Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Addition of NaOH to water Sodium hydroxide pellets are added to water. The

Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.

Addition of NaOH to water

Sodium hydroxide pellets are added to water.

The mixture is carefully stirred to dissolve the pellets.

The temperature of the water is recorded before and after adding sodium hydroxide.


Addition of NH4Cl to water

Ammonium chloride crystals are added to water.

The mixture is carefully stirred to dissolve the pellets.

The temperature of the water is recorded before and after adding ammonium chloride.


Results of Both Experiments

Experiment

Solid Initial temperature

(°C)

Final temperature (°C)

I NaOH 28 34

II NH4Cl 28 22


What conclusions about energy changes can be made from these 2 experiments?

In experiment I, heat energy was given out when the solid dissolved in water.

Thus, the temperature of the solution rose.

We say this change is exothermic.


What conclusions about energy changes can be made from these 2 experiments?

In experiment II, heat energy was taken in when the solid dissolved in water.

Thus, the temperature of the solution dropped.

We say this change is endothermic.


We have just seen that heat energy is given out during a physical process, such as when sodium hydroxide dissolves in water.

Heat energy may also be given out during a chemical reaction, such as when sodium hydroxide reacts with hydrochloric acid.


Exothermic Reactions

Reactions that give out heat energy to the surroundings are called exothermic reactions.


What are the characteristics of exothermic reactions?

When an exothermic reaction occurs,

a) heat is liberated and is transferred from

the chemicals to the surroundings, and

b) the temperature of the reaction mixture

rises. The container feels warm.


How does temperature change in an exothermic reaction?

Initially, the temperature of the reaction mixture rises until the highest temperature is reached.

When the reaction is completed, the temperature of the reaction mixture falls until it reaches room temperature.


Examples of exothermic reactions

• Combustion of fuels

• Rusting of iron

• Corrosion of metals

• Reaction between acid and alkali

(neutralisation),

• Respiration


Endothermic Reactions

Reactions that absorb heat from the

surroundings are called endothermic

reactions.


What are the characteristics of endothermic reactions?

When an endothermic reaction occurs,

a) heat is absorbed and is transferred from the

surroundings to the reactants, and

b) the temperature of the reaction mixture

falls. It feels cold.


Initially, the temperature of the reaction mixture falls until the lowest temperature is reached.

When the reaction is completed, the temperature of the reaction mixture rises until it reaches room temperature.

How does temperature change in an endothermic reaction?


• Photosynthesis

• Action of light on silver bromide in

photographic film

• Thermal decomposition

for example

Examples of endothermic reactions

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Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Addition of NaOH to water Sodium hydroxide pellets are added to water. The