Chemistry • Year 12 • Module 5 • Lesson 4
Equilibrium in Context: Analogies & Misconception Deep-Dive
Lock in the core vocabulary, sort correct from incorrect descriptions of dynamic equilibrium, and map the two key analogies before tackling harder application questions.
1. Term–definition match
Match each term on the left to its correct definition. Write the matching term in the right-hand column. Terms: dynamic equilibrium, static equilibrium, equilibrium constant (Keq), reaction quotient (Q), forward reaction rate, reverse reaction rate, closed system, Le Chatelier’s Principle, catalyst, collision theory. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A state where both forward and reverse reactions continue at equal, non-zero rates, so macroscopic properties remain constant. | |
| 1.2 | A state where a reaction has completely stopped — the forward rate is zero and the reverse rate is zero. | |
| 1.3 | The ratio of product to reactant concentrations at equilibrium, raised to stoichiometric powers; depends only on temperature. | |
| 1.4 | The same ratio as Keq but calculated at any point in time; compared with Keq to predict the direction of net reaction. | |
| 1.5 | The rate at which reactants convert to products. | |
| 1.6 | The rate at which products convert back to reactants. | |
| 1.7 | A container or vessel from which no matter can enter or leave; essential for equilibrium to be established. | |
| 1.8 | The principle stating that a system at equilibrium will shift to counteract any applied stress (disturbance). | |
| 1.9 | A substance that lowers activation energy for both forward and reverse reactions by the same amount, speeding up the rate of reaching equilibrium but not changing Keq. | |
| 1.10 | The model explaining that reactions occur when particles collide with sufficient energy and correct orientation. |
2. True or False — with correction
Circle T or F for each statement. If false, write a precise correction on the line below. 12 marks (1 for T/F, 1 for each correction)
2.1 At dynamic equilibrium, the concentrations of all species are equal to each other. T / F
2.2 At dynamic equilibrium, both the forward and reverse reactions are still occurring at equal, non-zero rates. T / F
2.3 Adding a catalyst to a system at equilibrium shifts the equilibrium position and increases the yield of products. T / F
2.4 The same equilibrium position is reached whether you start with all reactants, all products, or a mixture — provided the total atomic composition is the same. T / F
2.5 On a concentration-vs-time graph, equilibrium is reached at the point where the reactant and product concentration curves cross. T / F
2.6 Keq can be changed by adding more reactant to the equilibrium system. T / F
3. Fill the blanks — dynamic equilibrium in a saturated NaCl solution
Use the word bank to fill each blank. Each word is used once only. 8 marks (1 per blank)
When solid NaCl is added to water until no more dissolves, the system appears unchanged. Students who think nothing is happening are confusing this with ________ equilibrium. In reality, the system is at ________ equilibrium: Na+ and Cl− ions are constantly leaving the crystal ________ (a process called ________) at the same rate as ions from solution return to the crystal surface (a process called ________). Because these two rates are ________, the net change in the amount of dissolved NaCl is ________, and macroscopic properties appear constant — yet molecular activity is ________.
4. Map the escalator analogy to chemistry
The table below lists features of the escalator analogy for dynamic equilibrium. In the right column, write the matching chemical concept. 8 marks (1 each)
| Escalator analogy feature | Matching chemical concept |
|---|---|
| People on Floor 1 (reactant floor) | |
| People on Floor 2 (product floor) | |
| The up-escalator | |
| The down-escalator | |
| 50 people riding up per minute = 50 riding down per minute | |
| Constant number of people on each floor | |
| Adding 100 extra people to Floor 1 | |
| Fixed escalator speeds (cannot vary independently) |
5. Function recall
Answer each in 1–2 precise sentences using lesson language. 8 marks (2 each)
5.1 Why does adding a catalyst to an equilibrium system not change the equilibrium position?
5.2 What does it mean for a reaction to be at dynamic equilibrium, as opposed to static equilibrium?
5.3 Why do the concentrations of reactants and products remain constant at dynamic equilibrium?
5.4 On a concentration-vs-time graph, how do you correctly identify the point at which dynamic equilibrium has been established?
6. Build a concept map
Draw labelled arrows between the six terms below. Each arrow must carry a linking phrase (e.g. “equals at equilibrium”, “does not change”, “results from”). Aim for at least 6 labelled arrows. 6 marks (1 per correct labelled arrow)
Terms: forward reaction rate • reverse reaction rate • dynamic equilibrium • constant concentrations • Keq • temperature
Q1 — Term/definition matches
1.1 dynamic equilibrium • 1.2 static equilibrium • 1.3 equilibrium constant (Keq) • 1.4 reaction quotient (Q) • 1.5 forward reaction rate • 1.6 reverse reaction rate • 1.7 closed system • 1.8 Le Chatelier’s Principle • 1.9 catalyst • 1.10 collision theory.
Q2 — True / False with correction
2.1 False. At dynamic equilibrium the rates of forward and reverse reactions are equal, not the concentrations. Concentrations are constant but their values depend on Keq and can be very different from each other.
2.2 True.
2.3 False. A catalyst increases the rate of reaching equilibrium but does not change the equilibrium position or Keq. It lowers activation energy equally for forward and reverse reactions, leaving the ratio of rates (and therefore Keq) unchanged.
2.4 True.
2.5 False. Equilibrium is identified where all concentration curves simultaneously become horizontal (flat), not where they cross. The crossing point only coincides with equilibrium when Keq ≈ 1 (equal concentrations at equilibrium), which is a special case.
2.6 False. Keq depends only on temperature. Adding more reactant shifts the equilibrium position (Q < Keq, net forward reaction) but does not change Keq itself.
Q3 — Cloze paragraph
In order: static • dynamic • lattice • dissolution • recrystallisation • equal • zero • ongoing.
Q4 — Escalator analogy mapping
People on Floor 1 → concentration of reactants. People on Floor 2 → concentration of products. Up-escalator → forward reaction. Down-escalator → reverse reaction. 50 up = 50 down → forward rate equals reverse rate (dynamic equilibrium). Constant numbers on each floor → constant concentrations at equilibrium. Adding 100 people to Floor 1 → adding reactant (Le Chatelier’s shift — temporary increase in forward rate). Fixed escalator speeds → limitation of the analogy: in chemistry, reaction rates are not fixed but vary with concentration and temperature.
Q5 — Function recall
5.1 A catalyst lowers the activation energy for both forward and reverse reactions by the same amount. Because both rates increase by the same factor, if the system is already at equilibrium, the rates remain equal — no net shift occurs and Keq is unchanged.
5.2 At dynamic equilibrium, both forward and reverse reactions continue to occur simultaneously at equal, non-zero rates. Macroscopic properties (concentration, colour, pressure) appear constant not because nothing is happening, but because the rate of change in each direction cancels out. Static equilibrium, by contrast, means all reaction has completely stopped.
5.3 Concentrations remain constant because the rate at which each species is being produced exactly equals the rate at which it is being consumed. Forward production of products equals reverse consumption of products; forward consumption of reactants equals reverse production of reactants.
5.4 Equilibrium is established at the moment all concentration curves simultaneously become horizontal (cease to change). This is not necessarily where the curves cross — that only corresponds to equilibrium if Keq ≈ 1.
Q6 — Sample concept map
Correct arrows include: forward reaction rate equals at equilibrium → reverse reaction rate; this equality defines → dynamic equilibrium; dynamic equilibrium produces → constant concentrations; Keq is set by → temperature; Keq determines the ratio of → constant concentrations; temperature does not change → forward/reverse reaction rate equally (via catalyst). Accept any chemically valid linking phrases. Full marks for 6+ correctly directed, labelled arrows.