Chemistry • Year 12 • Module 5 • Lesson 2
Reversibility, Non-Equilibrium Systems & Entropy
Lock in the core vocabulary of reversibility and irreversibility, the ΔG° spectrum, entropy as a concept, and the thermodynamic conditions for spontaneity.
1. Term–definition match
The eleven definitions below are shuffled. In the right-hand column write the matching term from this list: reversible reaction, irreversible reaction, entropy (S), Gibbs free energy (ΔG), non-equilibrium system, spontaneous reaction, dynamic equilibrium, enthalpy (ΔH), ΔG°, open system, closed system. 11 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A reaction that proceeds in both the forward and reverse directions, reaching a state where the concentrations of all species remain constant. | |
| 1.2 | A measure of the disorder or dispersal of energy in a system; increases when gas molecules are produced or particles spread into solution. | |
| 1.3 | A reaction that proceeds without continuous external energy input; has ΔG < 0 in the direction considered. | |
| 1.4 | A thermodynamic quantity that combines enthalpy and entropy; at equilibrium its value is zero for the system. | |
| 1.5 | A reaction system where the reaction quotient Q does not equal the equilibrium constant K—net reaction is still occurring. | |
| 1.6 | A reaction that goes effectively to completion because the products are so thermodynamically stable that the reverse reaction is negligible; written with a single arrow (→). | |
| 1.7 | The standard Gibbs free energy change; measured under standard conditions (25°C, 100 kPa, 1 mol L−1) and used to compare reactions. | |
| 1.8 | A system that can exchange both matter and energy with the surroundings; cannot reach dynamic equilibrium. | |
| 1.9 | The state in a reversible reaction where the forward and reverse reaction rates are equal and macroscopic properties remain constant; written with a double arrow (⇌). | |
| 1.10 | A system that can exchange energy but not matter with its surroundings; required to reach dynamic equilibrium. | |
| 1.11 | The heat content of a system; related to bond breaking and forming; negative when a reaction releases heat to the surroundings. |
2. Fill in the blanks — the ΔG° spectrum
Complete the paragraph below using the word bank. Each word is used once. 9 marks
Whether a reaction is _____________ or _____________ is determined by the _____________ of ΔG°. When ΔG° is very large and negative (e.g. less than −100 kJ/mol), the products are so thermodynamically stable that the reaction goes to _____________—the system has essentially no _____________ at all. When ΔG° is only _____________ in size (e.g. within ±50 kJ/mol), both the forward and reverse directions are thermodynamically accessible, and the system reaches _____________ with an _____________ composition. At dynamic equilibrium, the Gibbs free energy of the system is at its _____________ value, and ΔG = _____________.
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 10 marks (1 for T/F, 1 for the correction where needed)
3.1 A reaction written with a double arrow (⇌) always produces equal concentrations of reactants and products at equilibrium. T / F
3.2 Combustion of methane is an irreversible reaction because it releases heat to the surroundings. T / F
3.3 An endothermic reaction can be spontaneous if the entropy increase is large enough to make ΔG negative. T / F
3.4 At dynamic equilibrium, both the forward and reverse reactions have stopped completely. T / F
3.5 Photosynthesis is a non-equilibrium system that requires continuous external energy input because its ΔG is large and positive. T / F
4. Function recall
Answer each question in 1–2 sentences using precise terms from Lesson 2. 10 marks (2 each)
4.1 What is the role of entropy (S) in determining whether a reaction is spontaneous?
4.2 What is the function of the Gibbs free energy equation (ΔG = ΔH − TΔS) in classifying chemical reactions?
4.3 Why must the Haber process be conducted in a closed system to reach dynamic equilibrium?
4.4 What does a large negative ΔG° tell you about the position of equilibrium and the value of Keq?
4.5 Why is the combustion of wood in a bushfire (e.g. a Black Summer fire in south-east Australia) considered an irreversible non-equilibrium process?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they are connected. Each arrow must carry a linking phrase (e.g. “determines”, “equals zero at”, “increases when”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: ΔG° magnitude • reversibility • entropy (S) • dynamic equilibrium • spontaneous reaction • Gibbs free energy (ΔG).
Q1 — Term–definition matches
1.1 reversible reaction • 1.2 entropy (S) • 1.3 spontaneous reaction • 1.4 Gibbs free energy (ΔG) • 1.5 non-equilibrium system • 1.6 irreversible reaction • 1.7 ΔG° • 1.8 open system • 1.9 dynamic equilibrium • 1.10 closed system • 1.11 enthalpy (ΔH).
Q2 — Cloze paragraph
In order: reversible • irreversible • magnitude • completion • equilibrium • moderate • equilibrium • intermediate • minimum • zero. (9 blanks; accept synonyms where the meaning is preserved.)
Q3 — True / false with correction
3.1 False. Correction: a double arrow indicates a reversible reaction that reaches dynamic equilibrium—the position of that equilibrium (the ratio of products to reactants) depends on Keq, which can range from very large to very small; concentrations are not necessarily equal.
3.2 False. Correction: combustion of methane is irreversible because it has an extremely large negative ΔG (−818 kJ/mol)—both the ΔH and the TΔS terms make ΔG very negative. Releasing heat alone (ΔH < 0) is not sufficient to make a reaction irreversible; many reversible reactions are also exothermic. It is the magnitude of ΔG that determines irreversibility.
3.3 True.
3.4 False. Correction: at dynamic equilibrium both forward and reverse reactions are still occurring simultaneously at equal, non-zero rates—the system appears static macroscopically but continues to exchange reactants and products at the molecular level.
3.5 True.
Q4.1 — Role of entropy in spontaneity
Entropy contributes a negative TΔS term to ΔG = ΔH − TΔS. A positive ΔS (increase in disorder, e.g. production of gas molecules) makes ΔG more negative, favouring spontaneity. If ΔS is positive and large enough, a reaction can be spontaneous even when ΔH is positive (endothermic).
Q4.2 — Function of the Gibbs equation
The equation ΔG = ΔH − TΔS combines both the heat change (ΔH) and the entropy change (ΔS) into a single value that predicts spontaneity: ΔG < 0 means the reaction is spontaneous in the forward direction; ΔG > 0 means it is non-spontaneous; ΔG = 0 means the system is at equilibrium. The magnitude of ΔG° also indicates how far the equilibrium lies towards products or reactants.
Q4.3 — Why closed system is needed for equilibrium
In a closed system, no reactants or products can escape to the surroundings. This allows the concentrations of N2, H2, and NH3 to reach constant values where the forward and reverse reaction rates are equal—dynamic equilibrium. In an open system, products (e.g. NH3) would escape, continuously driving the reaction in one direction and preventing equilibrium from being established.
Q4.4 — Large negative ΔG° and Keq
A large negative ΔG° means the products are overwhelmingly more thermodynamically stable than the reactants. The equilibrium position lies far to the products side, and Keq is very large (Keq >> 1). In the extreme case, the reaction goes essentially to completion and the system is described as irreversible.
Q4.5 — Bushfire combustion as irreversible non-equilibrium
Combustion of cellulose and other fuels in a bushfire has an extremely large negative ΔG—both ΔH < 0 (enormous heat release) and ΔS > 0 (CO2 gas and water vapour produced from solid biomass) make ΔG deeply negative. The reverse reaction (re-forming biomass from CO2 and H2O spontaneously) would require a ΔG of equal magnitude in the positive direction—essentially impossible. It is also an open system (gases escape into the atmosphere), preventing dynamic equilibrium.
Q5 — Sample concept map
A correct map should include arrows such as:
- Gibbs free energy (ΔG) —is determined by→ entropy (S) (via TΔS term)
- ΔG° magnitude —determines→ reversibility
- Gibbs free energy (ΔG) —equals zero at→ dynamic equilibrium
- spontaneous reaction —has→ Gibbs free energy (ΔG) < 0
- reversibility —when small ΔG°, reaches→ dynamic equilibrium
- entropy (S) —increases in→ spontaneous reaction (universe level)
Award 1 mark per correctly labelled and directionally correct arrow (max 6).