Chemistry: Chapter 15 - Energy Changes

15.1 Exothermic and Endothermic Changes
Energy cannot be created or destroyed.
However, it can be changed from one form to another.

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

Characteristics of exothermic reactions

1. Heat is liberated and is transferred from the chemicals to the surroundings, and
2. the temperature of the reaction mixture rises. 

Examples of exothermic reactions include:

• combustion of fuels
• rusting of iron
• corrosion of metals
• reaction between acid and alkali (neutralisation)
• respiration

b) Endothermic Reactions
Reaction that absorb heat from the surroundings are called endothermic reactions.

Characteristics of endothermic reaction:

1. Heat energy is absorbed and is transferred from the surroundings to the reactants, and
2. the temperature of the reaction mixture falls.

Examples of endothermic reactions include

• Photosynthesis
• Action of light on silver bromide in photographic film
• thermal decomposition

Heat Changes in a Reaction
The amount of energy involved in a reaction is known as the heat change or enthaly change of the reaction.

For exothermic reaction, energy change is negative because it lose energy to the surroundings.
For endothermic reaction, energy change is positice because it gain energy to the surroundings.

15.2 Combustion of Fuels
Fuels give out energy when burnt.

Imcomplete combustion occurs when there is a limited supply of air is used.
The combustion of fuels is an exothermic reaction.

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Hot, Exothermic

Cold, Endothermic