Dynamic equilibrium

IB Chemistry SL· 35 min read

1. Key Definitions and Characteristics of Dynamic Equilibrium★★☆☆☆⏱ 10 min

📘 Definition

Dynamic Equilibrium

A state of a reversible reaction in a closed system where the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the concentrations of reactants and products.

Example:

Evaporation and condensation of water in a sealed flask

For a system to reach dynamic equilibrium, it must be closed: no matter can enter or leave the system. Open systems continuously exchange matter with the surroundings, so they can never reach a steady equilibrium state.

Forward and reverse reaction rates are equal ( $r_{f} = r_{r}$ )
Concentrations of reactants and products remain constant over time
Equilibrium can be approached from either the forward or reverse direction
The process is dynamic: reactions continue to occur, rather than stopping

📐 Worked Example

A student claims that equilibrium is a static state where no reactions occur. Evaluate this claim.

1
The term 'dynamic' means that the forward and reverse reactions are still occurring at equal rates.
2
Because the rate of formation of products equals the rate of formation of reactants, there is no net change in observable properties like concentration, which can make equilibrium appear static.
3
Therefore the claim is incorrect: equilibrium is a dynamic, not static, process.

Exam tip:

Always mention both that reaction rates are equal and concentrations are constant when describing equilibrium. Examiners require both points for full marks.

2. Physical vs Chemical Equilibrium★★☆☆☆⏱ 8 min

📘 Definition

Physical Equilibrium

Dynamic equilibrium that occurs when a reversible physical change (rather than a chemical reaction) takes place.

Example:

Phase equilibrium between liquid water and water vapour in a sealed container

📘 Definition

Chemical Equilibrium

Dynamic equilibrium that occurs when a reversible chemical reaction takes place, with continuous interconversion between reactants and products.

Example:

Decomposition of dinitrogen tetroxide: \ce{N2O4(g) <=> 2NO2(g)}

Both types of equilibrium follow all the general characteristics of dynamic equilibrium. The only difference is whether the process occurring is a physical change or a chemical reaction.

📐 Worked Example

Classify the equilibrium formed when solid sodium chloride is in contact with a saturated solution of sodium chloride in a sealed beaker as physical or chemical, and justify your answer.

1
Identify the process: solid NaCl dissolves to form aqueous \ce{Na+} and \ce{Cl-} ions, while dissolved ions precipitate to reform solid NaCl.
2
No new chemical substance is formed: dissolving is a physical change, not a chemical reaction.
3
Therefore this is an example of physical equilibrium.

3. Graphical Representation of Equilibrium★★★☆☆⏱ 12 min

Dynamic equilibrium is commonly visualized using two types of graphs: concentration-time graphs, and rate-time graphs.

For a reaction starting with only reactants: reactant concentration decreases over time and becomes constant at equilibrium, while product concentration increases over time and becomes constant. In a rate-time graph, forward rate starts high and decreases, reverse rate starts at zero and increases, until the two meet at equilibrium.

📐 Worked Example

Sketch a rate-time graph for the reaction \ce{H2(g) + I2(g) <=> 2HI(g)} starting with only \ce{H2} and \ce{I2}. Label the point where equilibrium is reached.

1
Label the y-axis as reaction rate, and the x-axis as time.
2
Draw the forward reaction rate starting at a maximum value at $t = 0$ , decreasing with a flattening slope over time.
3
Draw the reverse reaction rate starting at 0 at $t = 0$ , increasing with a flattening slope over time.
4
Equilibrium is reached at the intersection of the two curves, after which both rates stay constant. Mark this point and label it 'Equilibrium'.

✓ Quick check

Check your understanding:

At equilibrium on a concentration-time graph, what is true?
- Both reactant and product concentrations are equal
- Both reactant and product concentrations are constant over time
- Reactant concentration is always higher than product concentration
- Product concentration is always higher than reactant concentration
Reveal answer
1 —
Concentrations are constant at equilibrium, but not necessarily equal. Equal reaction rates, not equal concentrations, is the key feature.

4. Reversible vs Irreversible Reactions★★☆☆☆⏱ 7 min

📘 Definition

Reversible vs Irreversible Reactions

Reversible reactions can proceed in both directions under the same conditions, and reach dynamic equilibrium. Irreversible reactions proceed only in the forward direction to completion, and cannot reach equilibrium.

Example:

Combustion of hydrocarbons is an irreversible reaction

Reversible reactions never go to completion, because products are continuously converted back to reactants. Irreversible reactions often go to completion in open systems because a gaseous product escapes, or a solid precipitate forms that cannot re-react.

📐 Worked Example

Explain why the reaction between hydrochloric acid and sodium hydroxide in an open beaker is irreversible.

1
The reaction is \ce{HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)}, and no reverse reaction occurs under the same conditions.
2
There is no interconversion back to reactants, so dynamic equilibrium cannot be established.
3
The reaction proceeds to completion, so it is classified as irreversible.

5. Common Pitfalls

Wrong move:

Claiming that concentrations of reactants and products are equal at equilibrium

Why:

Confuses equal reaction rates with equal concentrations

Correct move:

State that reaction rates are equal, and concentrations are constant, not equal

Wrong move:

Claiming that equilibrium can be reached in an open system

Why:

Matter can escape open systems, so net continuous change occurs

Correct move:

Dynamic equilibrium can only be established in a closed system

Wrong move:

Thinking that equilibrium is a static state where all reactions stop

Why:

The term 'dynamic' means ongoing change at the molecular level

Correct move:

Reactions continue to occur in both directions at equal rates, so no net change

Wrong move:

Classifying dissolving equilibrium as chemical equilibrium

Why:

Dissolving is a physical change that does not produce new chemical substances

Correct move:

Equilibrium for physical changes like dissolving or phase change are physical equilibrium

Wrong move:

Stating that equilibrium can only be reached starting from pure reactants

Why:

Equilibrium has the same final state regardless of starting direction

Correct move:

Equilibrium can be reached starting from reactants, products, or any mixture of both

6. Quick Reference Cheatsheet

Characteristic	Description
System requirement	Only closed systems can reach equilibrium
Reaction rates	$r_{f or w a r d} = r_{r e v er se} > 0$
Concentrations	Constant over time, not necessarily equal
Direction of approach	Same equilibrium from reactants or products
Physical equilibrium	Applies to reversible physical changes (phase, dissolving)
Chemical equilibrium	Applies to reversible chemical reactions

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Characteristics of dynamic equilibrium
2023 · 2
Interpret equilibrium concentration graph

Going deeper

syllabusIB Chemistry SL Syllabus: Reactivity 3

What's Next

Dynamic equilibrium is the foundational concept for all subsequent equilibrium topics in IB Chemistry SL. Understanding its core characteristics is essential for learning about the equilibrium constant ( $K_{c}$ ), calculating equilibrium concentrations, and predicting how systems respond to change via Le Chatelier's principle. These topics build directly on the ideas covered here, so mastering dynamic equilibrium first will make more advanced concepts much easier to understand and apply in exam questions.