# Dynamic equilibrium characteristics

> Chemistry · CIE A-Level AS
> Source: https://www.owlsprep.com/study/cie-9701-u7-dynamic-equilibrium-characteristics/

This sub-topic introduces the core properties of dynamic chemical equilibrium, the foundational concept for all subsequent equilibrium topics including acid-base, solubility and redox equilibria. You will learn to identify equilibrium systems and their key features.

**Prerequisites:** Reversible and irreversible chemical reactions; Basic rate of reaction concepts

## Learning objectives

- Define dynamic equilibrium correctly for closed systems
- Distinguish between static and dynamic equilibrium
- List and recall all key characteristics of dynamic equilibrium
- Interpret concentration-time and rate-time graphs for equilibrium systems
- Avoid common misconceptions tested in CIE exams

## 1. Defining 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 amounts of reactants and products.

*Example:* For $N_2O_4(g) \rightleftharpoons 2NO_2(g)$, at equilibrium the rate of $N_2O_4$ breakdown equals the rate of $N_2O_4$ reformation from $NO_2$.

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. Key Characteristics of Dynamic Equilibrium

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 ($rate_f = rate_r$)
- 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

> **tip**
>
> In CIE exams, you are often required to list these characteristics for 2-3 marks, so memorise all key points.

**Worked example:** A student starts with 1.0 mol of pure $PCl_5(g)$ in a sealed 1 $dm^3$ container at 500 K. For $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$, equilibrium $[PCl_5] = 0.4$ mol dm$^{-3}$. What equilibrium $[PCl_5]$ would be obtained starting from 1.0 mol $PCl_3$ + 1.0 mol $Cl_2$ 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 $PCl_5$ equals 1 mol $P$ + 5 mol $Cl$, which matches 1 mol $PCl_3$ + 1 mol $Cl_2$.
3. Therefore, the equilibrium concentration of $PCl_5$ will also be 0.4 mol dm$^{-3}$.

## 3. Interpreting Equilibrium Graphs

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 $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, 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, $rate_{forward} = rate_{reverse}$: this is the point equilibrium is reached. After this, both rates remain constant.

> **info**
>
> Always label the two rates and the equality point if asked to draw this graph in an exam.

## 4. Dynamic vs Static Equilibrium

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.

**Exam command terms**

Common CIE command terms for this topic:

- **Explain what is meant by dynamic equilibrium** — You must mention both equal rates *and* constant concentrations to get full marks *(2 mark question: 1 mark for equal forward/reverse rate, 1 mark for constant concentrations)*

- **State the characteristics of dynamic equilibrium** — List 3-4 core points, 1 mark per correct characteristic

## Common pitfalls

- **Wrong:** Claiming concentrations of reactants and products are equal at equilibrium.
  - Why it fails: Confuses 'constant' with 'equal' — equal concentrations only occur by chance for reactions with $K_c = 1$.
  - Correct: Always state that concentrations are constant, not equal.
- **Wrong:** Says equilibrium can be achieved in an open system.
  - Why it fails: Open systems exchange matter with the surroundings, so net change never stops.
  - Correct: State that dynamic equilibrium can only form in a closed system.
- **Wrong:** Claims no reactions occur at dynamic equilibrium.
  - Why it fails: Confuses dynamic equilibrium with static equilibrium. 'Dynamic' means ongoing change.
  - Correct: State that reactions continue in both directions at equal non-zero rates.
- **Wrong:** Thinks equilibrium state depends on the starting direction of the reaction.
  - Why it fails: Ignores the core characteristic that final equilibrium is the same for the same overall conditions.
  - Correct: Recognise that starting from reactants or products gives the same equilibrium.
- **Wrong:** Draws concentrations changing slowly after equilibrium on a concentration-time graph.
  - Why it fails: Forgets concentrations are constant at equilibrium, so their plots should be flat.
  - Correct: Draw all concentration lines as horizontal after equilibrium is established.

## 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 |

## 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.

- [Le Chatelier's principle](https://www.owlsprep.com/study/cie-9701-u7-le-chatelier-s-principle/)
- [Equilibrium constants](https://www.owlsprep.com/study/cie-9701-u7-equilibrium-constants/)
- [Reaction kinetics (AS)](https://www.owlsprep.com/study/cie-9701-u8-overview/)

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