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 idealised completed-process state: the selected effectively irreversible reaction has no ongoing forward or reverse conversion under the stated conditions; particles still undergo thermal motion. | |
| 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. 9 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 is to 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 forward 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 Does a rusted appearance alone show that an iron nail on a bench is at static equilibrium? Explain.
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. In the model considered here it requires a closed system. Ongoing matter exchange can maintain a non-equilibrium steady state even when measured concentrations are 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 • equal • non-zero • reversible • closed • 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. Ongoing loss of reactants or products can prevent the closed-system equilibrium being modelled. A maintained flow can produce a steady state, which is not necessarily equilibrium.
Q4.2, Opening a sealed sparkling water bottle
When the bottle is opened, CO2(g) escapes to the atmosphere, the system becomes open. The dissolution rate (CO2(g) → CO2(aq)) decreases as the CO2 pressure above the liquid falls. Outgassing now exceeds dissolution, so CO2 continues to leave the liquid until the drink goes flat.
Q4.3, What a rusted appearance can show
No. A rusted surface does not show that all available iron has reacted or that corrosion has stopped. In moist air, further iron can continue to oxidise. A completed effectively irreversible reaction can model static behaviour only when completion is stipulated; even then particles still undergo thermal motion. Dynamic equilibrium instead involves continuing forward and reverse conversion at equal non-zero rates.
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; after a limiting reactant is exhausted the selected process has no ongoing conversion (forward rate = reverse rate = 0 in this idealised completed-process model; thermal motion continues) rather than dynamic equilibrium (forward rate = reverse rate ≠ 0).
Q5, Sample concept map
Arrows should include (any chemically valid linking phrases accepted):
- closed systemis required for → dynamic equilibrium
- reversible reactionis required for → dynamic equilibrium
- dynamic equilibriummeans forward rate equals reverse rate at → forward rate and reverse rate
- forward rateequals → reverse rate (bidirectional, both non-zero)
- dynamic equilibriumresults in → constant concentrations
- closed systemprevents escape that would change → constant concentrations
Award 1 mark per correctly labelled, directionally accurate arrow (max 5).