Dynamic equilibrium characteristics

Chemistry· 15 min read

1. 1. Defining Dynamic Equilibrium★☆☆☆☆⏱ 5 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 amounts of reactants and products.

Example:

For $N_{2} O_{4} (g) ⇌ 2 N O_{2} (g)$ , at equilibrium the rate of $N_{2} O_{4}$ breakdown equals the rate of $N_{2} O_{4}$ reformation from $N O_{2}$ .

Dynamic equilibrium can only occur in closed systems, where no reactants or products can enter or leave. Open systems constantly lose matter (e.g. a beaker of open water evaporates all water over time), so equilibrium cannot be established.

📐 Worked Example

Explain why a sealed jar of water left at constant temperature contains a dynamic equilibrium between liquid water and water vapour.

1
First confirm the system is closed (lid prevents water vapour escaping) and the process is reversible:
2
$H_2O(l) \rightleftharpoons H_2O(g)$
3
At equilibrium, the rate of evaporation (liquid → gas) equals the rate of condensation (gas → liquid).
4
There is no net change in the mass of liquid water or water vapour pressure over time, but both processes still occur, so it is dynamic not static.

2. 2. Key Characteristics of Dynamic Equilibrium★★☆☆☆⏱ 5 min

All dynamic equilibrium systems share core characteristics, which are frequently tested in multiple choice and short answer CIE questions.

Only occur for reversible reactions in closed systems
Forward and reverse reaction rates are equal ( $r a t e_{f} = r a t e_{r}$ )
Concentrations of all reactants and products remain constant over time
Equilibrium can be approached from either direction (same final state for same conditions)
The equilibrium state is dynamic, not static: reactions continue in both directions

📐 Worked Example

A student starts with 1.0 mol of pure $P C l_{5} (g)$ in a sealed 1 $d m^{3}$ container at 500 K. For $P C l_{5} (g) ⇌ P C l_{3} (g) + C l_{2} (g)$ , equilibrium $[P C l_{5}] = 0.4$ mol dm $^{- 3}$ . What equilibrium $[P C l_{5}]$ would be obtained starting from 1.0 mol $P C l_{3}$ + 1.0 mol $C l_{2}$ at the same conditions? Explain your answer.

1
Recall that equilibrium state is independent of starting direction for the same total composition, temperature and pressure.
2
The total amount of phosphorus and chlorine is identical in both experiments: starting from 1 mol $P C l_{5}$ equals 1 mol $P$ + 5 mol $C l$ , which matches 1 mol $P C l_{3}$ + 1 mol $C l_{2}$ .
3
Therefore, the equilibrium concentration of $P C l_{5}$ will also be 0.4 mol dm $^{- 3}$ .

3. 3. Interpreting Equilibrium Graphs★★☆☆☆⏱ 4 min

Equilibrium is easily identified on graphs of concentration or rate against time. We can use these plots to confirm when equilibrium is established.

When equilibrium is established, all concentrations stop changing, so their plots become horizontal lines. On a rate-time graph, forward rate falls from its initial maximum, reverse rate rises from zero, and they become equal and constant at equilibrium.

📐 Worked Example

Describe the rate-time graph for $H_{2} (g) + I_{2} (g) ⇌ 2 H I (g)$ , starting with only $H_{2}$ and $I_{2}$ present. Mark when equilibrium is reached.

1
Initial state: only reactants are present, so forward rate starts at maximum, reverse rate starts at 0.
2
As reaction proceeds, reactant concentration falls, so forward rate decreases. Product concentration increases, so reverse rate increases.
3
When the two rate lines meet, $r a t e_{f or w a r d} = r a t e_{r e v er se}$ : this is the point equilibrium is reached. After this, both rates remain constant.

4. 4. Dynamic vs Static Equilibrium★★☆☆☆⏱ 3 min

A common exam question asks to compare dynamic and static equilibrium, so it is important to clearly distinguish the two.

Property	Dynamic Equilibrium	Static Equilibrium
Reactions ongoing	Yes (both directions proceed)	No (no reaction occurs)
Net concentration change	Zero	Zero
Reaction rates	Equal non-zero forward/reverse rates	Zero rate for all processes
Example	Water + vapour in sealed jar	Complete irreversible reaction at end point

📐 Worked Example

An irreversible reaction goes to completion, with no reverse reaction occurring. Is the final state static or dynamic equilibrium? Explain.

1
Recall static equilibrium is a state with no ongoing reaction and no net change.
2
An irreversible reaction that goes to completion has no ongoing reverse reaction, and no net change after completion.
3
Therefore the final state is a static equilibrium, not dynamic.

5. Common Pitfalls

Wrong move:

Claiming concentrations of reactants and products are equal at equilibrium.

Why:

Confuses 'constant' with 'equal' — equal concentrations only occur by chance for reactions with $K_{c} = 1$ .

Correct move:

Always state that concentrations are constant, not equal.

Wrong move:

Says equilibrium can be achieved in an open system.

Why:

Open systems exchange matter with the surroundings, so net change never stops.

Correct move:

State that dynamic equilibrium can only form in a closed system.

Wrong move:

Claims no reactions occur at dynamic equilibrium.

Why:

Confuses dynamic equilibrium with static equilibrium. 'Dynamic' means ongoing change.

Correct move:

State that reactions continue in both directions at equal non-zero rates.

Wrong move:

Thinks equilibrium state depends on the starting direction of the reaction.

Why:

Ignores the core characteristic that final equilibrium is the same for the same overall conditions.

Correct move:

Recognise that starting from reactants or products gives the same equilibrium.

Wrong move:

Draws concentrations changing slowly after equilibrium on a concentration-time graph.

Why:

Forgets concentrations are constant at equilibrium, so their plots should be flat.

Correct move:

Draw all concentration lines as horizontal after equilibrium is established.

6. Quick Reference Cheatsheet

Key Characteristic	Description
System requirement	Reversible reaction in closed system
Reaction rate rule	Forward rate = Reverse rate (rate ≠ 0)
Concentration rule	Concentrations are constant, not equal
Direction independence	Same equilibrium from any starting point
Equilibrium type	Dynamic, not static — reactions still occur

7. Frequently Asked

Do concentrations of reactants and products have to be equal at equilibrium?

No, they only have to be constant. Equal concentrations are rare, only occurring for reactions with an equilibrium constant of 1.

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
MCQ on equilibrium characteristics
2023 · 2
Explain dynamic equilibrium
2021 · 12
Identify correct equilibrium properties

Going deeper

syllabusOfficial CIE 9701 AS SyllabusFull syllabus reference for Unit 7

What's Next

Understanding dynamic equilibrium characteristics is the foundation for all upcoming equilibrium topics in CIE AS Chemistry. Next, you will learn how to quantify equilibrium using the equilibrium constant $K_{c}$ , calculate concentrations of species at equilibrium, and predict how changing conditions affect the position of equilibrium using Le Chatelier's Principle. These concepts are heavily tested in both multiple choice and structured questions, so make sure you are confident with the core characteristics before moving on.