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: What is dynamic equilibrium?
Dynamic equilibrium occurs in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. Reactants and products are constantly interconverting, but their concentrations remain constant over time. The system must be closed (no matter enters or leaves).
Q2: 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).
Q3: 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.
Q4: What is the reaction quotient (Q) and how is it used?
Q has the same formula as Kc but uses current (non-equilibrium) concentrations. Comparing Q to Kc: if Q < Kc → reaction shifts right (more products needed). If Q > Kc → reaction shifts left (more reactants needed). If Q = Kc → system is at equilibrium.
Q5: What is an ICE table and how is it used?
ICE = Initial, Change, Equilibrium. A systematic table to calculate equilibrium concentrations. List initial concentrations, define the change in terms of x (using stoichiometry), then express equilibrium concentrations. Substitute into the Kc expression and solve for x.
Sample Quiz Questions
Q1: At dynamic equilibrium, both forward and reverse reactions have stopped.
Answer: FALSE
At dynamic equilibrium, both reactions CONTINUE at equal rates. Reactions do NOT stop — "dynamic" means ongoing activity.
Q2: At equilibrium, the concentrations of reactants and products must be equal.
Answer: FALSE
At equilibrium, concentrations are CONSTANT but not necessarily equal. The ratio depends on the value of Kc.
Q3: 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.
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 · 10 flashcards · 9 quiz questions