What is dynamic equilibrium?

Forward and reverse reaction rates approaching dynamic equilibrium A plot of reaction rate against time. The forward rate starts high and falls. The reverse rate starts at zero and rises. The two curves meet at a constant value and remain equal beyond that point, which is dynamic equilibrium. rate t forward reverse equilibrium reached At equilibrium, forward and reverse rates are equal but non-zero.

NSWChemistrySyllabus dot point

Inquiry Question 1: What happens when chemical reactions do not go through to completion?

Investigate the differences between static and dynamic equilibrium, and reversible and non-reversible reactions, using practical examples

A focused answer to the HSC Chemistry Module 5 dot point on static and dynamic equilibrium. The definitions, the macroscopic vs molecular view, classic practical examples (NO2/N2O4, cobalt complexes, sealed water), and the worked HSC past exam questions.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-22

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

NESA wants you to distinguish static from dynamic equilibrium, contrast reversible and non-reversible reactions, and use practical chemical examples to demonstrate the concept. This is the foundation dot point for Module 5. Every later concept (Le Chatelier, Kc, Ksp, buffers) assumes you understand that equilibrium is dynamic, not static.

The answer

Static equilibrium

Static equilibrium is a state of balance in which no net change is occurring and no process is active at the molecular level. A pencil balanced on a table is in static equilibrium. Two unreactive gases mixed in a sealed flask sit in static equilibrium because nothing is reacting.

Key marker: no microscopic process is happening.

Dynamic equilibrium

Dynamic equilibrium is the state of a reversible reaction where the forward and reverse reactions occur at equal rates. Macroscopic properties (concentration, colour, pressure, mass) remain constant. At the molecular level, however, the forward and reverse reactions continue to occur.

Key marker: rates are equal, so net change is zero, but molecules continue to react.

Reversible vs non-reversible reactions

A non-reversible reaction proceeds to completion in one direction. Combustion of methane is non-reversible under normal conditions because the products (CO2 and H2O) do not spontaneously reform methane.

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

A reversible reaction can proceed in both directions. The double-headed arrow $\rightleftharpoons$ shows this. Given enough time in a closed system, a reversible reaction will reach dynamic equilibrium.

N_2 + 3H_2 \rightleftharpoons 2NH_3

Conditions required for dynamic equilibrium

The system must be closed (no matter escapes).
The reaction must be reversible.
The forward and reverse rates must be equal.
Macroscopic properties must be constant (concentration, colour, pressure).

Worked example

The dimerisation of nitrogen dioxide is a textbook HSC example.

2NO_{2(g)} \rightleftharpoons N_2O_{4(g)}

$NO_2$ is brown; $N_2O_4$ is colourless. Place pure $NO_2$ in a sealed flask. The brown colour fades over time but does not disappear, stabilising at a steady (paler) brown.

At the molecular level, two processes are happening continuously:

Forward: two $NO_2$ molecules collide and combine to form $N_2O_4$ .
Reverse: an $N_2O_4$ molecule dissociates back into two $NO_2$ molecules.

When the rates are equal, the concentrations stop changing and the system has reached dynamic equilibrium. If you heated the flask, the equilibrium would shift back toward $NO_2$ (the colour deepens), evidence that the system is dynamic and not frozen. The direction of that shift is predicted by Le Chatelier's principle.

A second canonical example is the equilibrium between liquid water and water vapour in a sealed bottle. At constant temperature, the rate of evaporation equals the rate of condensation. The water level stays constant even though molecules cross the phase boundary every second.

A third is the cobalt chloride equilibrium, often demonstrated in labs:

[Co(H_2O)_6]^{2+}_{(aq)} + 4Cl^-_{(aq)} \rightleftharpoons [CoCl_4]^{2-}_{(aq)} + 6H_2O_{(l)}

Pink (left) shifts toward blue (right) when chloride concentration rises or temperature increases, and back to pink when water is added. Colour changes prove the system is dynamic.

Examples in context

Example 1. Sealed soft-drink bottle from a Sydney bottling plant. A Coca-Cola bottle filled at the Northmead plant contains carbonated water in dynamic equilibrium: $CO_{2(g)} \rightleftharpoons CO_{2(aq)}$ . The gas above the liquid is pressurised at around 4 atm, dissolving carbon dioxide into solution as fast as it leaves. To an observer the bubbles stop forming after a few seconds and the system looks static, but molecular exchange continues at high rate. Open the bottle and the partial pressure of $CO_2$ above the liquid drops, the equilibrium shifts to the gas phase, and dissolved carbon dioxide leaves visibly as fizz. The HSC criterion of "macroscopic constancy plus continuing molecular activity" is satisfied while sealed.

Example 2. NSW HSC depth study with the cobalt chloride equilibrium. A favourite Stage 6 depth study is the colour-change demonstration $[Co(H_2O)_6]^{2+}_{(aq)} + 4Cl^-_{(aq)} \rightleftharpoons [CoCl_4]^{2-}_{(aq)} + 6H_2O_{(l)}$ , pink on the left, blue on the right. Students prepare a solution at room temperature with both colours present, evidence of dynamic equilibrium. Adding silver nitrate removes free chloride, the equilibrium shifts left, and the solution turns pink. Warming the test tube shifts equilibrium right (endothermic forward), turning it blue. The colour return after cooling shows the reaction has not been driven to completion, confirming reversibility and the dynamic character of the closed system.

Try this

Q1. Distinguish between static equilibrium and dynamic equilibrium, giving one example of each. [3 marks]

Cue. Static: no process occurring (e.g. a book at rest on a table); dynamic: equal forward and reverse rates with continuing molecular exchange (e.g. sealed $N_2O_4$ / $NO_2$ tube).

Q2. A sealed flask contains $N_2O_{4(g)} \rightleftharpoons 2NO_{2(g)}$ with $[N_2O_4] = 0.20$ mol L $^{-1}$ and $[NO_2] = 0.10$ mol L $^{-1}$ . The forward rate equals $1.0 \times 10^{-3}$ mol L $^{-1}$ s $^{-1}$ . State the reverse rate and justify. [2 marks]

Cue. At dynamic equilibrium forward rate equals reverse rate, so reverse rate $= 1.0 \times 10^{-3}$ mol L $^{-1}$ s $^{-1}$ .

Q3. A student watches a saturated $NaCl$ solution sitting over excess solid for 24 hours. (a) Identify the equilibrium present. (b) State the macroscopic and molecular observations. (c) Explain why the system is dynamic, not static. [1+2+2 marks]

Cue. (a) $NaCl_{(s)} \rightleftharpoons Na^+_{(aq)} + Cl^-_{(aq)}$ . (b) Macroscopic: mass of solid constant; molecular: ions continually dissolve and crystallise. (c) Equal rates of dissolution and precipitation at the molecular scale make it dynamic.

Exam-style practice questions

Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2020 HSC4 marksDistinguish between static and dynamic equilibrium, using a chemical example to support your answer.

Show worked answer →

A 4 mark answer needs clear definitions, contrast, and a worked chemical example.

Static equilibrium is a state of balance in which no further change occurs because the system is not reacting. A book resting on a table is the classic physical example. In chemistry, a non-reactive mixture (for example, a sealed flask of argon and helium gas) is in static equilibrium because no forward or reverse process is taking place.

Dynamic equilibrium is the state of a reversible chemical reaction in which the rate of the forward reaction equals the rate of the reverse reaction. Macroscopic properties (concentration, colour, pressure) remain constant, but reactions continue in both directions at the molecular level.

Chemical example. For the reaction $2NO_{2(g)} \rightleftharpoons N_2O_{4(g)}$ , a sealed flask of brown $NO_2$ gas gradually pales as colourless $N_2O_4$ forms. The colour eventually stabilises, not because the reaction has stopped, but because $NO_2$ molecules are dimerising into $N_2O_4$ at the same rate as $N_2O_4$ molecules are dissociating back to $NO_2$ .

Markers reward (1) the definitions, (2) the explicit "equal forward and reverse rates" criterion, (3) a named chemical example with the equation.

2017 HSC3 marksExplain how a sealed bottle of soft drink is an example of dynamic equilibrium.

Show worked answer →

A sealed bottle of soft drink contains dissolved $CO_2$ in liquid and gaseous $CO_2$ above the liquid. The equilibrium is:

CO_{2(aq)} \rightleftharpoons CO_{2(g)}

At equilibrium, the rate at which $CO_2$ molecules leave the solution equals the rate at which gaseous $CO_2$ molecules dissolve back into the liquid. No net change in $CO_2$ concentration occurs (the drink stays fizzy), but molecules continue to move between phases.

Opening the bottle disturbs the equilibrium by releasing gaseous $CO_2$ . The forward rate (dissolved to gas) then exceeds the reverse rate, so the drink slowly loses its dissolved gas and goes flat.

Markers reward (1) the equation, (2) explicit "equal rates" language, (3) a sentence on what happens when the system is disturbed.