Skip to main content
Chemistry 4% exam weight

Chemical Equilibrium and Le Chatelier's Principle

Part of the NECO SSCE study roadmap. Chemistry topic chem-7 of Chemistry.

→ See in Roadmap NECO SSCE Study Plans Build 1-Month Plan
By Last updated 4% exam weight

Chemical Equilibrium and Le Chatelier’s Principle

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

A reversible reaction reaches chemical equilibrium when the forward and reverse reaction rates become equal, so concentrations of reactants and products stay constant (though not equal) at a given temperature. The equilibrium constant quantifies this state.

  • Kc = [products]^coefficients ÷ [reactants]^coefficients (mol dm⁻³)
  • Kp = (P_products)^coeff ÷ (P_reactants)^coeff; linked by Kp = Kc(RT)^Δn, where Δn = moles of gaseous products − moles of gaseous reactants
  • Le Chatelier’s Principle: a system at equilibrium shifts to oppose any applied stress (concentration, pressure, temperature)
  • Pure solids and liquids are omitted from Kc and Kp expressions; a catalyst does NOT change Kc or the position of equilibrium
  • Large Kc (>10³) favours products; small Kc (<10⁻³) favours reactants

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Defining Equilibrium

Chemical equilibrium is dynamic: both forward and reverse reactions continue at equal rates, with no net change in concentrations. Two main types exist:

  • Homogeneous equilibrium — all species in the same phase (e.g., N₂(g) + 3H₂(g) ⇌ 2NH₃(g))
  • Heterogeneous equilibrium — species in different phases (e.g., CaCO₃(s) ⇌ CaO(s) + CO₂(g))

For heterogeneous systems, pure solids and pure liquids are excluded from the equilibrium expression because their “concentrations” are constant.

Equilibrium Constants

ExpressionUseKey rule
KcConcentrations in mol dm⁻³Omit pure solids/liquids
KpPartial pressures (atm or Pa)Gaseous species only
Q (reaction quotient)Any instant, same form as KcCompare with Kc to predict shift

The relationship Kp = Kc(RT)^Δn applies only to gaseous equilibria; Δn must count gaseous moles only. A large Kc means products are favoured; a small Kc means reactants dominate at equilibrium.

Le Chatelier’s Principle — Stresses and Shifts

  • Concentration: adding a reactant or removing a product shifts equilibrium forward
  • Pressure/volume: relevant only when gaseous moles differ between sides; increasing pressure shifts toward fewer gas moles
  • Temperature: the ONLY stress that changes the value of K; heating an endothermic reaction shifts it forward, heating an exothermic reaction shifts it backward
  • Catalyst: lowers activation energy for both directions equally — equilibrium is reached faster, but K and the position of equilibrium are unchanged

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Reaction Quotient vs Equilibrium Constant

The reaction quotient Q has the same mathematical form as Kc but uses current concentrations, not equilibrium ones. Comparing Q to Kc predicts the direction of net reaction: if Q < Kc, the reaction proceeds forward; if Q > Kc, it proceeds in reverse; if Q = Kc, the system is at equilibrium. This distinction is heavily tested in NECO Paper II calculations.

Worked Micro-Example

For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) at 500 K with [N₂] = 0.10 M, [H₂] = 0.20 M, [NH₃] = 0.040 M:

Kc = (0.040)² / [(0.10)(0.20)³] = 0.0016 / 0.0008 = 2.0 mol⁻² dm⁶

Δn = 2 − (1 + 3) = −2, so Kp = Kc(RT)^Δn = 2.0 × (0.0821 × 500)⁻² ≈ 2.4 × 10⁻⁵ atm⁻².

Common Exam Traps and Edge Cases

  1. Writing units inside Kc/Kp values — equilibrium constants are correctly dimensionless when derived from activities, but for school calculations omit them deliberately.
  2. Forgetting that Δn for Kp/Kc conversion counts only gaseous species — never include liquids or solids.
  3. Assuming concentrations at equilibrium are equal — they are constant, not equal; only the rates are equal.
  4. Saying “a catalyst shifts equilibrium” — it does not; it shortens the time taken to reach equilibrium only.

This topic connects directly to Industrial Chemistry (Haber process for NH₃, Contact process for SO₃), Acids and Bases (weak acid dissociation constant Ka), and Solubility (Ksp expressions). NECO typically tests this topic through 3–5 mark structured questions on writing expressions, calculating Kc/Kp, and predicting shifts.

Continue your study

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.

Sources & verification