HSC Chemistry — Module 5
Equilibrium Constant — Flashcards & Quiz
The equilibrium constant Kc quantifies the position of a reversible reaction at equilibrium and is a core skill in HSC Chemistry Module 5. You need to write Kc expressions correctly — products over reactants, raised to stoichiometric powers, with solids and pure liquids excluded. Magnitude interpretation matters: Kc much greater than 1 favours products, much less than 1 favours reactants. Always state the temperature when quoting Kc, since it is temperature-dependent and changes only when temperature changes.
Key Points
- Kc = [products]^coefficients / [reactants]^coefficients, for aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b.
- Solids and pure liquids are EXCLUDED from the Kc expression — their concentrations are effectively constant.
- Kc magnitude: Kc >> 1 = products favoured; Kc << 1 = reactants favoured; Kc ≈ 1 = significant amounts of both.
- Kc is temperature-dependent — the ONLY change that changes its value. Pressure, concentration and catalysts do not.
- Reaction quotient Q uses current (non-equilibrium) concentrations. Q < Kc → shifts right; Q > Kc → shifts left; Q = Kc → at equilibrium.
- Units of Kc depend on stoichiometry; many HSC problems use "unitless Kc" — follow the convention stated in the question.
Common Mistakes to Avoid
- Including solids and pure liquids in the Kc expression — only aqueous and gaseous species count.
- Forgetting to raise concentrations to their stoichiometric powers.
- Changing Kc when pressure, concentration, or catalyst changes — only TEMPERATURE changes Kc.
- Confusing Kc (equilibrium state) with Q (reaction quotient at any point).
- Misreading units: Kc often has units depending on the reaction, but HSC problems sometimes use unitless Kc — follow the question convention.
Exam Strategy
HSC Module 5 Kc questions ask you to (1) write the expression from a balanced equation, (2) interpret Kc magnitude, or (3) calculate new equilibrium positions after a change. Always write the balanced equation first, exclude solids/liquids, apply stoichiometric powers. For interpretation: Kc >> 1 = products favoured, Kc << 1 = reactants favoured.
Sample Flashcards
Q1: Write the expression for the equilibrium constant Kc.
For aA + bB ⇌ cC + dD: Kc = [C]^c × [D]^d / [A]^a × [B]^b. Only include aqueous and gaseous species. Solids and pure liquids are excluded (their concentrations are constant). Square brackets represent molar concentration (mol/L).
Q2: What does the value of Kc tell you about a reaction?
Kc >> 1: products are favoured at equilibrium (reaction goes mostly to completion). Kc << 1: reactants are favoured (very little product at equilibrium). Kc ≈ 1: neither strongly favoured, significant amounts of both.
Q3: How does temperature affect the value of Kc?
For exothermic reactions: increasing temperature DECREASES Kc (shifts toward reactants). For endothermic reactions: increasing temperature INCREASES Kc (shifts toward products). Temperature is the ONLY factor that changes Kc.
Q4: What units does Kc have?
Kc may have units or be unitless depending on the reaction. Units depend on the stoichiometry: if the total moles of products and reactants in Kc differ, Kc has units (combinations of mol/L). If they're equal, Kc is dimensionless. In practice, Kc values are often given without units at HSC level.
Sample Quiz Questions
Q1: Pure solids are included in the Kc expression.
Answer: FALSE
Pure solids and pure liquids are EXCLUDED from Kc expressions because their concentrations are constant and incorporated into the Kc value itself.
Q2: A large Kc value indicates that products are favoured at equilibrium.
Answer: TRUE
Kc >> 1 means product concentrations are much higher than reactant concentrations at equilibrium — the reaction goes largely to completion.
Q3: Changing the concentration of a reactant changes the value of Kc.
Answer: FALSE
Changing concentration shifts the equilibrium position but does NOT change the value of Kc. Only temperature changes Kc.
Revision Tip
Kc questions are mechanical — drill a Revizi deck of 10+ equations asking you to write the Kc expression, until the pattern is automatic.
Related Concepts
Last updated: March 2026 · 4 flashcards · 4 quiz questions