Endothermic and Exothermic Reactions

M3 Science Workbook
Energy Transfer

Teacher : C.Wilson-James
[email protected]

At the end of this unit you should be able to define endothermic and
exothermic reactions .
Give examples of endothermic and exothermic reaction

Explanation:
The overall net movement of energy determines if the
reaction is endothermic or exothermic. The reaction is:
● exothermic if more heat energy is released in

forming bonds in the products than is required in
breaking bonds in the reactants
● endothermic if less heat energy is released in
forming bonds in the products than is required in
breaking bonds in the reactant

Distinguish between Endothermic and Exothermic Reaction

An endothermic reaction is one that takes in energy from
the surroundings so the temperature of the surroundings
decreases. Examples include:
● electrolysis
● photosynthesis
● thermal decomposition

Endothermic reactions include thermal decompositions
and the reaction of citric acid and sodium
hydrogencarbonate. Some sports injury packs are based
on endothermic reactions.

An exothermic reaction is one that transfers energy to
the surroundings so the temperature of the surroundings
increases. Examples include:
● combustion
● neutralisation
● many oxidation reactions

Endothermic and Exothermic: During a chemical
reaction, energy is either transferred to or from its
surroundings.

Reactions which involve heat energy being released
into the surrounding environment are called
exothermic reactions.

By contrast, endothermic reactions draw heat energy
from their surroundings. This causes the temperature
of the surrounding environment to decrease.

Examples of Endothermic And Exothermic
Reaction

Everyday uses of exothermic reactions include
self-heating cans and hand warmers.
Depending on the reactants, the following reactions
can be either exothermic or endothermic:
● salts dissolving in water
● precipitation reactions

Chemical reactions can be thought of as happening in
two stages:
1. Energy is needed to break the bonds in the

reactants to form their separate atoms. This energy
is called the activation energy, and is an
endothermic process.
2. The newly separated atoms form new bonds to
make the products, and release energy to their
surroundings. This is an exothermic process.