# Dynamic equilibrium

> IB Chemistry SL · Reactivity 3: Equilibrium and Organic Chemistry
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u6-dynamic-equilibrium/

Dynamic equilibrium describes reversible reactions in closed systems where forward and reverse reaction rates are equal, with no net change in measurable system properties. It is the foundational concept for all further equilibrium topics in IB Chemistry SL.

**Prerequisites:** [Rates of reaction](https://www.owlsprep.com/study/ib-chemistry-sl-u5-reaction-rates/)

## Learning objectives

- Define dynamic equilibrium in closed systems
- Distinguish between physical and chemical equilibrium
- State the key characteristics of a system at dynamic equilibrium
- Interpret concentration-time and rate-time graphs for equilibrium systems

## Key Definitions and Characteristics of Dynamic Equilibrium

**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.

## Physical vs Chemical Equilibrium

**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

**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.

## Graphical Representation of Equilibrium

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'.

**Check your understanding**

Check your understanding:

1. 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

   *Answer:* Both reactant and product concentrations are constant over time

   *Why:* Concentrations are constant at equilibrium, but not necessarily equal. Equal reaction rates, not equal concentrations, is the key feature.

## Reversible vs Irreversible Reactions

**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.

## Common pitfalls

- **Wrong:** Claiming that concentrations of reactants and products are equal at equilibrium
  - Why it fails: Confuses equal reaction rates with equal concentrations
  - Correct: State that reaction rates are equal, and concentrations are *constant*, not equal
- **Wrong:** Claiming that equilibrium can be reached in an open system
  - Why it fails: Matter can escape open systems, so net continuous change occurs
  - Correct: Dynamic equilibrium can only be established in a closed system
- **Wrong:** Thinking that equilibrium is a static state where all reactions stop
  - Why it fails: The term 'dynamic' means ongoing change at the molecular level
  - Correct: Reactions continue to occur in both directions at equal rates, so no net change
- **Wrong:** Classifying dissolving equilibrium as chemical equilibrium
  - Why it fails: Dissolving is a physical change that does not produce new chemical substances
  - Correct: Equilibrium for physical changes like dissolving or phase change are physical equilibrium
- **Wrong:** Stating that equilibrium can only be reached starting from pure reactants
  - Why it fails: Equilibrium has the same final state regardless of starting direction
  - Correct: Equilibrium can be reached starting from reactants, products, or any mixture of both

## Cheatsheet

| Characteristic | Description |
| --- | --- |
| System requirement | Only closed systems can reach equilibrium |
| Reaction rates | $r_{forward} = r_{reverse} > 0$ |
| 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 |

## 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.

- [Position of equilibrium](https://www.owlsprep.com/study/ib-chemistry-sl-u6-position-of-equilibrium/)
- [Introduction to organic chemistry](https://www.owlsprep.com/study/ib-chemistry-sl-u6-introduction-to-organic-chemistry/)
- [Functional group chemistry](https://www.owlsprep.com/study/ib-chemistry-sl-u6-functional-group-chemistry/)

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