Chemistry • Year 12 • Module 5 • Lesson 1
Static vs Dynamic Equilibrium
Build HSC Band 5–6 extended-response technique: evaluate claims using graphical evidence, compare system types, and justify conclusions using equilibrium theory.
1. Extended response, graphical evidence and multi-criteria evaluation (Analyse/Evaluate)
8 marks Analyse–6
Scenario. A Year 12 student is investigating the equilibrium: SO2(g) + ½O2(g) ⇌ SO3(g) in a sealed industrial-style reactor at 600°C. She measures the concentrations of all three species every 10 minutes and obtains the following data:
| Time (min) | [SO2] (mol L−1) | [O2] (mol L−1) | [SO3] (mol L−1) |
|---|---|---|---|
| 0 | 0.80 | 0.40 | 0.00 |
| 10 | 0.60 | 0.30 | 0.20 |
| 20 | 0.48 | 0.24 | 0.32 |
| 30 | 0.42 | 0.21 | 0.38 |
| 40 | 0.40 | 0.20 | 0.40 |
| 50 | 0.40 | 0.20 | 0.40 |
| 60 | 0.40 | 0.20 | 0.40 |
(Hypothetical data for examination practice.)
Figure 1.1 Concentration vs time for SO2(g) + ½O2(g) ⇌ SO3(g) at 600°C in a sealed reactor.
Q1. Analyse and evaluate the student’s data and graph to determine whether the system reaches dynamic equilibrium or static equilibrium. Justify your conclusion using evidence from the table and graph. In your response you must:
- Define dynamic equilibrium and static equilibrium and state the two conditions required for dynamic equilibrium.
- Identify, using specific data values from the table, the point at which the system reaches its equilibrium state.
- Explain what the concentration–time graph reveals about what is happening at the molecular level at equilibrium.
- Evaluate whether the system is at static or dynamic equilibrium, with reference to at least two criteria that distinguish them.
- Explain one limitation of using constant concentrations to identify dynamic equilibrium, and propose additional evidence of continuing reaction.
2. Source critique, identifying the flaw (Analyse/Evaluate)
7 marks Analyse–6
“In a sealed flask, dinitrogen tetroxide and nitrogen dioxide reach a state of equilibrium. At this equilibrium point the reaction has come to a complete halt, both the forward and reverse processes have stopped. We know this because the concentrations of the two gases do not change when we measure them. Since concentrations are constant and the reaction has stopped, this system is no better than static equilibrium where a completed reaction leaves only products behind.”
Excerpt from a hypothetical Year 12 student’s chemistry revision notes.
Q2. The passage above contains at least two distinct scientific errors. For each error: (a) identify the incorrect claim, (b) explain the correct chemistry using lesson content, and (c) suggest what experimental evidence could be used to demonstrate that the correct understanding is true. 7 marks
You may use the table below to structure your response, or write continuously:
| Error # | The incorrect claim | Correct chemistry + experimental evidence |
|---|---|---|
| Error 1 | ||
| Error 2 |
Continuation space (optional):
Q1, Marking criteria (8 marks)
1 mark, Define dynamic equilibrium. Dynamic equilibrium occurs in a reversible reaction in a closed system when the forward reaction rate equals the reverse reaction rate, both rates are non-zero, and the concentration of every species remains constant.
1 mark, Define static equilibrium. In the idealised static/completed-process model, the selected effectively irreversible reaction has no ongoing forward or reverse conversion after completion. This does not mean that particles stop moving.
1 mark, State both conditions for dynamic equilibrium. (1) The reaction must be reversible (written with ⇌). (2) The system must be closed (no matter can enter or leave).
1 mark, Identify the equilibrium time from data. Equilibrium is established at t = 40 min: from t = 40 onward, [SO2] = 0.40, [O2] = 0.20, and [SO3] = 0.40 mol L−1all three values are unchanged at t = 50 and t = 60 min.
1 mark, Molecular-level explanation. Constant concentration does not mean the reaction has stopped. At t ≥ 40 min, the rate of SO2 reacting with O2 to form SO3 equals the rate of SO3 decomposing back to SO2 and O2; both reactions continue simultaneously at equal non-zero rates, so the net change in concentration is zero.
1 mark, Evaluate: two criteria distinguishing the models. Dynamic equilibrium has equal non-zero forward and reverse rates in a reversible system; the idealised completed-process model has no continuing conversion for the selected effectively irreversible reaction. Particle thermal motion continues in both models. Accept comparison of continuing opposing reactions and reversibility; do not require all reactants to be absent in a completed reaction, since an excess reactant may remain.
1 mark, Evidence limitation and additional observation. Constant concentrations and coexistence of reactants/products alone do not measure non-zero reaction rates; a kinetically halted system can also appear constant. Tracer exchange, with no material change in bulk composition, could provide evidence of continuing molecular conversion.
1 mark, Evidenced conclusion. The system is at dynamic equilibrium. Both conditions are met: the reaction SO2 + ½O2 ⇌ SO3 is reversible, and the sealed reactor is a closed system. The data is consistent with dynamic equilibrium in the stated reversible system at constant temperature, but bulk concentration measurements alone do not prove ongoing non-zero rates.
Q2, Marking criteria (7 marks)
Error 1 (3 marks): Claim that “the reaction has come to a complete halt, both forward and reverse processes have stopped.”
(a) Incorrect claim: at dynamic equilibrium, all molecular reaction activity ceases. (1 mark)
(b) Correct chemistry: at dynamic equilibrium, both the forward reaction (N2O4 → 2NO2) and the reverse reaction (2NO2 → N2O4) continue at equal, non-zero rates simultaneously. Constant macroscopic concentration is the result of these two processes exactly cancelling each other, not of the reaction stopping. The system is “busy” at the molecular level even though it appears static from the outside. (1 mark)
(c) Experimental evidence: isotope labelling, introducing a trace amount of 15N-labelled N2O4 without materially changing the mixture composition and, after a short time, detecting 15N atoms appearing in NO2 molecules would confirm that the forward reaction is still proceeding at equilibrium. (1 mark)
Error 2 (3 marks): Claim that the sealed N2O4/NO2 flask is “no better than static equilibrium.”
(a) Incorrect claim: the sealed N2O4/NO2 system is equivalent to static equilibrium. (1 mark)
(b) Correct chemistry: the two equilibria are fundamentally different. The idealised static/completed-process model has no continuing conversion for the selected effectively irreversible reaction. It does not imply all molecular motion stops or that every reactant has been consumed. In the sealed flask, N2O4 ⇌ 2NO2 is a reversible reaction in a closed system with measurable amounts of both N2O4 and NO2 present at equilibrium, and both reactions are occurring. This is the definition of dynamic, not static, equilibrium. (1 mark)
(c) Experimental evidence: the coexistence of N2O4 and NO2 does not by itself establish non-zero rates. Trace isotope exchange without a material bulk-composition change can support continuing conversion. A reversible response to a temperature change is also consistent with an equilibrium system; constant concentration alone is insufficient. (1 mark)
1 mark, Clarity and coherence of argument throughout.