Rust forms on iron and never turns back — but in a sealed bottle of fizzy drink, CO₂ is dissolving and escaping simultaneously at the molecular level, even though the pressure gauge reads the same every second.
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A rusted iron nail sits on a bench. A sealed bottle of sparkling water sits next to it. Both appear completely unchanged — nothing visible is happening in either system.
But chemists would say one of these is at static equilibrium and the other is at dynamic equilibrium. Before reading on — which is which, and what do you think the difference actually means at the particle level? Write your reasoning now. You will come back to this at the end of the lesson and evaluate whether your instinct was correct.
📚 Core Content
Wrong: At equilibrium, the concentrations of reactants and products are equal.
Right: At equilibrium, the forward and reverse reaction rates are equal, not the concentrations. The concentrations remain constant but are usually unequal. The equilibrium position depends on Keq and initial conditions — it can favour reactants, products, or be roughly equal.
Static equilibrium is the chemical equivalent of a finished race — the runners have stopped, the result is fixed, and nothing is going to change unless something external intervenes.
Static equilibrium describes the state of a system after an irreversible reaction has gone to completion. There are no reactants remaining to react further, and the products are stable under the conditions. At static equilibrium there is no molecular activity — the forward reaction rate has fallen to zero (reactants exhausted), and the reverse reaction does not occur because the reaction is irreversible.
From both a macroscopic and a microscopic perspective, everything has stopped. The system is truly at rest.
Examples of static equilibrium: burning magnesium ribbon in air (once Mg is consumed, MgO remains, no reverse reaction); neutralisation of a strong acid with a strong base to completion (NaCl and water form and remain); decomposition of CaCO₃ in an open system where CO₂ escapes.
Dynamic equilibrium is chemistry's most counterintuitive idea — a system that looks completely still from the outside is actually a scene of constant molecular activity, with reactions running simultaneously in both directions.
Dynamic equilibrium occurs in a reversible reaction in a closed system when the forward reaction rate equals the reverse reaction rate — and both rates are non-zero. The concentration of every species remains constant over time, but this constancy is not because nothing is happening — it is because reactants are being converted to products at exactly the same rate as products are being converted back to reactants.
Net change is zero, but molecular change is constant. This is the critical distinction: macroscopic constancy does not mean microscopic stillness.
Two conditions required for dynamic equilibrium:
Example: In a sealed container, N₂O₄(g) ⇌ 2NO₂(g) reaches dynamic equilibrium when the rate of N₂O₄ decomposing to NO₂ equals the rate of NO₂ combining to form N₂O₄. The brown colour of the mixture stabilises — not because the reaction has stopped, but because the two processes cancel each other out.
Whether a system can reach dynamic equilibrium is determined entirely by whether it is open or closed — and this distinction maps directly onto whether matter can enter or leave the system.
A closed system is one in which matter cannot enter or leave, although energy (heat) can be exchanged with the surroundings. Closed systems can reach dynamic equilibrium because concentrations can stabilise — there is no mechanism for reactants or products to escape. A sealed flask, a closed bottle, or a sealed reaction vessel are closed systems.
An open system is one in which matter can enter or leave. Open systems cannot reach dynamic equilibrium because products can escape (or reactants can be continuously added), preventing concentration from stabilising. A log fire, a car exhaust, and the human body are all open systems.
Static vs Dynamic Equilibrium — key differences at a glance
The approach to dynamic equilibrium has a characteristic graphical signature — and being able to read and draw this graph is a core HSC skill that appears repeatedly across Module 5.
A rate-vs-time graph for a reversible reaction approaching equilibrium has two curves:
Rate-vs-time forward rate starts high and falls; reverse rate starts at zero and rises; both meet at a non-zero equilibrium rate
Particle diagrams make the abstract concrete — by counting the number of reactant and product particles at different points in time, you can see equilibrium as a property of the whole system rather than any individual molecule.
A particle diagram for a reversible reaction approaching equilibrium shows three snapshots:
The key insight is that the ratio at equilibrium depends on the specific reaction. For some reactions (large Keq), almost all particles are products; for others (small Keq), almost all are reactants. The particle diagram does NOT show equal numbers of reactant and product particles unless Keq ≈ 1.
✏️ Worked Examples
(a) A sealed flask containing H₂(g) and I₂(g) has been left for several hours at 450°C. The colour has stopped changing.
(b) A campfire has burned all its wood fuel and the ash is sitting cold on the ground.
(c) A beaker of water is evaporating in a warm room.
The reaction H₂(g) + I₂(g) ⇌ 2HI(g) is reversible (⇌). The system is closed (sealed flask). The colour has stopped changing → macroscopic properties are constant. Both conditions for dynamic equilibrium are met.
→ Dynamic equilibrium.
Combustion of wood is an irreversible reaction (large negative ΔG — products far more stable). All fuel has been consumed — the reaction has gone to completion. No reverse reaction occurs. Forward rate = 0, reverse rate = 0.
→ Static equilibrium.
The beaker is open — water vapour can escape to the surroundings and is not contained. This is an open system. Evaporation continues without the reverse process (condensation) catching up — the system cannot reach dynamic equilibrium. The water will eventually all evaporate.
→ Neither — open system, non-equilibrium.
Summary: (a) Dynamic equilibrium — reversible reaction in a closed system with stable macroscopic properties. (b) Static equilibrium — irreversible reaction gone to completion, all molecular activity has ceased. (c) Neither — open system, cannot reach dynamic equilibrium, water will completely evaporate.
(a) Which curve represents the forward reaction rate and which represents the reverse? (b) At what point on the graph is dynamic equilibrium first established? (c) What would the graph look like if, after equilibrium was established, more reactant were added to the closed system?
Curve A starts high (maximum reactant concentration, maximum forward rate) and decreases as reactants are consumed → Curve A is the forward reaction rate.
Curve B starts at zero (no products initially, reverse rate = 0) and increases as products accumulate → Curve B is the reverse reaction rate.
Dynamic equilibrium is first established at the point where Curve A and Curve B meet and become equal — where both rates have the same non-zero value. This is the point where the curves intersect and both become horizontal.
Adding more reactant increases the concentration of reactants → forward rate increases immediately (Curve A spikes upward). Reverse rate is initially unchanged. Forward rate > reverse rate → system is no longer at equilibrium.
Over time, forward rate decreases (reactants consumed) and reverse rate increases (more products forming) until they equalise again at a new higher equilibrium rate.
A sudden upward spike in Curve A, followed by both curves settling to a new constant equal value — slightly higher than the original equilibrium rate.
Summary: (a) Curve A = forward rate; Curve B = reverse rate. (b) Equilibrium is established where the curves first intersect and both become horizontal. (c) Adding reactant causes a temporary spike in the forward rate curve; both curves then re-equalise at a new, slightly higher constant value.
🧪 Activities
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
❓ Multiple Choice
1. Which of the following correctly distinguishes dynamic equilibrium from static equilibrium?
2. A sealed flask containing SO₂(g), O₂(g), and SO₃(g) at 600°C shows identical concentrations when measured every 10 minutes. Which statement best explains this?
3. Which of the following is a necessary condition for dynamic equilibrium to be established?
4. On a rate-vs-time graph for a reversible reaction starting with pure reactants, which of the following correctly describes the reverse reaction rate curve?
5. A student claims: "The rusted nail and the sealed bottle of sparkling water are both at equilibrium because neither appears to be changing." Evaluate this claim.
✍️ Short Answer
6. Distinguish between an open system and a closed system. Explain why dynamic equilibrium can only be established in a closed system. Give one example of each type of system. 4 MARKS
7. A rate-vs-time graph for the reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) shows the forward rate curve touching the x-axis (reaching zero) at equilibrium. Identify and explain the error in this graph. Draw a corrected qualitative description of what the graph should look like. 4 MARKS
8. Real-World Application: A sealed bottle of sparkling water contains dissolved CO₂ in equilibrium with CO₂ gas in the headspace: CO₂(g) ⇌ CO₂(aq). A student opens the bottle and it goes flat within minutes.
(a) Explain why the sealed bottle is at dynamic equilibrium but the opened bottle is not. Use the concepts of open/closed system and forward/reverse rates. (3 marks)
(b) If the bottle is resealed immediately after opening, will it return to exactly the same equilibrium position? Explain your reasoning. (2 marks)5 MARKS
Go back to your Think First response at the top of this lesson. Now that you've studied static and dynamic equilibrium:
1. CO₂ in sealed can: Dynamic equilibrium. CO₂(g) ⇌ CO₂(aq) is a reversible process in a closed system; the rate of dissolution equals the rate of escape; constant pressure confirms stable concentrations.
2. Zinc + HCl, open flask: Static equilibrium (approaching it). The reaction Zn + 2HCl → ZnCl₂ + H₂ is irreversible and has gone to completion; H₂ gas has escaped (open system); all reactants consumed.
3. N₂O₄/NO₂ sealed, stable brown: Dynamic equilibrium. The reaction 2NO₂(g) ⇌ N₂O₄(g) is reversible; the sealed flask is a closed system; stable brown colour indicates constant concentrations; both reactions continue at equal rates.
4. Coffee cooling: Neither. This is a physical process (heat transfer) in an open system approaching thermal equilibrium with the room — not a chemical equilibrium in the Module 5 sense.
5. NaCl in saturated solution, sealed flask: Dynamic equilibrium. NaCl(s) ⇌ Na⁺(aq) + Cl⁻(aq) is reversible; sealed flask is closed; rate of dissolution = rate of recrystallisation; the crystal appears unchanged but ion exchange continues at the surface.
1. Forward rate curve: starts at a maximum value (maximum concentration of A and B → maximum collision frequency → maximum rate). Decreases progressively as A and B are consumed and their concentrations fall, reducing the frequency of effective forward collisions.
2. Reverse rate curve: starts at zero because there are no product molecules (C) present at t = 0 — no reverse collisions can occur. As C accumulates, the frequency of reverse collisions increases and the reverse rate rises.
3. Dynamic equilibrium is established when both curves intersect and become horizontal — both rates are equal and non-zero, and neither changes over time.
4. If the flask is opened: C escapes, removing product. This immediately reduces the reverse rate (fewer C molecules → fewer reverse collisions). Forward rate momentarily exceeds reverse rate → net forward reaction → more A and B consumed. As C escapes faster than it is produced, the system never reaches a stable equilibrium — it cannot be at dynamic equilibrium because it is now an open system.
1. B — Dynamic equilibrium is defined by equal, non-zero forward and reverse rates. Static equilibrium is characterised by all rates being zero (reaction complete). Option A is the most common wrong answer — equal rates, not equal concentrations.
2. C — All three species present (not gone to completion) + sealed flask (closed system) + constant concentrations → dynamic equilibrium. Option D is wrong: static equilibrium would mean only products remain with reactants exhausted.
3. C — Dynamic equilibrium requires both conditions: reversible reaction AND closed system. Option A is wrong (equal concentrations not required — equal rates required). Option D is wrong (open system prevents equilibrium).
4. A — Starting with pure reactants means no products initially, so reverse rate = 0. As products form, reverse rate increases and levels off at the same non-zero value as the forward rate at equilibrium.
5. D — The student is partially correct (both are at equilibrium) but misses the crucial distinction: the nail is at STATIC equilibrium (irreversible oxidation complete, all molecular activity zero) while the sparkling water is at DYNAMIC equilibrium (reversible CO₂ dissolution in a closed system, both rates non-zero and equal).
Q6 (4 marks): Open system: matter can enter or leave the system; e.g. a beaker of water evaporating in an open room [1]. Closed system: matter cannot enter or leave (energy exchange permitted); e.g. a sealed flask of gases [1]. Dynamic equilibrium requires a closed system because the concentrations of all species must remain constant for the rates to remain equal [1]. In an open system, products escape (or reactants are added), so one concentration continuously changes — the rates cannot equalise and remain equal indefinitely [1].
Q7 (4 marks): Error: the forward rate curve should not reach zero at equilibrium [1]. At dynamic equilibrium, both the forward and reverse rates are non-zero — the forward reaction (PCl₅ decomposing) and reverse reaction (PCl₃ + Cl₂ recombining) both continue simultaneously [1]. Corrected the forward rate curve starts at a maximum, decreases, and levels off at a constant non-zero value [1]; the reverse rate curve starts at zero, increases, and levels off at the same non-zero value as the forward rate [1]. The equilibrium point is where both curves meet and become horizontal — not where either reaches zero.
Q8 (5 marks): (a) Sealed bottle: closed system — CO₂ cannot escape; CO₂(g) ⇌ CO₂(aq) reaches dynamic equilibrium where rate of CO₂ dissolving = rate of CO₂ escaping from solution [1]. Both rates are non-zero and equal, so concentration remains constant [1]. Opened bottle: now an open system — CO₂(g) escapes to the atmosphere and is not replaced [1]. The reverse rate (re-dissolving) drops; the forward rate (escaping from solution) exceeds it; CO₂ continuously leaves and the equilibrium cannot be maintained — concentration of dissolved CO₂ falls [1]. (b) Will not return to exactly the same equilibrium [1]. Some CO₂ gas has permanently escaped to the atmosphere. When resealed, the total amount of CO₂ in the system (solution + headspace) is less than before. The new equilibrium will have a lower dissolved CO₂ concentration — the drink will be slightly less fizzy than the original [1].
Climb platforms, hit checkpoints, and answer questions on Static vs Dynamic Equilibrium. Quick recall from lessons 1–1.
Tick when you've finished all activities and checked your answers.