# Reversible reactions and equilibria

> Chemistry · Edexcel IGCSE 4CH1 (2017 spec)
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s3-reversible-reactions-and-equilibria/

This guide covers all Edexcel IGCSE 4CH1 content for reversible reactions and equilibria, including core reaction examples, dynamic equilibrium characteristics (Higher only), and temperature/pressure shift rules.

**Prerequisites:** [Basic reaction equation writing](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-chemical-formulae-equations/); [Exothermic and endothermic reactions](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s3-energetics/)

## Learning objectives

- Identify reversible reactions using the ⇌ symbol
- Describe the reversible dehydration of hydrated copper(II) sulfate and thermal decomposition of ammonium chloride
- Explain the characteristics of dynamic equilibrium in closed systems (Higher only)
- Predict how temperature and pressure changes shift equilibrium position (Higher only)
- Recall that catalysts do not alter equilibrium position (Higher only)

## Reversible Reactions (Core Content, All Tiers)

**Reversible Reaction** — A chemical reaction that can proceed in both the forward (reactants → products) and reverse (products → reactants) directions, indicated by the ⇌ symbol in balanced equations.

You must recall two key reversible reactions for the exam: the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride.

**Worked example:** Write the balanced equation for the reversible dehydration of hydrated copper(II) sulfate, including state symbols.

1. 1. Identify the reactant: hydrated copper(II) sulfate is blue crystalline $CuSO_4 \cdot 5H_2O(s)$
2. 2. Identify products on heating: white anhydrous copper(II) sulfate $CuSO_4(s)$ and water vapour $H_2O(g)$
3. 3. Add the reversible symbol (the equation is already balanced):
4. $$CuSO_4 \cdot 5H_2O(s) \rightleftharpoons CuSO_4(s) + 5H_2O(g)$$

**Worked example:** Describe the reversible change observed when ammonium chloride is heated in a sealed test tube.

1. 1. Heating white solid ammonium chloride causes it to thermally decompose into colourless ammonia and hydrogen chloride gases
2. 2. As the gases cool at the top of the sealed tube, they react to reform solid white ammonium chloride, which deposits on the glass
3. $$NH_4Cl(s) \rightleftharpoons NH_3(g) + HCl(g)$$

> **Exam tip:** You may be asked to link copper sulfate dehydration to the test for water: adding water to white anhydrous copper sulfate turns it blue, confirming water is present.

*Calculator:* allowed

## Dynamic Equilibrium (Higher/Chemistry Only)

**Dynamic Equilibrium** — State reached in a closed (sealed) system where the rate of the forward reaction equals the rate of the reverse reaction, so concentrations of reactants and products remain constant over time.

Note that 'dynamic' means both reactions are still occurring constantly, not that the reaction has stopped. The equal reaction rates mean no overall observable change is seen at equilibrium.

> **info**
>
> You must explicitly mention a closed/sealed system when describing how equilibrium is reached in exam answers to get full marks.

**Worked example:** A student heats hydrated copper sulfate in an open beaker and claims equilibrium is reached. Explain why they are incorrect.

1. 1. Dynamic equilibrium requires a closed system, where no substances can enter or leave.
2. 2. In an open beaker, water vapour produced from dehydration escapes into the air, so the reverse reaction (reforming hydrated copper sulfate) cannot occur at the same rate as the forward reaction.
3. 3. Equilibrium is therefore never achieved in an open container for this reaction.

*Calculator:* allowed

## Factors Affecting Equilibrium Position (Higher/Chemistry Only)

The position of equilibrium describes whether the mixture contains more reactants or products at equilibrium. Changes to temperature or pressure can shift this position, while catalysts have no effect.

**Catalyst effect on equilibrium** — A catalyst speeds up both the forward and reverse reaction rates equally, so it does not change the position of equilibrium, only reduces the time taken to reach equilibrium.

1. Temperature change: An increase in temperature shifts equilibrium in the direction of the endothermic reaction (absorbs heat). A decrease in temperature shifts equilibrium in the direction of the exothermic reaction (releases heat).
2. Pressure change (only applies to gas-phase reactions): An increase in pressure shifts equilibrium towards the side with fewer moles of gas. A decrease in pressure shifts equilibrium towards the side with more moles of gas.

**Worked example:** For the reversible reaction below, predict the effect of (a) increasing temperature, (b) increasing pressure: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$  ΔH = -92 kJ/mol (forward reaction exothermic)

1. (a) Increasing temperature: Shift occurs in the endothermic direction. Since forward is exothermic, reverse is endothermic, so equilibrium shifts left, forming more $N_2$ and $H_2$.
2. (b) Count moles of gas on each side: Left side = 1 + 3 = 4 moles of gas, right side = 2 moles of gas. Increasing pressure shifts to fewer moles, so equilibrium shifts right, forming more $NH_3$.

> **Exam tip:** Always state your reasoning when predicting shifts: e.g., 'shift left because the reverse reaction is endothermic' rather than just writing 'shifts left' to secure full marks.

*Calculator:* allowed

## Key Recap and Concept Check

**Summary**

- Core tier: Recall ⇌ symbol for reversible reactions, copper sulfate and ammonium chloride reversible reaction examples
- Higher tier: Equilibrium requires closed system, forward/reverse rates equal, concentrations constant
- Higher tier: Catalysts do not change equilibrium position, only speed up reaching equilibrium
- Higher tier: Temp shift towards endothermic when increased, pressure shift towards fewer gas moles when increased

**Check your understanding**

1. Which symbol indicates a reversible reaction?

   - →
   - ↔
   - ⇌
   - =

   *Answer:* ⇌

   *Why:* Correct. The ⇌ symbol is used exclusively for reversible reactions in Edexcel IGCSE Chemistry.

2. (Higher only) Which statement about dynamic equilibrium is true?

   - Reactions stop when equilibrium is reached
   - Forward reaction rate is higher than reverse rate
   - Concentrations of reactants and products are equal
   - Equilibrium only occurs in closed systems

   *Answer:* Equilibrium only occurs in closed systems

   *Why:* Correct. A closed sealed system is required so no reactants or products can escape, allowing rates to equalise.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Using → instead of ⇌ for reversible reactions
  - Why it fails: The single arrow indicates a reaction that goes to completion, not reversible, so you lose the mark for correct symbol use.
  - Correct: Always use ⇌ for all reversible reaction equations, including the two named core examples.
- **Wrong:** Stating that reactions stop at dynamic equilibrium
  - Why it fails: Equilibrium is dynamic, meaning both forward and reverse reactions are still occurring at equal rates, with no net change.
  - Correct: Explicitly state that both reactions continue at the same rate at equilibrium.
- **Wrong:** Claiming catalysts shift equilibrium position
  - Why it fails: Catalysts speed up forward and reverse rates equally, so there is no change to equilibrium position or product yield.
  - Correct: State that catalysts only reduce the time taken to reach equilibrium, with no effect on the equilibrium position.
- **Wrong:** Forgetting to state a closed system is required for equilibrium
  - Why it fails: Exam answers require explicit mention of a closed/sealed system to award full marks for equilibrium conditions.
  - Correct: Always include reference to a closed system when explaining how equilibrium is achieved.
- **Wrong:** Applying pressure shift rules to reactions with no gaseous reactants/products
  - Why it fails: Pressure only affects equilibrium of gas-phase reactions, not reactions with only solid or liquid reactants/products.
  - Correct: First check for gas state symbols before applying pressure shift rules, and count only moles of gas on each side of the equation.

## Cheatsheet

| Content Area | Core Tier Requirement | Higher Tier Requirement |
| --- | --- | --- |
| Reversible reaction symbol | Recall ⇌ means reversible | Same as core |
| Named reactions | Describe copper sulfate dehydration and ammonium chloride heating reversibility | Same as core |
| Dynamic equilibrium | Not assessed | Closed system, equal forward/reverse rates, constant reactant/product concentrations |
| Catalyst effect | Not assessed | No change to equilibrium position, only speeds up reaching equilibrium |
| Temperature shift | Not assessed | Increase temp → shift to endothermic direction; decrease temp → shift to exothermic direction |
| Pressure shift | Not assessed | Increase pressure → shift to side with fewer gas moles; decrease pressure → shift to side with more gas moles |

## What's next

Now that you have mastered reversible reactions and equilibria, you can move on to applying these rules to context-based exam questions, and link this content to other physical chemistry topics such as reaction rates and energetics. This knowledge will also form a strong foundation if you progress to A-Level Chemistry, where you will learn about quantitative equilibrium calculations. Be sure to practice past paper questions specifically for this topic to test your ability to apply shift rules correctly under timed exam conditions.

- [Energetics (S3_T04)](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s3-energetics/)

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