Chemistry • Year 12 • Module 5 • Lesson 1
Static vs Dynamic Equilibrium
Lock in the core vocabulary, the static vs dynamic distinction, the two conditions for dynamic equilibrium, and the signature shape of rate-vs-time graphs.
1. Term–definition match
The ten definitions below are shuffled. Write the matching term from this list in the right-hand column: static equilibrium, dynamic equilibrium, closed system, open system, macroscopic property, forward reaction rate, reverse reaction rate, reversible reaction, irreversible reaction, net change. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | An equilibrium state where both the forward and reverse reaction rates are equal and non-zero; concentrations remain constant but molecular activity continues. | |
| 1.2 | An equilibrium state where an irreversible reaction has gone to completion; both forward and reverse rates are zero and no molecular activity occurs. | |
| 1.3 | A system in which matter cannot enter or leave, although energy exchange with surroundings is permitted. | |
| 1.4 | A system in which matter can enter or leave; cannot support dynamic equilibrium. | |
| 1.5 | An observable bulk property such as colour, pressure, or concentration that remains constant at equilibrium. | |
| 1.6 | The rate at which reactants are converted to products; starts at its maximum when only reactants are present. | |
| 1.7 | The rate at which products are converted back to reactants; starts at zero when only reactants are present. | |
| 1.8 | A reaction that can proceed in both directions and is written with a double-headed arrow (⇌). | |
| 1.9 | A reaction that proceeds in one direction only and cannot be driven backwards under normal conditions; goes to completion. | |
| 1.10 | The overall change in a system; equals zero at equilibrium even though molecular activity may continue. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 12 marks (1 for T/F, 1 for each correction)
2.1 At dynamic equilibrium, the concentrations of reactants and products are equal. T / F
2.2 Static equilibrium occurs when an irreversible reaction has gone to completion; the forward rate and the reverse rate are both zero. T / F
2.3 Dynamic equilibrium can be established in an open system provided the reaction is reversible. T / F
2.4 On a rate-vs-time graph for a reversible reaction starting with pure reactants, the reverse rate curve starts at zero and increases until both rates meet at a constant non-zero value. T / F
2.5 The forward reaction rate curve on a rate-vs-time graph falls to zero when dynamic equilibrium is reached. T / F
2.6 A sealed bottle of sparkling water in which CO2(g) ⇌ CO2(aq) is an example of dynamic equilibrium in a closed system. T / F
3. Fill the blank — approaching equilibrium
Complete the passage below using the word bank. Each word is used once only. 10 marks
Word bank: closed • concentrations • decreases • dynamic • equal • forward • increases • non-zero • products • reversible
A system reaches equilibrium when the reaction rate equals the reverse reaction rate and both rates are . This can only occur if the reaction is and the system is . At the start of the reaction, the rate is at its maximum because reactant concentrations are high. As reactants are consumed, the forward rate . Simultaneously, as accumulate, the reverse rate from zero. Equilibrium is established when the of all species become constant over time.
4. Function recall
Answer each in 1–2 sentences using precise lesson terms. 8 marks (2 each)
4.1 What is the function of a closed system in establishing dynamic equilibrium?
4.2 What happens at the particle level when a sealed bottle of sparkling water is opened?
4.3 Why does the rusted nail on a bench represent static equilibrium rather than dynamic equilibrium?
4.4 Why must a reaction be reversible for dynamic equilibrium to be established?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “requires”, “produces”, “prevents”, “equals at”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: closed system · reversible reaction · dynamic equilibrium · forward rate · reverse rate · constant concentrations.
Q1 — Term–definition matches
1.1 dynamic equilibrium • 1.2 static equilibrium • 1.3 closed system • 1.4 open system • 1.5 macroscopic property • 1.6 forward reaction rate • 1.7 reverse reaction rate • 1.8 reversible reaction • 1.9 irreversible reaction • 1.10 net change.
Q2 — True/False with correction
2.1 False. Correction: at dynamic equilibrium the rates of the forward and reverse reactions are equal, not the concentrations. Concentrations remain constant but are usually unequal.
2.2 True.
2.3 False. Correction: dynamic equilibrium cannot be established in an open system. It requires a closed system so that concentrations can stabilise — in an open system, products escape (or reactants are added continuously) and concentrations cannot remain constant.
2.4 True.
2.5 False. Correction: at dynamic equilibrium, the forward rate curve levels off at a constant non-zero value equal to the reverse rate. It does not reach zero — that would indicate static equilibrium, where the reaction has stopped entirely.
2.6 True.
Q3 — Cloze paragraph
In order: dynamic • forward • non-zero • reversible • closed • forward • decreases • products • increases • concentrations.
Q4.1 — Function of a closed system
A closed system prevents matter from entering or leaving, which allows the concentrations of all species to stabilise over time. Without this containment, reactants or products could escape and concentrations could never reach a steady state — so dynamic equilibrium could not be established.
Q4.2 — Opening a sealed sparkling water bottle
When the bottle is opened, CO2(g) escapes to the atmosphere — the system becomes open. The reverse reaction rate (CO2 redissolving) decreases because CO2 gas concentration falls. The forward rate (CO2 escaping from solution) now exceeds the reverse rate, so CO2 continues to leave the liquid until the drink goes flat.
Q4.3 — Rusted nail = static equilibrium
The oxidation of iron is an irreversible reaction: once Fe2O3 forms, the reverse reaction (reducing iron oxide back to iron) does not occur spontaneously under normal conditions. Both the forward and reverse rates are zero — the system is at rest at the molecular level, which is the defining property of static equilibrium.
Q4.4 — Why reversibility is required
Dynamic equilibrium requires a non-zero reverse rate. If the reaction is irreversible, there is no reverse process to oppose the forward reaction; the forward reaction runs to completion and the system reaches static equilibrium (forward rate = reverse rate = 0) rather than dynamic equilibrium (forward rate = reverse rate ≠ 0).
Q5 — Sample concept map
Arrows should include (any chemically valid linking phrases accepted):
- closed system — is required for → dynamic equilibrium
- reversible reaction — is required for → dynamic equilibrium
- dynamic equilibrium — means forward rate equals reverse rate at → forward rate and reverse rate
- forward rate — equals → reverse rate (bidirectional, both non-zero)
- dynamic equilibrium — results in → constant concentrations
- closed system — prevents escape that would change → constant concentrations
Award 1 mark per correctly labelled, directionally accurate arrow (max 5).