Dynamic equilibrium

IB Chemistry Higher Level· R2: How much / how fast / how far?· 15 min read

1. Reversible vs Irreversible Reactions & Closed Systems★★☆☆☆⏱ 5 min

Most chemical reactions do not go to 100% completion (all reactants converted to products). They can proceed in both the forward direction (reactants → products) and reverse direction (products → reactants), making them reversible. Irreversible reactions only proceed in one direction, usually because products escape the system or the equilibrium position lies extremely far toward products.

📘 Definition

Reversible reaction

⇌

A reaction that can proceed in both forward and reverse directions under the same reaction conditions

Example:

N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)

📐 Worked Example

Classify each reaction as reversible or irreversible: (1) Burning of methane in an open container, (2) Dissolution of solid copper(II) sulfate in water, (3) Decomposition of calcium carbonate in an open beaker.

1
Step 1: Analyze each reaction for ability to reverse under reaction conditions
2
1. Burning methane produces CO₂ and water vapor that escape the open system. They cannot spontaneously reform methane and oxygen, so:
3
Result: Irreversible
4
1. Copper(II) sulfate dissolves into ions, and water can be evaporated to reform solid copper(II) sulfate under the same conditions, so:
5
Result: Reversible
6
1. Decomposition of CaCO₃ in an open beaker releases CO₂ gas that escapes. No reverse reaction can occur, so:
7
Result: Irreversible

Exam tip:

Always note if the reaction system is open or closed: irreversible reactions most often occur in open systems where products are lost.

2. The Dynamic Nature of Equilibrium★★★☆☆⏱ 5 min

When a reversible reaction occurs in a closed system (no matter enters or leaves), over time the rate of the forward reaction decreases as reactants are consumed, and the rate of the reverse reaction increases as products form. Eventually, the two rates become equal: this is dynamic equilibrium.

📘 Definition

Dynamic equilibrium

A steady state of a closed reversible reaction system where the rate of the forward reaction equals the rate of the reverse reaction, so concentrations of all reactants and products remain constant over time.

A common misunderstanding is that the reaction stops at equilibrium. In fact, both forward and reverse reactions continue to occur at equal rates, which is why equilibrium is described as dynamic. There is no net change in concentrations because any change from the forward reaction is canceled by an equal change from the reverse reaction.

📐 Worked Example

Explain why the equality below confirms that concentrations are constant at equilibrium: $r a t e_{f or w a r d} = r a t e_{r e v er se}$

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Step 1: The forward reaction consumes reactants and forms products, so it changes concentrations of all species
2
Step 2: The reverse reaction consumes products and reforms reactants, so it causes the opposite change in concentrations
3
Step 3: When rates are equal, the amount of reactant consumed per second by the forward reaction equals the amount of reactant formed per second by the reverse reaction
4
Conclusion: No net change in concentration occurs over time, so concentrations remain constant

3. Key Properties of Equilibrium Systems★★★☆☆⏱ 5 min

Can only be achieved in a closed system (no exchange of matter with surroundings)
Is reversible: can be reached starting from either reactants or products, with the same equilibrium position under the same conditions
Is dynamic: forward and reverse reactions continue at equal rates
Concentrations of all reactants and products are constant (not equal) over time
Can be disturbed by changes in temperature, concentration, or pressure per Le Chatelier's principle

📐 Worked Example

1.0 mol H₂ and 1.0 mol I₂ react in a closed 1 dm³ container at 400°C to form 2HI, reaching equilibrium with [HI] = 1.56 mol dm⁻³. What is the equilibrium [HI] when 2.0 mol HI is placed in the same container at the same temperature?

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Step 1: A core property of equilibrium is that the same equilibrium position is reached from either direction (reactants or products) if conditions (temperature, volume, total starting moles) are identical.
2
Step 2: Starting from 2.0 mol HI is the reverse of starting from 1.0 mol H₂ + 1.0 mol I₂, so the final equilibrium composition is identical.
3
Result: Equilibrium [HI] = 1.56 mol dm⁻³

✓ Quick check

Test your understanding of core properties

Which statement is true for a system at dynamic equilibrium?
- A. The forward reaction stops
- B. Concentrations of reactants and products are equal
- C. Rate of forward reaction = rate of reverse reaction
- D. Equilibrium cannot be disturbed
Reveal answer
C. Rate of forward reaction = rate of reverse reaction —
A is wrong: reactions continue. B is wrong: concentrations are constant, not equal. D is wrong: changing conditions disturbs equilibrium.

4. Common Pitfalls

Wrong move:

Claiming equilibrium means reactant and product concentrations are equal

Why:

Confuses 'equal rates' of reaction with 'equal concentrations' of species

Correct move:

At equilibrium, rates of forward and reverse reactions are equal, so concentrations are constant (not necessarily equal)

Wrong move:

Stating dynamic equilibrium can be achieved in an open system

Why:

Ignores that matter lost from an open system prevents a steady state

Correct move:

Dynamic equilibrium can only form in a closed system with no exchange of matter with surroundings

Wrong move:

Claiming the reaction stops at equilibrium

Why:

Misinterprets no net change as no reaction occurring at all

Correct move:

Equilibrium is dynamic: both forward and reverse reactions continue at equal rates

Wrong move:

Assuming the same equilibrium position forms regardless of starting conditions

Why:

Forgets that equilibrium position depends on temperature, pressure, and initial amounts

Correct move:

The same equilibrium position from either direction only occurs when temperature, volume, and total moles of starting material are identical

5. Quick Reference Cheatsheet

Property	Key Feature of Dynamic Equilibrium
Reaction type	Must be reversible
System requirement	Closed system (no matter exchange)
Rate relationship	$r a t e_{f or w a r d} = r a t e_{r e v er se}$
Concentration behavior	Constant over time, not equal
Dynamic nature	Reactions continue to occur
Starting direction	Same equilibrium from reactants or products (same conditions)

6. Frequently Asked

Does equilibrium mean reactant and product concentrations are equal?

No. Dynamic equilibrium only requires that the rate of the forward reaction equals the rate of the reverse reaction. Concentrations of all species are constant, not equal.

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
Identify equilibrium property
2023 · 2
Explain dynamic nature of equilibrium
2024 · 1
Classify reversible/irreversible reaction

Going deeper

syllabusOfficial IB Chemistry HL 2025 SyllabusFull syllabus learning outcomes

What's Next

Dynamic equilibrium is the foundational concept for all equilibrium topics in IB Chemistry HL. Mastering its core properties and definitions is critical to applying Le Chatelier's principle, calculating equilibrium constants, and solving more complex problems for acid-base equilibrium, solubility product, and buffer systems. The misconceptions addressed here are common exam traps, so reviewing them regularly will help you avoid losing easy marks. Build on this foundation with the topics below to master all IB HL equilibrium learning outcomes.