← Module 5: Equilibrium and Acid Reactions

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 OpusReviewed by Better Tuition Academy6 min answerUpdated 2026-05-18

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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.

Common traps

Saying the reaction has "stopped" at equilibrium. This loses marks immediately. The reaction continues at the molecular level. Always write that the forward and reverse rates are equal.

Confusing dynamic equilibrium with equal concentrations. Concentrations at equilibrium are usually not equal; what is equal is the forward and reverse rate. The position of equilibrium depends on Kc.

Forgetting the closed system requirement. An open beaker of evaporating water never reaches equilibrium because water vapour escapes. Always specify "closed" or "sealed."

Calling a reversible reaction "reversed." A reversible reaction goes in both directions simultaneously at equilibrium, not first forward and then backward.

In one sentence

Dynamic equilibrium is the state of a closed reversible reaction in which the forward and reverse reactions occur at equal rates, so macroscopic properties stay constant while molecules continue to react, distinguishing it from static equilibrium where no process occurs at all.

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

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

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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.

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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.