Exothermic and Endothermic Reactions
CIE IGCSE Chemistry· 5.1 Chemical Energetics· 45 min read
1. Core: Definitions and Key Characteristics★★☆☆☆⏱ 10 min
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.
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.
2. Core: Common Examples and Practical Identification★★☆☆☆⏱ 8 min
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
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.
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
Reveal answer
C —Neutralisation, combustion and metal-acid reactions are all exothermic. All thermal decomposition reactions require heat input, so they are endothermic.
3. Core: Energy Level Diagrams★★★☆☆⏱ 12 min
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.
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'.
4. Extended Only: Enthalpy Change from Bond Energies★★★★☆Extended only⏱ 15 min
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).
To calculate overall enthalpy change (ΔH): Calculate total energy required to break all bonds in reactants (positive value, endothermic)
Calculate total energy released when all new bonds in products are formed (negative value, exothermic)
Add the two values: ΔH = total bond breaking energy + total bond forming energy. A negative ΔH = exothermic, positive ΔH = endothermic
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
- 3
Count all bonds in products: 2 C=O bonds in CO₂, 4 O-H bonds in 2H₂O.
- 4
- 5
- 6
ΔH is negative, so the combustion of methane is exothermic, as expected.
5. Common Pitfalls
Wrong move:
Confusing heat transfer direction, stating exothermic reactions absorb heat from surroundings.
Why:
Students mix up the prefixes exo- and endo-. Exo- means 'out' (heat exits the reaction system), endo- means 'in' (heat enters the system).
Correct move:
Use the mnemonic: EXO = EXIT (heat leaves the reaction), ENDO = ENTER (heat enters the reaction).
Wrong move:
Assuming all reactions that require initial heating are endothermic.
Why:
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 move:
Check if the reaction continues to release heat after the heat source is removed to confirm it is exothermic.
Wrong move:
Drawing product energy levels lower than reactant levels on endothermic reaction diagrams.
Why:
Students often copy the exothermic diagram pattern for both reaction types.
Correct move:
Remember endothermic reactions absorb energy, so products store more energy than reactants, meaning their level on the diagram is higher.
Wrong move:
Forgetting to count the number of moles of each molecule when calculating ΔH from bond energies.
Why:
For example, using 1 O=O bond instead of 2 for 2 moles of O₂ leads to incorrect ΔH values.
Correct move:
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 move:
Classifying physical changes like melting or boiling as endothermic chemical reactions.
Why:
These processes absorb heat, but no new chemical substance is formed, so they are not reactions.
Correct move:
Only classify processes that produce new chemical substances as reactions; explicitly note physical changes if asked in exam questions.
6. Quick Reference 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 |
7. Frequently Asked
Are physical changes like melting or boiling classed as endothermic reactions?
Melting and boiling are endothermic processes (they absorb heat) but they are not chemical reactions, as no new chemical substance is formed. Examiners may ask you to classify both reactions and physical changes, so read questions carefully.
Do all endothermic reactions need to be heated continuously?
Most endothermic reactions require a continuous heat source to proceed, as they absorb heat from the surroundings to form higher-energy products. Exothermic reactions may need initial heating to reach activation energy, but will release heat once started without an external heat source.
Going deeper
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.