Exothermic and endothermic reactions
Introduced| English | Meaning |
|---|---|
| exothermic/eɡzəˈðɜːmɪk/ | Describing a process that transfers heat to its surroundings. |
| endothermic/ˌendəʊˈθɜːmɪk/ | Describing a process that takes in heat from its surroundings. |
| combustion/kəmˈbʌstʃn/ | An exothermic reaction of a fuel with an oxidising agent, usually oxygen. |
| neutralisation/ˌnjuːtrəlaɪˈzeɪʃn/ | A reaction between an acid and a base that forms a salt, usually with water. |
| thermal decomposition/ˈθɜːml ˌdiːkɒmpəˈzɪʃn/ | The breakdown of a compound into simpler substances when heated. |
Reactions that heat or cool
- Strike a match and it gives out heat; dissolve some salts and the beaker turns cold.
- Reactions either release or absorb energy. These are exothermic 放热反应 and endothermic 吸热反应.
An exothermic reaction:
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Exothermic reactions release energy.
Burning (combustion) is an example of a(n):
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Combustion releases heat — exothermic.
In an exothermic reaction, the products are at a higher energy than the reactants.
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False. In an exothermic reaction the products are at lower energy; the difference is released to the surroundings as heat.
Exothermic reactions
- An exothermic reaction releases energy to the surroundings (they get hotter).
- Examples: combustion 燃烧 (burning), neutralisation 中和, most everyday reactions.
In an exothermic reaction, the products are at lower energy than the reactants — energy is released.
Exothermic & endothermic
ΔH = products − reactants
Exothermic drops to lower-energy products (releases heat); endothermic climbs to higher.
An endothermic reaction makes the surroundings:
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It absorbs energy, cooling the surroundings.
A reaction that absorbs energy and cools its surroundings is ______.
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Endothermic reactions take in energy.
Endothermic reactions
- An endothermic reaction absorbs energy from the surroundings (they get colder).
- Examples: thermal decomposition 热分解, photosynthesis, some dissolving.
Burning wood is exothermic, releasing heat to the surroundings
Telling them apart
- Exothermic: surroundings warm up; products at lower energy than reactants.
- Endothermic: surroundings cool down; products at higher energy than reactants.
Exothermic = exits (releases) energy; endothermic = enters (absorbs). A quick check: if the beaker gets hot, energy is leaving the reaction (exothermic); if it gets cold, energy is being taken in (endothermic).
An exothermic reaction warms the surroundings; an endothermic reaction cools them
You've got it
- exothermic reactions release energy (surroundings get hotter; products lower energy)
- endothermic reactions absorb energy (surroundings get colder; products higher energy)
- combustion is exothermic; thermal decomposition is endothermic
Write from an endothermic decomposition
- Calcium carbonate is heated and breaks down into calcium oxide and carbon dioxide.
- The surroundings cool if the heat is not replaced. Burning methane heats the surroundings.
Is the decomposition exothermic or endothermic?
Thermal decomposition absorbs energy.
What kind of energy change is this decomposition?
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Heat is absorbed from the surroundings.