C7 Energy changes

C7.1

What is the difference between endothermic and exothermic?

Exothermic products have a lower energy content than endothermic products, which have a higher energy content. Endothermic reactions transfer energy from the environment and reduce the temperature; exothermic reactions transfer energy to the environment and increase the temperature.

Name an endothermic reaction.

Thermal decomposition is an example of an endothermic reaction.

Name an exothermic reaction.

Neutralisation and combustion are examples of exothermic reactions.

Required practical: finding out if a reaction is endothermic or exothermic

Mix different combinations and stir every 30 seconds for 5 minutes (eg). Record the initial and final temperature. Find out if the temperature decreased (endothermic) or increased (exothermic).

C7.2

Types of handwarmers

Handwarmers only used once are based on the energy transferred during the oxidation of iron. Sodium chloride handwarmers are disposable, used once, but last hours. Handwarmers lasting many hours are based on the formation of crystals in a solution; to reuse it, put it in boiling water to redissolve the crystals.

What else are exothermic reactions used for?

They are also used in self-heating cans. The reaction for this is:

calcium oxide + water = calcium hydroxide

The uses of endothermic reactions

Endothermic reactions are used in ice/cold packs which reduce pain. The water and ammonium nitrate are kept separate; when the water inside the bag breaks the ammonium nitrate is released. Instant cold packs are…