QCE Chemistry — Unit 3
Equilibrium Constant — Flashcards & Quiz
The equilibrium constant Kc quantifies the ratio of product to reactant concentrations at equilibrium, and QCE Chemistry Unit 3 tests both its calculation and its interpretation. You need to write Kc expressions correctly (excluding solids and pure liquids), relate magnitude to reaction favourability, and explain why Kc changes only with temperature. Combine this with Le Chatelier reasoning for most exam applications.
Key Points
- For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b. Products on top, reactants on bottom, raised to stoichiometric coefficients.
- Exclude pure solids and pure liquids from Kc — their activity is constant and already absorbed into the constant.
- Magnitude interpretation: Kc >> 1 products favoured; Kc << 1 reactants favoured; Kc ≈ 1 significant amounts of both.
- Kc is temperature-dependent only. Pressure, concentration, and catalysts do NOT change Kc.
- Reaction quotient Q has the same form as Kc but uses non-equilibrium concentrations; Q < Kc means system shifts right, Q > Kc means shifts left.
- The relationship Kp = Kc (RT)^Δn converts between concentration and pressure equilibrium constants for gas-phase reactions.
Common Mistakes to Avoid
- Including solids or pure liquids in the Kc expression — they are always omitted.
- Forgetting to raise concentrations to their stoichiometric powers.
- Claiming adding a catalyst changes Kc — it only speeds up reaching equilibrium.
- Changing Kc for pressure or concentration changes — only temperature changes Kc.
- Confusing Kc (equilibrium state) with Q (any moment in time).
Exam Strategy
QCAA Unit 3 Kc questions typically ask you to write the expression, calculate the value from equilibrium concentrations, or interpret the magnitude. Method: (1) balance the equation first, (2) write Kc omitting solids/liquids, (3) substitute concentrations raised to their coefficients, (4) interpret in context. For Le Chatelier follow-ups, remember only temperature changes Kc — position shifts with concentration/pressure but Kc stays the same.
Sample Flashcards
Q1: Write the general expression for the equilibrium constant Kc.
For aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b. Concentrations are in mol L⁻¹ at equilibrium. Pure solids and pure liquids are excluded from the expression. A large Kc (≫1) means products are favoured; a small Kc (≪1) means reactants are favoured.
Q2: How does the value of Kc relate to the position of equilibrium?
Kc ≫ 1: equilibrium lies far to the right (products favoured). Kc ≈ 1: significant amounts of both reactants and products present. Kc ≪ 1: equilibrium lies far to the left (reactants favoured). Kc is constant at a given temperature — it changes ONLY when temperature changes, not with concentration or pressure changes.
Q3: What is Kp and how does it differ from Kc?
Kp is the equilibrium constant expressed in terms of partial pressures of gaseous species: Kp = (pC)^c(pD)^d / (pA)^a(pB)^b. Units depend on the reaction. Kp is related to Kc by Kp = Kc(RT)^Δn where Δn = moles of gaseous products − moles of gaseous reactants. If Δn = 0, Kp = Kc.
Q4: How does the equilibrium constant change when a reaction equation is reversed or multiplied?
Reversed: K_reverse = 1/K_forward. Multiplied by factor n: K_new = (K_original)ⁿ. Halved: K_new = √K_original. These relationships apply to both Kc and Kp. When combining equations (Hess's law style), multiply the K values.
Sample Quiz Questions
Q1: The value of Kc changes when the concentration of a reactant is increased.
Answer: FALSE
Kc is constant at a given temperature. Changing concentration shifts the position of equilibrium but does NOT change the value of Kc. Only temperature changes Kc.
Q2: A large Kc value indicates that products are favoured at equilibrium.
Answer: TRUE
When Kc ≫ 1, the numerator (products) is much larger than the denominator (reactants), meaning the equilibrium position favours product formation.
Q3: Pure solids and liquids are included in the Kc expression.
Answer: FALSE
Pure solids and pure liquids have constant concentrations and are excluded from Kc expressions. Only aqueous and gaseous species are included.
Revision Tip
Writing Kc expressions and calculating values is drillable — build a Revizi flashcard deck with 10+ balanced equations asking you to write the Kc expression and calculate it from given data.
Related Concepts
Last updated: March 2026 · 4 flashcards · 4 quiz questions