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E3.1 · Explain dynamic equilibrium in physical and chemical systems

Learn to explain dynamic equilibrium in physical and chemical systems through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Chemical Systems and Equilibrium

How opposite processes can continue at equal rates

A liquid in a sealed container may appear unchanged even though particles are still moving between the liquid and gas. Similarly, the colour of a reacting mixture may become steady while reactions continue. These observations can be explained by dynamic equilibrium: opposite processes continue, but at equal rates. To understand this idea, first recall that particles are always moving and that some changes can occur in both directions. Equilibrium does not mean that particles stop or that both sides contain equal amounts.

What you will learn

Explain what is meant by dynamic equilibrium.
Use particle-level reasoning to describe equilibrium in a physical system.
Explain how a reversible chemical reaction can reach equilibrium in a closed system.
Distinguish equal forward and reverse rates from equal amounts of substances.

1. Prerequisite bridge: reversible changes

A physical change alters a substance’s form or state without changing what substance it is. For example, liquid water can evaporate to form water vapour. Evaporation is the change from liquid to gas. Condensation is the reverse change, from gas to liquid.

In a chemical change, atoms rearrange to form different substances. A reversible reaction can proceed in both directions under the same conditions. The forward reaction forms products from reactants; the reverse reaction forms reactants from products. A double arrow is used to show that both directions are possible.

A system is the part of the surroundings being considered. A closed system can exchange energy with its surroundings but does not allow matter to enter or leave. A closed system is important for many examples of equilibrium because substances must remain available for both opposing processes.

\mathrm{reactants} \rightleftharpoons \mathrm{products}

Physical changes do not turn a substance into a different substance.
Reversible chemical reactions can proceed in both directions.
A closed system does not exchange matter with its surroundings.

2. Physical equilibrium: particles keep changing state

Consider a sealed container with liquid water and water vapour above it. At first, some liquid particles have enough motion to leave the surface and enter the gas. This is evaporation. Gas particles also strike the liquid surface and return to the liquid. This is condensation.

As vapour builds up, more gas particles are available to condense. In a sealed container at a steady temperature, the rate of condensation can eventually become equal to the rate of evaporation. Rate means how much change occurs in a given amount of time. At that point, the system is at dynamic equilibrium.

Dynamic means that change continues. Water particles still evaporate, and water-vapour particles still condense. Equilibrium means that these opposite changes occur at equal rates. Because equal amounts change in opposite directions over the same time, the overall amounts of liquid and vapour stay constant. They do not have to be equal to each other.

This equilibrium is not reached in an open container in the same way if water vapour continually escapes. For the sealed-container example, the closed system allows vapour to remain available for condensation.

rate of evaporation = rate of condensation

At dynamic equilibrium, evaporation and condensation continue.
Their rates are equal, so the observable amounts remain constant.
Constant amounts do not mean equal amounts of liquid and vapour.

3. Chemical equilibrium: forward and reverse reactions continue

A similar pattern occurs in a reversible chemical reaction in a closed system. Initially, a mixture may contain mostly reactants. The forward reaction then forms products. As products build up, the reverse reaction can also take place because product particles are present.

Consider colourless dinitrogen tetroxide gas, which can form brown nitrogen dioxide gas. In a sealed container, the forward reaction changes some dinitrogen tetroxide into nitrogen dioxide. The reverse reaction changes nitrogen dioxide back into dinitrogen tetroxide. If the temperature and other conditions remain steady, the system can reach equilibrium when the forward and reverse reaction rates are equal.

At chemical equilibrium, both reactions continue at the same rate. The concentrations of reactants and products then remain constant over time. Concentration describes the amount of a substance in a given volume. Constant concentrations do not mean that reactant and product concentrations are equal. Their amounts depend on the system and conditions.

The mixture may show a steady brown colour once equilibrium is established. This visible steadiness does not mean that the reaction has stopped. It means that nitrogen dioxide is being formed and used up at equal rates. The particle-level changes continue even when the overall observation remains unchanged.

\mathrm{N_2O_4(g)} \rightleftharpoons 2\,\mathrm{NO_2(g)}

Chemical equilibrium is dynamic: the forward and reverse reactions continue.
Equal reaction rates keep concentrations constant over time.
Reactant and product concentrations need not be equal.

4. Reading equilibrium statements carefully

To identify dynamic equilibrium, check for three ideas: the system allows the opposing processes to continue, the forward and reverse rates are equal, and observable amounts or concentrations remain constant. A statement that a mixture looks unchanged describes its overall appearance; it does not show that particles have stopped reacting.

Rates describe change per unit time, while concentrations describe amounts in a volume. Equal rates are not the same as equal concentrations. For example, a forward reaction can use reactants at the same rate that the reverse reaction forms them, even when there is much more reactant than product.

Equilibrium is a condition of the system, not a claim that every particle behaves identically. Individual particles continue to move and take part in changes. The balance is seen in the overall behaviour of the system.

Ask whether opposite processes continue and whether their rates are equal.
A steady observation or concentration is consistent with ongoing particle changes.
Equal rates do not require equal quantities.

Worked example

Interpreting a steady colour in a sealed vessel

A sealed vessel contains the reversible gas reaction between colourless dinitrogen tetroxide and brown nitrogen dioxide. After some time, the brown colour stops becoming more intense, and the measured concentrations remain constant. Explain what the observations mean at the particle level. Do not assume that the two gases have equal concentrations.

Identify the opposing reactions
The double arrow means nitrogen dioxide can form from dinitrogen tetroxide, and the reverse reaction can reform dinitrogen tetroxide.
$\mathrm{N_2O_4(g)} \rightleftharpoons 2\,\mathrm{NO_2(g)}$
Interpret the steady observation
The unchanged colour and constant concentrations show that the overall amounts are no longer changing. They do not show that the particles have stopped reacting.
Describe the equilibrium condition
At dynamic equilibrium, the forward and reverse reactions continue at equal rates. Nitrogen dioxide is formed as quickly as it is used up, so its concentration stays constant. The concentrations of the two gases do not have to be equal.
$\text{forward rate} = \text{reverse rate}$

Answer: The vessel is at dynamic equilibrium: both reactions continue, their rates are equal, and the concentrations remain constant. The steady brown colour does not mean that the reaction stopped or that the two gas concentrations are equal.

Check: This interpretation connects the visible observation to ongoing particle changes and the defining equal-rate condition.

Common mistakes and how to avoid them

Equilibrium means the forward and reverse reactions have stopped.

Correction: Dynamic equilibrium means both reactions continue at equal rates.

Equal forward and reverse rates mean equal reactant and product concentrations.

Correction: Rates describe how quickly amounts change. Concentrations can remain constant at unequal values.

A steady colour or unchanged amount proves that no particle-level change is occurring.

Correction: The overall appearance can stay constant while particles continue to react or change state.

Physical equilibrium and chemical equilibrium describe the same kind of change.

Correction: Both involve opposing processes at equal rates, but physical equilibrium involves a physical change, while chemical equilibrium involves a reversible chemical reaction.

Lesson summary

Dynamic equilibrium occurs when opposing processes continue at equal rates.
In a physical system, evaporation and condensation can occur at equal rates.
In a chemical system, forward and reverse reactions can occur at equal rates.
At equilibrium, observable amounts or concentrations remain constant, but they do not have to be equal.

Check your understanding

Question 1

In a sealed container at equilibrium, liquid water continues to evaporate and water vapour continues to condense. What must be true?

The evaporation and condensation rates are equal.
There is no water vapour in the container.
The liquid and gas contain equal amounts of water.
All water particles have stopped moving.

Show answer and explanation

The evaporation and condensation rates are equal.

Dynamic equilibrium requires the opposing physical processes to continue at equal rates. It does not require equal amounts or stopped motion.

Question 2

At chemical equilibrium, the concentrations of reactants and products remain constant. Which statement best explains why?

The forward and reverse reaction rates are equal.
The reactant and product concentrations are equal.
The forward reaction has stopped.
The products have disappeared.

Show answer and explanation

The forward and reverse reaction rates are equal.

Equal forward and reverse rates balance the amounts formed and used up. Concentrations stay constant, but need not be equal.

Question 3

A reaction mixture has a steady colour. Which conclusion is justified?

The overall amounts may be constant while forward and reverse reactions continue.
Every particle is motionless.
The reactant and product amounts must match.
The reaction has permanently stopped.

Show answer and explanation

The overall amounts may be constant while forward and reverse reactions continue.

A steady appearance is consistent with dynamic equilibrium. It does not establish equal concentrations or stopped reactions.

Key terms

Dynamic equilibrium: A condition in which opposing processes continue at equal rates, leaving overall amounts constant.
Evaporation: The change from liquid to gas.
Condensation: The change from gas to liquid.
Rate: How much change occurs in a given amount of time.
Concentration: The amount of a substance in a given volume.
Closed system: A system that does not allow matter to enter or leave.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation E3.1. It is a study resource, not an official curriculum publication.

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