# Exothermic and Endothermic Reactions

> CIE IGCSE Chemistry · 0620 2026-2028
> Source: https://www.owlsprep.com/study/cie-0620-u5-exothermic-and-endothermic-reactions/

This guide covers core and extended content for exothermic and endothermic reactions in CIE IGCSE Chemistry 0620 Unit 5, including definitions, examples, energy diagrams, and extended-only bond energy calculations.

**Prerequisites:** [Basic knowledge of chemical reactions and covalent/ionic bonding](https://www.owlsprep.com/study/cie-0620-u1-chemical-bonding/); Understanding of basic energy transfer concepts

## Learning objectives

- Define exothermic and endothermic reactions in terms of heat transfer to/from surroundings
- Identify common examples of each reaction type from core syllabus lists
- Interpret and draw energy level diagrams for both reaction types (core)
- Calculate enthalpy change (ΔH) from bond energy values (extended only)
- Classify reactions as exothermic or endothermic using experimental temperature change data

## Core: Definitions and Key Characteristics

**Exothermic Reaction** — A chemical reaction that transfers heat energy from the reacting system to the surroundings, leading to a measurable increase in the temperature of the surroundings.

*Example:* Neutralisation of dilute hydrochloric acid and sodium hydroxide

Endothermic reactions are the opposite: they absorb heat energy from the surroundings, so the temperature of the surroundings decreases during the reaction. In all reactions, energy is conserved: the total amount of energy in the system and surroundings remains the same.

**Worked example:** A student adds ammonium nitrate crystals to water in a polystyrene cup. The temperature of the mixture falls from 22°C to 11°C. State if this process is exothermic or endothermic, and justify your answer.

1. First, observe the change in temperature of the surroundings (the water mixture in the cup).
2. The temperature decreased, meaning heat was removed from the surroundings by the dissolving process.
3. Therefore, the process is endothermic.

> **Exam Tip**
>
> Always reference temperature change data in justifications, as this is the key evidence examiners look for when you classify reactions as exothermic or endothermic from experimental results.

## Core: Common Examples and Practical Identification

- **Exothermic reaction examples (syllabus specified):** combustion of fuels, neutralisation of acids and alkalis, aerobic respiration, reactions of reactive metals with dilute acids
- **Endothermic reaction examples (syllabus specified):** thermal decomposition of calcium carbonate, photosynthesis, reaction of citric acid with sodium hydrogencarbonate

**Worked example:** Classify each of the following processes as exothermic or endothermic: a) Burning natural gas to heat a home, b) Thermal decomposition of lead nitrate, c) Respiration in muscle cells.

1. a) Burning is a combustion reaction, which releases heat to the surroundings → exothermic.
2. b) Thermal decomposition requires continuous heat input to proceed, so it absorbs heat from surroundings → endothermic.
3. c) Respiration breaks down glucose to release energy for cells → exothermic.

**Check your understanding**

1. Which of the following is an endothermic reaction?

   - A: Neutralisation of hydrochloric acid and sodium hydroxide
   - B: Combustion of ethanol
   - C: Thermal decomposition of calcium carbonate
   - D: Reaction of magnesium with dilute sulfuric acid

   *Why:* Neutralisation, combustion and metal-acid reactions are all exothermic. All thermal decomposition reactions require heat input, so they are endothermic.

## Core: Energy Level Diagrams

**Energy Level Diagram** — A graphical representation of a reaction, with reaction progress plotted on the x-axis and total energy of chemicals plotted on the y-axis. It shows the relative energy of reactants and products.

For exothermic reactions, products have lower total energy than reactants. The vertical gap between reactant and product energy levels is the amount of heat released to the surroundings. For endothermic reactions, products have higher total energy than reactants, so the gap represents the amount of heat absorbed from the surroundings.

**Worked example:** Describe the key features of an energy level diagram for the endothermic reaction of photosynthesis, where carbon dioxide and water react to form glucose and oxygen.

1. Draw a horizontal x-axis labelled 'Reaction Progress' and vertical y-axis labelled 'Energy'.
2. Draw a horizontal line on the left for reactants (CO₂ + H₂O) at a lower energy level than the horizontal line on the right for products (glucose + O₂).
3. Draw an upward arrow between the two lines, labelled 'Energy absorbed from surroundings'.

> **Exam tip**
>
> Examiners often ask you to add activation energy labels to these diagrams. Remember that activation energy is the minimum energy required for a reaction to start, and it is always shown as a hump above the reactant energy level for both reaction types.

## Extended Only: Enthalpy Change from Bond Energies

**Bond Energy** — The amount of energy required to break 1 mole of a specific covalent bond in the gaseous state, measured in kJ/mol. Breaking bonds is endothermic (requires energy input), forming bonds is exothermic (releases energy).

1. To calculate overall enthalpy change (ΔH): Calculate total energy required to break all bonds in reactants (positive value, endothermic)
2. Calculate total energy released when all new bonds in products are formed (negative value, exothermic)
3. Add the two values: ΔH = total bond breaking energy + total bond forming energy. A negative ΔH = exothermic, positive ΔH = endothermic

**Worked example:** Use the given bond energies to calculate ΔH for the reaction: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies (kJ/mol): C-H = 414, O=O = 498, C=O = 803, O-H = 464.

1. Count all bonds in reactants: 4 C-H bonds in CH₄, 2 O=O bonds in 2O₂.
2. $$Total bond breaking energy = (4 × 414) + (2 × 498) = 1656 + 996 = 2652 kJ/mol$$
3. Count all bonds in products: 2 C=O bonds in CO₂, 4 O-H bonds in 2H₂O.
4. $$Total bond forming energy = (2 × -803) + (4 × -464) = -1606 - 1856 = -3462 kJ/mol$$
5. $$ΔH = 2652 + (-3462) = -810 kJ/mol$$
6. ΔH is negative, so the combustion of methane is exothermic, as expected.

> **warning**
>
> Always draw the displayed formula of all reactants and products before counting bonds, to avoid missing or double-counting bonds in molecules with multiple identical bonds.

## Common pitfalls

- **Wrong:** Confusing heat transfer direction, stating exothermic reactions absorb heat from surroundings.
  - Why it fails: Students mix up the prefixes exo- and endo-. Exo- means 'out' (heat exits the reaction system), endo- means 'in' (heat enters the system).
  - Correct: Use the mnemonic: EXO = EXIT (heat leaves the reaction), ENDO = ENTER (heat enters the reaction).
- **Wrong:** Assuming all reactions that require initial heating are endothermic.
  - Why it fails: Exothermic reactions often need initial heating to reach activation energy, but will release heat once started without an external heat source (e.g. burning wood).
  - Correct: Check if the reaction continues to release heat after the heat source is removed to confirm it is exothermic.
- **Wrong:** Drawing product energy levels lower than reactant levels on endothermic reaction diagrams.
  - Why it fails: Students often copy the exothermic diagram pattern for both reaction types.
  - Correct: Remember endothermic reactions absorb energy, so products store more energy than reactants, meaning their level on the diagram is higher.
- **Wrong:** Forgetting to count the number of moles of each molecule when calculating ΔH from bond energies.
  - Why it fails: For example, using 1 O=O bond instead of 2 for 2 moles of O₂ leads to incorrect ΔH values.
  - Correct: Multiply the bond energy for each bond by the number of that bond per molecule, then by the number of moles of the molecule in the balanced equation.
- **Wrong:** Classifying physical changes like melting or boiling as endothermic chemical reactions.
  - Why it fails: These processes absorb heat, but no new chemical substance is formed, so they are not reactions.
  - Correct: Only classify processes that produce new chemical substances as reactions; explicitly note physical changes if asked in exam questions.

## Cheatsheet

| Property | Exothermic Reaction | Endothermic Reaction |
| --- | --- | --- |
| Heat transfer direction | Released to surroundings | Absorbed from surroundings |
| Surrounding temperature change | Increases | Decreases |
| ΔH value (Extended) | Negative | Positive |
| Product vs reactant energy level | Products < Reactants | Products > Reactants |
| Key syllabus examples | Combustion, neutralisation, respiration | Thermal decomposition, photosynthesis, citric acid + sodium hydrogencarbonate |

## What's next

Now that you have mastered exothermic and endothermic reactions, you can build on this foundation for other topics in CIE IGCSE Chemistry 0620. Next, you will learn about activation energy, which explains why some reactions need initial heat input to start even if they are exothermic. For extended students, you can practice more bond energy calculation questions to build speed and accuracy for Paper 4 exams. This knowledge will also be critical when you study rates of reaction in the next unit, as reaction energy changes directly relate to how quickly reactions proceed. You will also apply these concepts later when learning about combustion of organic fuels and thermal decomposition of metal carbonates.

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