# Reversible Reactions and Equilibrium

> Chemistry · CIE IGCSE 0620
> Source: https://www.owlsprep.com/study/cie-0620-u6-reversible-reactions-and-equilibrium/

This guide covers core reversible reaction and dynamic equilibrium concepts for CIE IGCSE Chemistry 0620, plus extended Le Chatelier’s principle content to help you score full marks on related exam questions.

**Prerequisites:** [Knowledge of basic chemical reaction notation and word equations](https://www.owlsprep.com/study/cie-0620-u2-chemical-equations/); [Understanding of exothermic and endothermic reactions](https://www.owlsprep.com/study/cie-0620-u6-energy-changes/)

## Learning objectives

- Define reversible reactions and use the ⇌ symbol correctly
- Describe dynamic equilibrium and its required conditions in closed systems
- Recall common IGCSE reversible reaction examples and their observations
- Apply Le Chatelier's principle to predict equilibrium position changes (Extended only)

## What are Reversible Reactions?

**Reversible Reaction** — A chemical reaction where products can react to reform the original reactants, proceeding in both forward and reverse directions simultaneously.

*Notation:* A + B ⇌ C + D

*Example:* Heating blue hydrated copper(II) sulfate forms white anhydrous copper sulfate and water; adding water reforms the blue hydrate.

Unlike irreversible reactions that go to completion (all reactants converted to products), reversible reactions never fully use up all reactants in a closed system. The double arrow ⇌ is always used instead of a single → to show both directions of the reaction.

> **tip**
>
> Always use the correct double arrow symbol for reversible reactions in your exam answers; losing a mark for incorrect notation is easily avoidable.

**Worked example:** Write the word equation for the reversible reaction of heating ammonium chloride, which decomposes to form ammonia and hydrogen chloride gases that react to reform ammonium chloride when cooled.

1. Identify the forward reaction: heating ammonium chloride produces ammonia + hydrogen chloride
2. Identify the reverse reaction: ammonia + hydrogen chloride react to form ammonium chloride on cooling
3. Combine the two reactions using the reversible arrow symbol
4. Final word equation: ammonium chloride ⇌ ammonia + hydrogen chloride

## Dynamic Equilibrium (Core)

**Dynamic Equilibrium** — A state in a closed reversible reaction system where the rate of the forward reaction equals the rate of the reverse reaction, so concentrations of reactants and products remain constant.

Two key conditions are required for dynamic equilibrium: first, the system must be closed (no substances can enter or leave, so no escape of gaseous products for example). Second, temperature must be kept constant, as temperature changes alter reaction rates and shift equilibrium position. At equilibrium, both forward and reverse reactions are still happening, but their rates are equal so there is no visible change to the system.

**Worked example:** A student heats pink hydrated cobalt(II) chloride in a sealed test tube. The solid turns blue as anhydrous cobalt(II) chloride and water form. After 2 minutes, the colour stops changing even though heating continues. Explain this observation.

1. Recognize the reaction is reversible and takes place in a closed (sealed) system
2. At the start of heating, the forward reaction rate is faster than the reverse, so more blue anhydrous cobalt chloride forms, turning the solid blue
3. Over time, the rate of the reverse reaction increases until it equals the rate of the forward reaction
4. The system reaches dynamic equilibrium, so concentrations of pink and blue cobalt compounds stay constant, so no further colour change is observed

## Le Chatelier's Principle (Extended only)

**Le Chatelier's Principle** — If a change in conditions (concentration, temperature, pressure) is applied to a system at dynamic equilibrium, the system shifts its equilibrium position to counteract the change.

This principle lets you predict which direction the equilibrium will move (favour forward reaction to make more products, or favour reverse to make more reactants) when conditions are altered. You only need to know the effect of three key changes for the exam:

1. **Temperature change**: If you increase temperature, equilibrium shifts in the endothermic direction (absorbs extra heat). If you decrease temperature, equilibrium shifts in the exothermic direction (releases heat).
2. **Concentration change**: If you increase the concentration of a reactant, equilibrium shifts right to use up the extra reactant and make more products. If you remove a product, equilibrium also shifts right to replace it.
3. **Pressure change (gaseous reactions only)**: If you increase pressure, equilibrium shifts to the side with fewer moles of gas to reduce pressure. If you decrease pressure, equilibrium shifts to the side with more moles of gas.

**Worked example:** The reaction between nitrogen and hydrogen to make ammonia is reversible: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$. The forward reaction is exothermic. Predict the effect of increasing temperature on the equilibrium yield of ammonia.

1. Identify the change applied: temperature is increased
2. Recall Le Chatelier's principle: the system shifts to counteract the temperature increase, so favours the endothermic direction
3. The forward reaction is exothermic, so the reverse reaction is endothermic
4. Equilibrium shifts to the left, so the yield of ammonia decreases

## Key Reversible Reaction Examples for Exams

You are expected to recall two common reversible reactions for IGCSE 0620 exams, both of which are often tested in practical or theory questions:

- **Hydrated copper(II) sulfate decomposition**: Blue hydrated copper(II) sulfate ⇌ white anhydrous copper(II) sulfate + water. Forward reaction is endothermic.
- **Ammonium chloride decomposition**: White solid ammonium chloride ⇌ ammonia gas + hydrogen chloride gas. Forward reaction is endothermic.

> **exam_tip**
>
> Memorize the colours and states of all reactants and products for these two reactions, as observation questions are very common for this topic.

## Common pitfalls

- **Wrong:** Using a single arrow (→) for reversible reactions
  - Why it fails: The single arrow implies the reaction goes to completion, which is incorrect for reversible reactions, and examiners deduct marks for wrong notation.
  - Correct: Always use the double reversible arrow (⇌) for all reversible reaction equations.
- **Wrong:** Stating that reactions stop at equilibrium
  - Why it fails: Equilibrium is dynamic, meaning forward and reverse reactions are still occurring at equal rates, not stopping.
  - Correct: Explicitly state that both reactions continue at equal rates, so concentrations of reactants and products stay constant.
- **Wrong:** Applying pressure change rules to reactions with only solids/liquids
  - Why it fails: Pressure only affects the volume of gases, so changes in pressure have no effect on equilibrium of non-gaseous reactions.
  - Correct: Only use pressure change rules for reactions where all reacting substances are gases, and count only moles of gas on each side of the equation.
- **Wrong:** Confusing endothermic/exothermic directions for temperature change shifts
  - Why it fails: This leads to incorrect predictions of equilibrium shift direction, costing you marks.
  - Correct: First note which direction is exothermic (given in the question), then remember increased temperature favours the endothermic direction to absorb extra heat.
- **Wrong:** Forgetting equilibrium only occurs in closed systems
  - Why it fails: If a system is open (e.g. gas can escape), the reverse reaction cannot occur so equilibrium will never be reached.
  - Correct: Always confirm the system is closed before stating an equilibrium will form.

## Cheatsheet

| Concept | Tier | Key Exam Fact |
| --- | --- | --- |
| Reversible reaction symbol | Core | Use ⇌ not → to show both reaction directions |
| Dynamic equilibrium conditions | Core | Closed system, constant temperature; forward + reverse rates equal |
| Le Chatelier: Temperature increase | Extended | Shifts equilibrium to endothermic direction |
| Le Chatelier: Reactant concentration increase | Extended | Shifts equilibrium right to make more products |
| Le Chatelier: Pressure increase (gases) | Extended | Shifts to side with fewer moles of gas |
| Hydrated copper sulfate reaction | Core | Blue ⇌ white solid + water; forward is endothermic |

## What's next

Now that you have mastered reversible reactions and equilibrium, you are ready to apply these concepts to industrial chemical processes that are core to the CIE IGCSE Chemistry 0620 syllabus. Understanding Le Chatelier's principle will let you explain the optimal conditions chosen for the Haber process (used to manufacture ammonia) and the Contact process (used to make sulfuric acid), both of which are high-frequency exam topics. You will also use this knowledge when studying reversible reactions in acid-base chemistry later in the course. Make sure you practice predicting equilibrium shifts for different reaction scenarios, as these questions are often worth 2-3 marks in extended papers and are easy to score full marks on if you follow the rules correctly.

- [Rate of Reaction (CIE IGCSE 0620)](https://www.owlsprep.com/study/cie-0620-u6-rate-of-reaction/)

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