CHEMISTRY · Stage 8 · Chapter 9: Chemical Reactions 2

Chemistry 9.3 Energy changes — Exothermic and Endothermic Reactions

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Lesson summary

9.3 Energy changes — Exothermic and Endothermic Reactions — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define exothermic and endothermic changes, give examples and use temperature change to decide which a change is.Fuels — kerosene, charcoal and gas transfer e…

Energy transfer
Hydrochloric acid and sodium hydroxide
Citric acid and sodium hydrogencarbonate
Reducing errors
Breaking bonds
Forming bonds

Part 1 of 7Energy changes in reactions

Fuels — kerosene, charcoal and gas transfer energy from the fuel when they burn, and the energy does something useful such as cooking.
Energy transfer — all chemical reactions involve energy changes, and energy is never created or destroyed.
- Energy is transferred between the chemicals and the surroundings.
Bonds — energy is needed to break bonds in the reactants and is released when new bonds form in the products.

Part 2 of 7Exothermic reactions

1Exothermic1 point

= a change that transfers thermal energy to the surroundings.

The temperature of the reacting mixture rises at first, then energy is transferred to the surroundings.

2Examples1 point

— all burning (combustion) reactions, some neutralisation reactions, respiration and many metal-with-acid reactions.

Hand warmers and self-heating food cans use exothermic reactions.

3Energy

— the products have less stored energy than the reactants.

Part 3 of 7Endothermic reactions

Endothermic = a change that transfers thermal energy from the surroundings.
- The temperature of the reacting mixture falls at first, then the mixture warms to room temperature.
Examples — the reaction of citric acid with sodium hydrogencarbonate, thermal decomposition and photosynthesis.
Energy — the products have more stored energy than the reactants.

Feature	Exothermic	Endothermic
Energy transfer	To surroundings	From surroundings
Temperature of mixture	Rises	Falls
Product energy	Lower than reactants	Higher than reactants
Example	Burning methane	Heating copper carbonate

Part 4 of 7Using temperature change

Method — put a measured volume of one reactant in an insulated plastic cup, record its temperature, add the second reactant, stir, and record the highest or lowest temperature.
Temperature change = final temperature - starting temperature.
- A positive change shows an exothermic reaction and a negative change shows an endothermic reaction.
Hydrochloric acid and sodium hydroxide — the temperature rises from 21 °C and 17 °C to 52 °C, so the neutralisation is exothermic.
- One hour later the mixture is back at 21 °C as energy is transferred to the surroundings.
Citric acid and sodium hydrogencarbonate — the mixture fizzes and the temperature falls, so the reaction is endothermic.
Reducing errors — a lid and a polystyrene cup reduce heat loss, and repeating and averaging improves reliability.

Part 5 of 7Explaining with bond breaking and making

Breaking bondsit needs energy, so it is endothermic.
Forming bondsit releases energy, so it is exothermic.
Exothermic overallmore energy is released making new bonds than is needed to break the old ones.
Endothermic overallmore energy is needed to break the old bonds than is released making new ones.
Energy level diagramthe product line is below the reactants for exothermic and above for endothermic, and the gap is the energy transferred.
Activation energymany reactions need a small input to start, such as striking a match, even when they are exothermic.

Part 6 of 7Changes of state and dissolving

Changes of state — melting and evaporation are endothermic, whereas condensing and freezing are exothermic.
- Sweat evaporating transfers energy from the skin, which cools you down.
- Melting ice takes energy from your hand, and the particles use it to leave their places and slide over each other.
Dissolving — it can be exothermic or endothermic.

Substance	Temperature on dissolving	Type
Ammonium chloride	Decreases	Endothermic
Ammonium nitrate	Decreases	Endothermic
Copper sulfate	Increases	Exothermic
Potassium nitrate	Increases	Exothermic

Part 7 of 7Using an endothermic change: cold packs

Structurean outer bag holds liquid water and an inner bag holds ammonium nitrate powder.
Actionbreaking the inner bag makes the ammonium nitrate dissolve and the mixture cools.
Usethe warm injured body part transfers energy to the cold water, so the injury cools and feels better.

Last updated: 13 October 2026