Chemistry
Equilibrium, acids, redox and organic chemistry — full Paper 1 + Paper 2 practice External Assessments with fully worked solutions.
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Reversible Reactions and the Nature of Dynamic Equilibrium
Reversible Reactions: Definition and Characteristics
A reversible reaction is a chemical reaction in which the products can react with one another to reform the original reactants. In contrast to an irreversible reaction — where conversion of reactants to products is essentially complete and proceeds in one direction only — reversible reactions can proceed simultaneously in both the forward direction (reactants → products) and the reverse direction (products → reactants). This bidirectionality is represented in chemical equations using a double-headed equilibrium arrow (⇌) rather than a single arrow.
A classic and industrially significant example from Australian and global chemistry is the Haber–Bosch process for ammonia synthesis:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Here, nitrogen gas and hydrogen gas can combine to form ammonia, but simultaneously, ammonia molecules can decompose back into nitrogen and hydrogen. Neither reaction ever fully stops as long as all species are present.
Key characteristics of reversible reactions include:
- Both the forward and reverse reactions occur simultaneously under the same conditions.
- The reaction does not go to completion; a mixture of reactants and products is always present at equilibrium (unless one component is continuously removed).
- The system is sensitive to changes in conditions such as temperature, pressure, and concentration — a feature exploited extensively in Queensland's industrial chemistry contexts (e.g. ammonia manufacture at Gibson Island, Brisbane).
- The enthalpy change (ΔH) of the reverse reaction is equal in magnitude but opposite in sign to that of the forward reaction. If the forward reaction is exothermic, the reverse is endothermic by the same amount.
It is important not to confuse reversible with instantaneous or fast. A reversible reaction may be very slow in one or both directions; reversibility refers to the direction the reaction can proceed, not its rate.
Open, Closed, and Isolated Systems
Understanding equilibrium requires precise terminology about the system and its surroundings. In QCAA Chemistry, students must be able to distinguish three types of thermodynamic systems:
| System Type | Matter exchange with surroundings? | Energy exchange with surroundings? | Equilibrium possible? |
|---|---|---|---|
| Open | Yes | Yes | No — true equilibrium cannot be established |
| Closed | No | Yes | Yes — chemical equilibrium can be reached |
| Isolated | No | No | Theoretically yes, but rare in practice |
An open system allows both matter and energy to transfer between the system and its surroundings. A beaker of hydrochloric acid reacting with marble chips (calcium carbonate) left open to the atmosphere is an open system: CO₂ gas escapes, so the reverse reaction cannot occur at a meaningful rate and equilibrium is never established. This is why many reactions encountered in general chemistry appear to go to completion — a product (gas or precipitate) is effectively removed from the system.
A closed system prevents the transfer of matter while allowing energy exchange. This is the essential condition for chemical equilibrium. A sealed vessel containing N₂O₄ and NO₂ gases is a closed system; neither reactant nor product can escape, so both forward and reverse reactions can occur, and equilibrium is eventually reached.
An isolated system permits neither matter nor energy exchange. True isolated systems are theoretical constructs (a perfect thermos flask approaches this). In practice, chemists work with closed systems when studying equilibrium.
Applied example: Consider a sealed plastic soft-drink bottle. The bottle is a closed system: CO₂ dissolved in the drink is in equilibrium with CO₂ gas in the headspace (CO₂(aq) ⇌ CO₂(g)). Opening the bottle converts it to an open system — CO₂ escapes to the atmosphere and equilibrium is disrupted, causing the drink to go flat. This everyday observation illustrates why the closed-system condition is essential for equilibrium to persist.
A nitrogen-fertiliser plant runs the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Which expression correctly represents the equilibrium constant Kc for this reaction?
- Kc = [NH₃]² / ([N₂][H₂]³)
- Kc = [N₂][H₂]³ / [NH₃]²
- Kc = 2[NH₃] / ([N₂] + 3[H₂])
- Kc = [NH₃] / ([N₂][H₂])
Show the worked answer
Answer: A
A — Kc is products over reactants, each raised to its stoichiometric coefficient: [NH₃]² / ([N₂][H₂]³).
All 20 practice exams
- Exam 1 — Chemical equilibrium: writing Kc expressions, calculating Kc from equilibrium concentrations and ICE tables, and interpreting the magnitude of Kc; Le Chatelier's Principle applied to concentration, pressure, volume and temperature changes in the Haber process, with collision-theory reasoning; Bronsted-Lowry acid-base theory, conjugate pairs, pH/pOH calculations for strong and weak acids/bases, Ka, and buffer behaviour (Henderson-Hasselbalch)
- Exam 2 — Carbonate equilibria in seawater: Kc expressions, Qc vs Kc, and Le Chatelier reasoning for rising atmospheric CO2; Bronsted-Lowry acid-base theory: conjugate pairs, Ka1/Ka2 of carbonic acid, pH/pOH and the bicarbonate buffer; Quantitative acid-base chemistry: weak-acid pH, dilution, strong-acid/base pH, Henderson-Hasselbalch
- Exam 3 — Unit 3: chemical equilibrium (Kc, Le Chatelier) applied to chlorine/HOCl systems; Unit 3: Bronsted-Lowry acid-base theory, pH/pOH, Ka/Kb, buffer solutions (HOCl/OCl- pool buffering); Unit 3: volumetric analysis - acid-base titration of pool dosing acids/bases with full stoichiometry
- Exam 4 — Volumetric analysis: back-titration determination of ethanoic acid in commercial vinegar (n=cV, dilution scaling, excess-reagent logic); Acid-base equilibrium: Bronsted-Lowry conjugate pairs, weak-acid pH, Ka/pKa, Kw and pOH; Titration-curve interpretation: half-equivalence (pH = pKa), equivalence-point pH for weak acid + strong base, indicator selection
- Exam 5 — QCAA Chemistry Units 3 & 4 External Assessment (50% of subject result), simulated across Paper 1 (Section A: 20 MC + Section B: 8 short-response, 38 marks) and Paper 2 (Section C: 8 short/extended-response, 52 marks) for a grand total of 110 marks; Unit 3 Topic 2 redox and electrochemistry anchored on the stimulus context: silver electroplating of a copper bracelet in a AgNO3 electrolytic cell - half-equations, anode/cathode identification, ion movement, Faraday's-law mass calculations, E°cell, and galvanic vs electrolytic distinction; Unit 3 Topic 1 chemical equilibrium: Kc expressions and calculations, ICE tables, Le Chatelier's Principle (concentration/pressure/temperature) explained through collision theory
- Exam 6 — Unit 3 Redox: oxidation-state assignment, half-equation balancing, combining half-equations for full redox, oxidising/reducing agent identification; Unit 3 Electrochemistry: galvanic vs electrolytic cells, standard electrode potentials E°, E°cell, spontaneity, Faraday's laws (Q=It, n=Q/zF), industrial electrolytic refining of copper and the anode-sludge/anode-impurity rationale; Unit 3 Equilibrium: Kc expressions and calculations, ICE tables, Le Chatelier's Principle (concentration, pressure, temperature) with collision-theory reasoning
- Exam 7 — Unit 3 redox: standard reduction potentials, galvanic cell EMF, spontaneity and reducing-agent strength (galvanic-corrosion / sacrificial-anode context); Unit 3 redox: oxidation-state assignment, half-equation balancing, electrolytic cells and Faraday's law; Unit 3 equilibrium: Kc expressions and calculations, ICE tables, Le Chatelier's Principle (concentration/pressure/temperature)
- Exam 8 — Unit 3 Topic 2 (Oxidation and reduction): standard electrode potentials, E°cell calculation, ranking oxidising/reducing agent strength, galvanic cell construction and labelling, electrolysis - anchored to a rechargeable-battery electrode-pair design context; Unit 3 Topic 1 (Equilibrium, acids and redox): Kc expression and ICE-table calculations, Le Chatelier's Principle via collision theory, Bronsted-Lowry theory, pH/pOH/Ka/Kb, weak-acid and buffer calculations, acid-base titration analysis and indicator selection; Unit 4 Topic 1 (Structure of organic materials): the 12 hydrocarbon-derivative classes, IUPAC nomenclature, structural and cis-trans isomerism, intermolecular forces and physical-property trends, polymers, proteins, carbohydrates and lipids
- Exam 9 — Unit 4 industrial synthesis — biodiesel production from waste vegetable oil by base-catalysed transesterification with methanol (the stimulus context anchoring the paper); Unit 4 organic structure — esters, the 12 hydrocarbon-derivative classes, IUPAC naming, isomerism, IMF-based physical properties, and triglyceride/lipid macromolecules; Unit 4 organic reactions — esterification/transesterification, combustion, addition (Markovnikov), oxidation of alcohols, nucleophilic substitution, elimination, and multi-step synthesis pathways
- Exam 10 — Unit 3 Chemical Equilibrium: Kc expressions and calculations, ICE tables, Le Chatelier's Principle (concentration/pressure/temperature), collision-theory mechanisms; Unit 3 Acids: Bronsted-Lowry theory, conjugate pairs, pH/pOH, Kw, Ka/Kb, weak-acid and weak-base calculations, buffers, acid-base titration analysis and indicator selection; Unit 3 Redox: oxidation-state assignment, half-equation balancing, galvanic vs electrolytic cells, standard electrode potentials and E°cell, spontaneity, electroplating and aluminium production
- Exam 11 — Organic synthesis & esterification (aspirin: salicylic acid -> acetylsalicylic acid); Reaction-type identification, balanced equations & reaction conditions; Atom economy & green chemistry evaluation
- Exam 12 — QCAA Chemistry Units 3 & 4 (General Senior Syllabus 2025 v1.3) full External Assessment, contextualised on the comparison of polylactic acid (PLA) with polyethylene (PE) in biodegradable packaging; Paper 1 Section A: 20 single-best-answer multiple-choice (1 mark each) spanning equilibrium/Kc, Le Chatelier, Bronsted-Lowry acid-base, pH/pOH/Ka, redox oxidation states and E°cell, organic structure/IUPAC naming/isomerism, addition vs condensation polymers, and stoichiometry; Paper 1 Section B: 8 short/extended-response questions (38 marks) on dynamic equilibrium and Kc, weak-acid pH from Ka, acid-base titration and standardisation, redox half-equations and galvanic cells, organic nomenclature and intermolecular forces, condensation vs addition polymerisation mechanisms
- Exam 13 — Unit 3: chemical equilibrium (Kc, Le Chatelier, reaction quotient) applied to esterification and industrial systems; Unit 3: Bronsted-Lowry acid-base theory, pH/pOH, Ka, weak-acid dissociation and buffer calculations; Unit 3: volumetric analysis - monoprotic and diprotic acid-base titrations with concordant titres
- Exam 14 — Markovnikov addition and acid-catalysed hydration of unsymmetrical alkenes (regioselectivity via carbocation stability); IUPAC nomenclature, isomerism and structure of the 12 hydrocarbon-derivative classes; Six core organic reactions: combustion, nucleophilic substitution, elimination, addition, oxidation, esterification
- Exam 15 — Multi-step organic synthesis pathway design from an alkane to an ester fragrance compound (reaction-type, reagent and condition selection at each step); Six core organic reaction types: combustion, free-radical/nucleophilic substitution, elimination, addition (Markovnikov), oxidation, esterification; Organic structure, IUPAC nomenclature, isomerism and intermolecular-force-based physical property trends across hydrocarbon derivative classes
- Exam 16 — Chemical equilibrium: Kc expressions, ICE tables and Le Chatelier applied to the contact process (SO2 oxidation over V2O5) as a yield-vs-rate compromise; Bronsted-Lowry acid-base theory: pH/pOH, strong vs weak acids/bases, Ka/Kb, buffers and acid-base titration stoichiometry; Oxidation-reduction: oxidation states, half-equations, galvanic cell EMF from standard electrode potentials, and electrolysis (Faraday's laws, electroplating, aluminium)
- Exam 17 — Unit 3 — Chemical equilibrium: Kc expressions and ICE calculations, Le Chatelier (concentration/pressure/temperature) and collision-theory mechanism; Unit 3 — Acids and bases: Bronsted-Lowry conjugate pairs, strong/weak distinction, pH/pOH, Ka/Kb, buffers and Henderson-Hasselbalch, acid-base titration and indicator choice; Unit 3 — Redox and electrochemistry: oxidation-state assignment, half-equation balancing in acid, galvanic cell labelling and E°cell, ranking oxidising/reducing agents, electrolysis and Faraday stoichiometry, aluminium/copper industrial cells
- Exam 18 — QCAA Chemistry Units 3 & 4 (2025 v1.3) full External Assessment; Stimulus context: nylon-6,6 condensation polymerisation from hexanedioic acid + 1,6-diaminohexane, amide bond, contrast with addition polymerisation; Unit 3: chemical equilibrium (Kc, ICE tables, Le Chatelier, collision theory)
- Exam 19 — Unit 3 Topic 2 Oxidation & Reduction anchored on the permanganate-iron(II) redox titration for water-quality testing (half-equation balancing in acidic conditions, MnO4-/Fe2+ 1:5 stoichiometry, oxidising/reducing agent identification, self-indicating endpoint, E°cell); Unit 3 Topic 1 Equilibrium, Acids & Redox: Kc expressions and ICE calculations, Le Chatelier's Principle via collision theory, Brønsted-Lowry conjugate pairs, pH/pOH, weak-acid Ka and percent ionisation, buffers, volumetric analysis and titration-curve interpretation; Unit 4 Topic 1 Properties & Structure of Organic Materials: IUPAC nomenclature of the 12 derivative classes, structural and cis-trans isomerism, intermolecular forces explaining physical-property trends, addition/condensation polymers and proteins
- Exam 20 — Unit 3 Equilibrium: Kc expressions, Le Chatelier (concentration/pressure/temperature); Unit 3 Acids & bases: Bronsted-Lowry, pH/pOH, Ka/Kb, weak vs strong; Buffer chemistry: composition, mechanism, Henderson-Hasselbalch, pH change on acid/base addition (pharmaceutical formulation context)
All 20 revision notes
- Reversible Reactions and the Nature of Dynamic Equilibrium
- Kc Expressions and the Significance of Kc Values
- Predicting Equilibrium Shifts Using Le Chatelier's Principle
- Bronsted-Lowry Theory, Conjugate Pairs and Acid-Base Strength
- pH, pOH, Ka and Kb Calculations for Acids and Bases
- Buffer Solutions: Composition, Function and pH Resistance
- Acid-Base Titrations: Procedure, Calculations and Indicator Selection
- Assigning Oxidation States and Identifying Redox Reactions
- Writing and Balancing Half-Equations and Full Redox Equations
- Galvanic Cells: Construction, Labelling and E°cell Calculations
- Electrolytic Cells, Electroplating and Industrial Electrolysis
- The 12 Hydrocarbon Derivative Classes: Naming, Structure and Isomerism
- Intermolecular Forces and Their Effect on Physical Properties
- Polymers: Addition Polymers, Polyesters, Polyamides and Biodegradability
- Proteins, Carbohydrates and Lipids as Organic Macromolecules
- Combustion, Substitution, Elimination and Addition Reactions of Organic Compounds
- Oxidation of Alcohols and Fischer Esterification
- Designing Multi-Step Organic Synthesis Pathways
- Industrial Chemical Syntheses and Green Chemistry Principles
- Limiting Reagents, Theoretical Yield and Percentage Yield Calculations