Reactions That Release or Absorb Heat
Distinguish between exothermic reactions (release energy, temperature rises) and endothermic reactions (absorb energy, temperature falls), with everyday and industrial examples
What a learner can do afterwards
- Defines exothermic and endothermic in terms of energy transfer to and from the surroundings
- Gives two examples of each type from everyday life (e.g. combustion, hand warmers; photosynthesis, cold packs)
- Explains energy changes during changes of state (melting is endothermic, freezing is exothermic)
- Interprets a temperature change in an experiment to classify the reaction type
1 · Read
Every chemical reaction moves energy between the chemicals and their surroundings. When a reaction releases energy into the surroundings, you feel the temperature rise around it. We call that an exothermic reaction. When a reaction absorbs energy from the surroundings, you feel the temperature fall. We call that an endothermic reaction. The energy doesn't vanish either way. It just moves from one place to another.
Burning wood, striking a match, and mixing wet cement are all exothermic: you can feel the heat they give off. Inside a sports cold pack, a solid dissolves in water, and that change is endothermic: it pulls heat from your hand, so the pack feels cold. Photosynthesis in a leaf is endothermic too. It takes in energy from sunlight and stores it in sugar.
Changes of state work the same way, even though no new substance forms. Melting ice absorbs energy from its surroundings, so melting is endothermic. Water freezing into ice releases energy into its surroundings, so freezing is exothermic.
In an experiment, your thermometer tells the story. If the temperature climbs after you mix two substances, the reaction is exothermic. If it drops, the reaction is endothermic.
Exothermic reactions release energy and warm things up; endothermic reactions absorb energy and cool things down.
Readings 'Hot packs and cold packs' and 'Energy from sunlight' are original to this dataset.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.