QCE Chemistry · Unit 3
QCE Chemistry Unit 3 Topic 1: Chemical Equilibrium — Flashcards & Quiz
QCE Chemistry Unit 3 Topic 1 explores how reversible reactions reach a state of dynamic equilibrium and how conditions shift that balance. These free flashcards and true/false questions cover reversible reactions, dynamic equilibrium, equilibrium constants (Kc and Kp), Le Chatelier's principle, the effects of concentration, temperature and pressure changes, catalysts and equilibrium, the Haber process and industrial applications, and acid-base equilibria including pH calculations. Every card is aligned to the QCAA Senior Chemistry syllabus so you revise exactly what examiners assess.
Key Terms
- Dynamic equilibrium
- A state in a reversible reaction where the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant over time despite ongoing molecular activity. QCAA Chemistry Unit 3 Topic 1 EA questions test whether students understand that equilibrium is dynamic, not static.
- Le Chatelier's principle
- The principle that if a system at equilibrium is subjected to a disturbance (change in concentration, pressure, volume or temperature), it will shift to partially counteract the change and establish a new equilibrium. QCAA external assessments require students to predict shift direction AND explain the reasoning for each type of disturbance.
- Equilibrium constant (Keq)
- A numerical value expressing the ratio of product concentrations to reactant concentrations at equilibrium, each raised to their stoichiometric coefficients. QCAA Unit 3 Topic 1 EA questions require students to write Keq expressions from balanced equations, noting that pure solids and liquids are excluded.
- Collision theory
- The model explaining that reactions occur when particles collide with sufficient energy (activation energy) and correct orientation. QCAA Chemistry assessments require students to link each rate factor (temperature, concentration, surface area, catalyst) to its effect on collision frequency or energy.
- ICE table
- A systematic method (Initial, Change, Equilibrium) for calculating equilibrium concentrations from initial conditions and stoichiometry. QCAA Unit 3 Topic 1 EA calculation questions commonly require the ICE table approach to determine unknown equilibrium values.
- Catalyst
- A substance that increases the rate of both forward and reverse reactions by providing an alternative reaction pathway with lower activation energy, without being consumed. QCAA Chemistry EA questions specifically test whether students understand that catalysts increase rate but do NOT shift equilibrium position or change Keq.
Sample Flashcards
Q1: Define dynamic equilibrium in a chemical system.
Dynamic equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the reverse reaction. Macroscopic properties (concentrations, colour, pressure) remain constant, but both reactions continue at the molecular level. It is dynamic because reactions have not stopped, and it is an equilibrium because there is no net change in composition.
Q2: What conditions are necessary for a system to reach equilibrium?
1) The system must be closed (no exchange of matter with surroundings). 2) The reaction must be reversible. 3) Sufficient time must pass for forward and reverse rates to equalise. Temperature must be constant for the position of equilibrium to be fixed. If matter can enter or leave, equilibrium cannot be established.
Q3: 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.
Q4: 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.
Q5: State Le Chatelier's principle.
If a change is imposed on a system at equilibrium, the system will respond by shifting the position of equilibrium to partially counteract the change and re-establish equilibrium. The system adjusts to minimise the effect of the disturbance. Changes include alterations to concentration, pressure/volume, or temperature.
Q6: How does changing concentration affect equilibrium position?
Increasing the concentration of a reactant shifts equilibrium toward products (right) to consume the added reactant. Increasing product concentration shifts equilibrium toward reactants (left). Decreasing concentration of a substance shifts equilibrium toward the side that produces that substance. The value of Kc remains unchanged.
Q7: How does changing pressure (or volume) affect a gaseous equilibrium?
Increasing pressure (decreasing volume) shifts equilibrium toward the side with FEWER moles of gas. Decreasing pressure (increasing volume) shifts toward MORE moles of gas. If both sides have equal moles of gas, pressure changes have no effect on equilibrium position. Kc remains unchanged.
Q8: How does changing temperature affect equilibrium position and the value of Kc?
Temperature is the ONLY factor that changes Kc. For an exothermic forward reaction (ΔH < 0): increasing temperature shifts equilibrium LEFT (Kc decreases). For an endothermic forward reaction (ΔH > 0): increasing temperature shifts equilibrium RIGHT (Kc increases). The system shifts to absorb the added heat.
Sample Quiz Questions
Q1: At dynamic equilibrium, the forward and reverse reactions have stopped.
Answer: FALSE
At dynamic equilibrium, both forward and reverse reactions continue at equal rates — they have NOT stopped. The concentrations remain constant because the rates are balanced.
Q2: Dynamic equilibrium can only be established in a closed system.
Answer: TRUE
A closed system (no exchange of matter with surroundings) is required for equilibrium. In an open system, products or reactants can escape, preventing equilibrium.
Q3: 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.
Q4: 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.
Q5: Increasing the concentration of a product shifts equilibrium toward the reactants.
Answer: TRUE
By Le Chatelier's principle, the system counteracts the increase in product concentration by shifting equilibrium to the left (toward reactants) to consume the added product.
Why It Matters
Chemical equilibrium and rates is a pivotal topic in QCE Chemistry Unit 3 because it underpins both the acid-base chemistry in Topic 2 and the electrochemistry in Unit 4. The external exam tests your ability to apply Le Chatelier's principle to predict equilibrium shifts, interpret equilibrium constant expressions and analyse rate graphs. Quantitative problems involving equilibrium constants and rate calculations appear regularly, so both conceptual understanding and mathematical confidence are essential for strong performance. Equilibrium concepts also have real-world relevance in industrial chemistry — the Haber process for ammonia synthesis is a common exam context that tests your ability to apply Le Chatelier's principle to optimise yield. QCAA exam questions frequently present an equilibrium system with changed conditions and ask you to predict the direction of shift, explain the reasoning, and describe the effect on the equilibrium constant.
Key Concepts
Rates of Reaction and Collision Theory
Understand how temperature, concentration, surface area and catalysts affect reaction rate through collision theory. Be able to draw and interpret rate graphs (concentration vs time, rate vs concentration) and explain the difference between average rate and instantaneous rate using tangent lines on concentration-time curves.
Chemical Equilibrium and Le Chatelier's Principle
Know the characteristics of dynamic equilibrium (forward and reverse rates equal, concentrations constant). Apply Le Chatelier's principle to predict how changes in concentration, pressure, volume and temperature shift the equilibrium position — and critically, explain why adding a catalyst does not shift equilibrium.
Equilibrium Constants (Keq and Kc)
Write equilibrium constant expressions from balanced equations, noting that pure solids and liquids are excluded. Interpret the magnitude of K: large K favours products, small K favours reactants. Practise calculating K from equilibrium concentrations — ICE tables are the standard method for QCAA exam problems.
Industrial Applications of Equilibrium
Apply equilibrium principles to industrial processes like the Haber process (ammonia synthesis) and the Contact process (sulfuric acid). Evaluate how conditions are chosen to balance yield against rate — this is a common extended response topic asking you to justify industrial compromises.
Common Mistakes to Avoid
- Stating that a catalyst shifts the equilibrium position — QCAA Chemistry Unit 3 Topic 1 EA marking guides explicitly require students to explain that a catalyst increases the rate of BOTH forward and reverse reactions equally, so it speeds up attainment of equilibrium without changing the position or Keq value.
- Confusing equilibrium position shift with a change in Keq — only temperature changes alter the value of Keq. Changes in concentration, pressure or volume shift the position but leave Keq unchanged. QCAA assessments test this distinction directly.
- Forgetting to exclude pure solids and liquids from the Keq expression — QCAA Chemistry EA questions penalise students who include solid or liquid species in equilibrium constant calculations because their concentrations are constant.
- Using Le Chatelier's principle to predict effects of adding an inert gas at constant volume — adding an inert gas at constant volume does not change the partial pressures of reacting gases and therefore causes no shift. QCAA examiners specifically target this misconception.
Study Tips
- Build ICE tables (Initial, Change, Equilibrium) for at least five different equilibrium problems until the method becomes automatic.
- Practise Le Chatelier's principle by writing the change, predicting the shift direction and explaining the reasoning for each of the five possible disturbances.
- Draw rate-versus-time and concentration-versus-time graphs from memory for reactions with and without catalysts.
- Link collision theory to every rate factor — always explain why the factor increases the frequency or energy of effective collisions.
- Use flashcards with spaced repetition to drill equilibrium constant expressions and Le Chatelier's predictions — these are rapid-recall skills that free up exam time for more complex calculations.
- Before your exam, work through the practice questions in this set at least twice using spaced repetition. Testing yourself repeatedly is the most effective revision strategy for long-term retention.
Related Topics
Frequently Asked Questions
What does QCE Chemistry Unit 3 Topic 1 cover?
Unit 3 Topic 1 covers reversible reactions, dynamic equilibrium, equilibrium constants (Kc and Kp), Le Chatelier's principle, effects of changing conditions, catalysts, industrial applications and acid-base equilibria.
How many flashcards are in this set?
This free set contains 20 flashcards and 20 true/false quiz questions covering all key concepts in Unit 3 Topic 1, aligned to the QCAA Senior Chemistry syllabus.
Are these flashcards aligned to the Queensland QCE syllabus?
Yes — every flashcard and quiz question is mapped to QCAA syllabus objectives for QCE Chemistry Unit 3 Topic 1: Chemical Equilibrium.
Last updated: March 2026 · 20 flashcards · 20 quiz questions · Content aligned to the QCAA Syllabus