5.1 Endothermic and Exothermic Reactions

FORM 3

CHAPTER 5
Thermochemistry:
5.1 Endothermic and
Exothermic Reactions

Name : Chong Xian Li
Class : C38

Heat Change in Chemical Reaction

❑ Heat or energy change occurs in most of the
chemical reactions.

❑ When a chemical reaction occurs, heat is
absorbed or released.

❑ Heat is absorbed to break the bond in a
compound. Conversely, heat is
released when the bond is formed.

❑ Chemical reactions can be classified into two
types, exothermic and
endothermic reactions.

❑ The chemical reactions are applied in the
making of hot and cold packs.

Let’s start!!

Endothermic and
Exothermic Reaction

Endothermic Reaction

❑ A reaction that causes the temperature of the
surroundings to decrease.

❑ Absorb heat energy from its surrounding to
break chemical bonds.

❑ Heat energy -> Chemical energy

Example Photosynthesis Electrolysis Evaporation

Baking cake Melting of ice Dissolving ammonium
nitrate powder in water

Exothermic Reaction

❑ Reaction that releases heat to the surroundings.
❑ When this reaction occurs, the reactant will lose heat

to the surroundings.
❑ Since heat is released from the reactant to the

surroundings, the surrounding temperature is raised.
❑ The contents in the container become hot. As a result,

the container also feels hot when touched or shows a
higher temperature when measured with a
thermometer.
❑ The total energy content of the product is less than
the total energy content of the reactants.

Example Respiration Explosion of a bomb Burning

Neutralization Rusting Thermite reaction

COLD PACK
AND

HOT PACK

Cold Pack

➢ Cold packs contain two separate compartments
– one is filled with water and the other is filled
with solid ammonium nitrate. These
compartments are separated by a thin film.

➢ When the pack is squeezed, the thin film that
separates the compartments will break and
allows ammonium nitrate to dissolve in water.

➢ The endothermic reaction produces instant cold
effect to help reduce swelling and muscle
spasms.

Hot Pack

1. Instant heat packs are used to heat hands, feet
or other body parts that are injured.

2. The pack uses the principle of exothermic
reaction that releases heat and warms up body
parts that are in contact with it.

3. The pack contains water and a type of salt,
such as magnesium sulphate or calcium
chloride that is packed separately.

When the heat pack is pressed,

• a thin membrane that separates the salt and water will break, causing
the water and the salt to mix

• the salt will dissolve in the water and energy is absorbed to break the
ionic bonds of the salt

• the free ions that are produced will then form bonds with water
molecules and this releases energy

Instant hot packs to treat muscle pains or to warm hands:

• Hot packs contain sodium acetate.

• When a hot pack is squeezed, the sodium acetate crystallises instantly.

• Crystallisation of sodium acetate is an exothermic reaction.

• The exothermic reaction gives out heat to increase blood flow and relieve
the injured tissues.

Chong Xian Li

THANKS!

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