C2.5 Exothermic and

Endothermic Reactions

C2.5 Exothermic and endothermic

reactionsChemical reactions involve energy transfers.

Many chemical reactions involve the release of

energy. For other chemical reactions to occur,

energy must be supplied.

BTEOTSSSBAT:

Know the difference between endothermic and

exothermic reactions

Know some example of endothermic and

exothermic reactions

Key terms

Reaction

Energy

Transfer

Surroundings

Exothermic

Endothermic

Reversible reaction

What happens when a chemical

reaction takes place?

What happens to the bonds?

Exothermic Reaction

• Exothermic reactions transfer energy to

the surroundings.

• The energy is usually transferred as heat

energy, causing the reaction mixture and

its surroundings to become hotter.

• Can you think of any examples?

• Combustion (burning)

• Many oxidation reactions,

for example rusting

• Neutralisation reactions

between acids and alkalis

• Hand warmers

Screaming Jelly babies

Screaming jelly babies!

potassium chlorate, a

strong oxidising agent, rapidly

oxidises the sugar in the jelly

baby

https://www.youtube.com/watch?v=aC-KOYQsIvU

Hot Ice: supersaturated sodium acetate

Endothermic Reaction

• These are reactions that take in energy

from the surroundings.

• The energy is usually transferred as heat

energy, causing the reaction mixture and

its surroundings to get colder.

• Can you think of any examples?

• The thermal

decomposition of

calcium carbonate

• Sports injury cold

packs

Endothermic

or

Exothermic?

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Thermal decomposition of marble?

Endothermic

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Combustion of methane?

Exothermic

Photosynthesis?

Endothermic

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Neutralisation of an acid and an alkali?

Exothermic

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2006 20

Rusting of iron?

Exothermic

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2006 21

Thermal decomposition of copper

carbonate?

Endothermic

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2006 22

Respiration?

Exothermic

A reusable hand warmer contains a solution of sodium acetate trihydrate.

When the hand warmer is activated, the sodium acetate trihydrate

crystallises and heat is released. The word equation for this reaction is

shown below:

1. What type of reaction is this?

(1 mark)

i. Using the information above, explain why hand warmers that

contain sodium acetate trihydrate are reusable.

(2 marks)

Exothermic

The reaction is reversible. (1 mark)

So the solid sodium acetate trihydrate that is formed when the

hand warmer is activated can be converted back into sodium acetate trihydrate

solution (1 mark).

ii. Suggest how the hand warmer could be reset after use, so that it

is ready to be used again. Explain your answer.

(2 marks)

The conversion of sodium acetate trihydrate solution to solid sodium

acetate trihydrate is exothermic, so the reverse reaction (the conversion

of solid sodium acetate trihydrate back to sodium acetate trihydrate

solution) must be endothermic (1 mark).

This means the hand warmer could be reset by heating it (1 mark).