# Dynamic equilibrium

> IB Chemistry Higher Level · IB Diploma Programme Chemistry HL (2025 syllabus)
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u5-dynamic-equilibrium/

This module covers the core definition and properties of dynamic chemical equilibrium, the difference between reversible and irreversible reactions, and common misconceptions tested in IB Chemistry HL exams.

**Prerequisites:** [Reaction rate fundamentals](https://www.owlsprep.com/study/ib-chemistry-hl-u4-reaction-rates/)

## Learning objectives

- Distinguish between reversible and irreversible reactions
- Explain the dynamic nature of chemical equilibrium
- Describe key properties of equilibrium systems in closed systems
- Identify common misconceptions about dynamic equilibrium

## Reversible vs Irreversible Reactions & Closed Systems

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.

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

*Notation:* ⇌

*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. 2. 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. 3. 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.

## The Dynamic Nature of Equilibrium

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.

**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: $rate_{forward} = rate_{reverse}$

1. 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

> **tip**
>
> If an exam question asks you to explain dynamic equilibrium, you must mention that *forward and reverse reactions continue at equal rates* to get full marks. Only mentioning constant concentrations will not earn all marks.

## Key Properties of Equilibrium Systems

- 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?

1. 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

**Check your understanding**

Test your understanding of core properties

1. 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

   *Why:* A is wrong: reactions continue. B is wrong: concentrations are constant, not equal. D is wrong: changing conditions disturbs equilibrium.

## Common pitfalls

- **Wrong:** Claiming equilibrium means reactant and product concentrations are equal
  - Why it fails: Confuses 'equal rates' of reaction with 'equal concentrations' of species
  - Correct: At equilibrium, rates of forward and reverse reactions are equal, so concentrations are constant (not necessarily equal)
- **Wrong:** Stating dynamic equilibrium can be achieved in an open system
  - Why it fails: Ignores that matter lost from an open system prevents a steady state
  - Correct: Dynamic equilibrium can only form in a closed system with no exchange of matter with surroundings
- **Wrong:** Claiming the reaction stops at equilibrium
  - Why it fails: Misinterprets no net change as no reaction occurring at all
  - Correct: Equilibrium is dynamic: both forward and reverse reactions continue at equal rates
- **Wrong:** Assuming the same equilibrium position forms regardless of starting conditions
  - Why it fails: Forgets that equilibrium position depends on temperature, pressure, and initial amounts
  - Correct: The same equilibrium position from either direction only occurs when temperature, volume, and total moles of starting material are identical

## Cheatsheet

| Property | Key Feature of Dynamic Equilibrium |
| --- | --- |
| Reaction type | Must be reversible |
| System requirement | Closed system (no matter exchange) |
| Rate relationship | $rate_{forward} = rate_{reverse}$ |
| Concentration behavior | Constant over time, not equal |
| Dynamic nature | Reactions continue to occur |
| Starting direction | Same equilibrium from reactants or products (same conditions) |

## 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.

- [The equilibrium constant](https://www.owlsprep.com/study/ib-chemistry-hl-u5-the-equilibrium-constant/)
- [AHL: Advanced stoichiometry and titration calculations](https://www.owlsprep.com/study/ib-chemistry-hl-u5-ahl-advanced-stoichiometry-and-titration/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-hl-u5-dynamic-equilibrium/