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Year 12 Chemistry Module 5 ⏱ ~35 min Lesson 1 of 18

️ Static vs Dynamic Equilibrium

In 1864, Scottish chemist Alexander Williamson demonstrated to the Chemical Society of London that ether synthesis was reversible, measuring that the same equilibrium concentration of diethyl ether formed whether he started from ethanol or from ether itself, proving that two opposing reactions were running simultaneously at identical rates.

Today's hook, In 1864, Alexander Williamson showed the Royal Chemical Society that the same ratio of ether to ethanol formed regardless of starting material, proving both reactions ran at once. That single experiment defined what 'dynamic equilibrium' means.

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Worksheets

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Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Before You Read

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.

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📐 Formulas at a Glance

Static equilibrium: forward rate = 0, reverse rate = 0

Reaction has gone to completion; no molecular activity Irreversible reaction only

Dynamic equilibrium: forward rate = reverse rate ≠ 0

Concentrations constant but molecular activity continues Requires: closed system + reversible reaction

No calculation formulas this lesson, equilibrium is conceptual here.

Learning Intentions

Know

Key facts

The definition of static and dynamic equilibrium
The two conditions required for dynamic equilibrium
The distinction between open and closed systems

Understand

Concepts

Why dynamic equilibrium requires molecular activity in both directions
Why an open system cannot reach dynamic equilibrium
How to read and draw rate-vs-time and concentration-vs-time graphs

Can do

Skills

Classify systems as static equilibrium, dynamic equilibrium, or neither
Describe particle diagrams at three stages: start, intermediate, equilibrium
Explain the sparkling water and rusted nail examples in full chemical language

Scan these before reading

Static equilibrium

A state where no net change occurs and no opposing process is running underneath; e.g., a balanced beam, or a reaction that has gone to completion.

Dynamic equilibrium

A state where forward and reverse reaction rates are equal and concentrations remain constant.

Open system

A system that can exchange both matter and energy with surroundings.

Closed system

A system where matter cannot enter or leave, though energy may be exchanged.

Macroscopic property

An observable bulk property (colour, pressure, concentration) that stays constant at equilibrium.

Rate of reaction

The change in concentration of a reactant or product per unit time.

Misconceptions to Fix

Predict then reveal+8 XP

1 · Predict

2 · Reveal

3 · Compare

A sealed bottle of sparkling water sits on a bench. The CO&sub2; pressure gauge reads the same every minute. Predict: is this system at static equilibrium or dynamic equilibrium, and what is happening at the molecular level?

Confidence 50%

Static equilibrium

Static Equilibrium, When a Reaction Finishes

Irreversible reactions · Forward rate = Reverse rate = 0

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 a system in which no net change is occurring and there is no opposing process running underneath. It covers physical systems held at rest by balanced forces, such as a balanced beam, and it covers chemical systems in which the reaction has finished.

The chemical case is the one this module builds on: after an irreversible reaction has gone to completion, there are no reactants remaining to react further and the products are stable under the conditions. The forward rate has fallen to zero because the reactants are exhausted, and the reverse reaction does not occur.

Be precise about what has stopped. The chemical change has stopped, not the molecules. Particles still move, vibrate and collide, they simply no longer produce any net reaction. That is exactly what separates this from dynamic equilibrium, where two opposing reactions are still running at equal rates.

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.

Static Equilibrium

Irreversible reaction gone to completion

Forward rate = 0; reverse rate = 0

Only products remain; reactants fully consumed

No molecular activity at equilibrium

Can occur in open or closed systems

Dynamic Equilibrium

Reversible reaction (⇌) in a closed system

Forward rate = reverse rate ≠ 0

Both reactants AND products present at constant concentrations

Constant molecular activity; macroscopic stillness

Requires a closed system only

Must know

Static equilibrium does NOT mean equal amounts of reactants and products, it means the reaction has finished and only products remain. The term "equilibrium" here simply means the macroscopic state is stable.

Common error

Students often assume static equilibrium is just a slow version of dynamic equilibrium. It is not, they are fundamentally different. In static equilibrium the reaction has stopped entirely. In dynamic equilibrium, molecular-level reactions continue at equal rates in both directions simultaneously.

What to write in your book

Static equilibrium: irreversible reaction gone to completion; forward rate = reverse rate = 0; no molecular activity; products only remain
Examples: burning Mg ribbon, neutralisation of strong acid/base, CaCO₃ decomposition in open system
The term "equilibrium" here means macroscopic state is stable, NOT that concentrations are equal

At static equilibrium, the reaction continues at a very slow rate.

Dynamic equilibrium

Dynamic Equilibrium, The Busy Standstill

Reversible reactions in closed systems · Both rates non-zero and equal

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:

The reaction must be reversible (written with ⇌)
The system must be closed (no matter enters or leaves)

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.

Dynamic Equilibrium

Reversible reaction (⇌) in a closed system

Forward rate = reverse rate (both non-zero)

Concentrations constant, not necessarily equal

Ongoing molecular interchange (reactants ⇌ products)

Cannot occur in open systems

Static Equilibrium

Irreversible reaction that has stopped completely

Forward rate = 0; reverse rate = 0

Only products remain; no molecular activity

Macroscopic AND microscopic activity both zero

Open or closed systems

Must know

Dynamic equilibrium requires a CLOSED system. If the system is open, if products can escape or reactants can be added from outside, the system cannot reach dynamic equilibrium because the concentrations cannot stabilise.

Common error

"Equilibrium means equal concentrations of reactants and products." This is one of the most persistent misconceptions in Year 12 Chemistry and it is wrong. At dynamic equilibrium, the RATES of the forward and reverse reactions are equal, the concentrations can be very different from each other. A reaction with K_eq = 10⁶ is at dynamic equilibrium with almost entirely products present.

Insight

The sparkling water in the hero is a near-perfect example. In a sealed bottle, CO₂(g) ⇌ CO₂(aq), CO₂ is dissolving into the liquid at the same rate as it is escaping back into the gas space. Open the bottle, the system is now open, CO₂ escapes without being replaced, and the dynamic equilibrium is destroyed. The drink goes flat.

What to write in your book

Dynamic equilibrium: forward rate = reverse rate ≠ 0; reversible reaction in a closed system
Concentrations are CONSTANT, not equal; molecular activity is continuous
Two conditions: (1) reversible reaction (⇌) and (2) closed system
Example: N₂O₄(g) ⇌ 2NO₂(g) in a sealed flask, stable brown colour despite ongoing reactions

At dynamic equilibrium, which of the following is correct?

Open vs closed systems

Open vs Closed Systems

The gateway condition for dynamic equilibrium

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.

Open System

Yes, matter enters or leaves

Yes

Log fire, human body, open beaker

Closed System

No, matter is contained

Yes

Sealed flask, sealed bottle, industrial reactor

HSC tip

In HSC questions asking whether a system can reach dynamic equilibrium, always check two things: (1) Is the reaction reversible? (2) Is the system closed? Both must be true.

Common error

Students sometimes confuse open/closed with isolated systems. An isolated system allows neither matter nor energy to exchange. For Module 5, you only need open (matter can leave) and closed (matter cannot leave), isolated is not tested here.

What to write in your book

Closed system: matter cannot enter/leave (energy can); sealed flask, sealed bottle
Open system: matter can enter/leave; log fire, human body, open beaker
Dynamic equilibrium ONLY possible in a CLOSED system
HSC checklist: (1) reversible? (2) closed system? Both required for dynamic equilibrium

Which of the following can reach dynamic equilibrium?

VISUAL SUMMARY

Feature-by-feature comparison of static (irreversible) versus dynamic (reversible) equilibrium.

Static vs Dynamic Equilibrium, key differences at a glance

Rate-vs-time graphs

Rate-vs-Time Graphs at Equilibrium

Core HSC graphical skill, appears repeatedly in Module 5

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:

At time zero: the forward rate is at its maximum (high reactant concentration → frequent collisions); the reverse rate is zero (no products yet).
As the reaction proceeds: the forward rate decreases as reactants are consumed; the reverse rate increases as products accumulate.
At equilibrium: both curves meet at the same non-zero value and remain horizontal.

The forward rate falls and the reverse rate rises until they become equal at equilibrium.

Rate-vs-time: forward rate starts high and falls; reverse rate starts at zero and rises; both meet at a non-zero equilibrium rate

Must know

On a rate-vs-time graph, equilibrium is where the two curves MEET AND BECOME EQUAL, not where either curve reaches zero. Both rates remain non-zero at equilibrium. A curve touching the x-axis would represent static equilibrium, not dynamic.

Common error

Students draw the forward rate curve falling to zero at equilibrium. This is wrong, both rates are non-zero and equal at equilibrium. If the forward rate fell to zero, the system would be at static equilibrium.

What to write in your book

Rate-vs-time graph: forward rate starts max → decreases; reverse rate starts 0 → increases
Equilibrium point: where both curves MEET at same NON-ZERO value and go horizontal
Both rates non-zero at equilibrium, never draw forward rate touching zero unless it's static equilibrium

On a rate-vs-time graph, the equilibrium point is where the forward rate _____ the reverse rate, and both become _____.

Cross-lesson links: The rate-vs-time graph you read in Card 4 reappears in L03 (collision theory at equilibrium) and L07 (Haber process graphs). The static-vs-dynamic distinction introduced here underpins every equilibrium shift question in L05–L11. Particle diagrams from this card are tested in L04 (misconception deep-dive).

Particle diagrams

Particle Diagrams, Modelling Equilibrium

Three snapshots: start · intermediate · equilibrium

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:

What you see

All particles are reactant molecules

Mix of reactants and products; ratio changing

Ratio of reactants to products is stable

What it means

Forward rate at maximum; reverse rate = 0

Forward rate still > reverse rate

Forward rate = reverse rate (both non-zero)

The key insight is that the ratio at equilibrium depends on the specific reaction. For some reactions (large K_eq), almost all particles are products; for others (small K_eq), almost all are reactants. The particle diagram does NOT show equal numbers of reactant and product particles unless K_eq ≈ 1.

Must know

When drawing or interpreting particle diagrams, label your three snapshots explicitly: t = 0 (start), t = intermediate (approaching equilibrium), t = equilibrium (constant ratio). The particle counts in the equilibrium snapshot must be consistent with the K_eq of the specific reaction.

Insight

The same equilibrium position is reached regardless of whether you start with all reactants, all products, or a mixture of both, as long as the total amounts of atoms present are the same. This is a profound property of dynamic equilibrium that distinguishes it from static equilibrium.

What to write in your book

Three snapshots: t=0 (all reactants), t=intermediate (ratio changing), t=equilibrium (stable ratio)
Equilibrium ratio does NOT have to be equal, depends on K_eq
Same equilibrium reached from either direction (reactants or products side)

A reaction has K_eq = 10⁶. At equilibrium, a particle diagram would show:

Worked examples and activities

✏️ Worked Examples

Worked Example 1, Identifying static vs dynamic equilibrium

For each scenario, identify whether the system is at static equilibrium, dynamic equilibrium, or neither. Justify your answer.

(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.

Worked Example 2, Interpreting a rate-vs-time graph

A rate-vs-time graph shows two curves for a reversible reaction. Curve A starts at a high value and decreases to a constant non-zero value. Curve B starts at zero and increases to the same constant non-zero value as Curve A.

(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.

Copy Into Your Books

Definitions

Static equilibrium: irreversible reaction gone to completion; forward rate = reverse rate = 0
Dynamic equilibrium: reversible reaction in a closed system where forward rate = reverse rate ≠ 0
Closed system: matter cannot enter or leave (energy exchange permitted)
Open system: matter can enter or leave; cannot achieve dynamic equilibrium

Conditions for Dynamic Equilibrium

Reversible reaction (written with ⇌)
Closed system (no matter escapes)
Sufficient time for rates to equalise
Both forward and reverse rates are equal AND non-zero

Rate-vs-Time Graph Key Features

Forward rate starts at maximum, decreases as reactants consumed
Reverse rate starts at zero, increases as products accumulate
Equilibrium: both curves meet at same non-zero value and become horizontal
After equilibrium: both rates remain constant and equal (non-zero)

Common Exam Errors, Avoid These

Saying "equilibrium means equal concentrations", WRONG; rates are equal
Saying "the reaction has stopped" at dynamic equilibrium, WRONG; both rates non-zero
Drawing forward rate curve falling to zero, WRONG at dynamic equilibrium
Saying open system can reach dynamic equilibrium, WRONG

Activities

Identifying Equilibrium Systems

For each system below, classify it as static equilibrium, dynamic equilibrium, or neither. Then write a justification of one to two sentences explaining your answer.

Classification

Your answer

Justification

Your answer

✓

Practice questions

Multiple Choice

+5 XP

A fresh set drawn from this lesson's question bank, with feedback shown immediately. +5 XP per correct · +25 XP all correct

Short Answer

UnderstandRecall(4 marks) 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.

ApplyApply(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.

AnalyseAnalyse(5 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)

Show all answers

Short Answer Model Answers

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 [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 is less than before. The new equilibrium will have a lower dissolved CO₂ concentration, the drink will be slightly less fizzy [1].

✓

Retrieve and reflect

Check what actually stuck

Extended Response

Compare and contrast static equilibrium and dynamic equilibrium. In your response, discuss: (a) the types of reactions that lead to each; (b) the molecular-level activity at each type of equilibrium; (c) how each would appear on a rate-vs-time graph; and (d) use specific chemical examples for each. (8 marks)

How did your thinking change?

Look back at what you wrote in Think First. Recall Williamson's 1864 experiment: he measured the same ether-to-ethanol ratio whether he started from pure ethanol or pure ether, the forward and reverse reactions reaching the same equilibrium from opposite directions. Can you now explain in full chemical language why his result proves both reactions were running simultaneously? What did you get right? What surprised you?

I have completed this lesson

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