algebraically.
To check whether a mixture IS at equilibrium, calculate Kc from its concentrations and compare with the known value — if they differ, the system is not yet at equilibrium and will shift toward it.

Interpreting the size of a Kc value

A LARGE Kc (much greater than 1) means the equilibrium mixture favours PRODUCTS — most reactant has converted, since the products-over-reactants ratio is large.
A SMALL Kc (much less than 1) means the equilibrium mixture favours REACTANTS — very little has converted to product, since the ratio stays small.

Exam-style PCl₃(g) + Cl₂(g) ⇌ PCl₅(g) at 300 °C Calculate Kc if [PCl₃] = 0.016 mol L⁻¹, [Cl₂] = 0.021 mol L⁻¹, [PCl₅] = 0.00013 mol L⁻¹. Kc = [PCl₅] ÷ ([PCl₃][Cl₂]) Kc = 0.00013 ÷ (0.016 × 0.021) Kc = 0.00013 ÷ 0.000336 = 0.39 (2 s.f.) ✓ Substitute directly into the expression you wrote first — a small Kc here means the equilibrium mixture favours the REACTANTS, since very little PCl₅ has formed relative to what remains. ✕

Confusing "shifts" with "changes Kc"

A change in concentration or pressure moves the POSITION of equilibrium, but the Kc value at that temperature is unchanged — the system simply settles at new concentrations that still satisfy the same Kc. Only a change in TEMPERATURE changes the value of Kc itself.

Reading a Kc trend to find exothermic/endothermic

Kc rises as temperature rises → the forward reaction is ENDOTHERMIC (favoured by added heat, so more product forms).
Kc falls as temperature rises → the forward reaction is EXOTHERMIC.

Practice for this standard covers the whole topic — flashcards and quiz.