Page 125 - ICSE Chemistry 8

• Ammonia and hydrogen chloride combine together to form ammonium chloride which is a
solid.
NH (g) + HCl(g) NH Cl(s)
3
4
Ammonia Hydrogen chloride Ammonium chloride
The physical states in a chemical reac on are represented as solid state (s), liquid state (l), gaseous
state (g) and aqueous solu on (aq).

5. Change in energy: Some reac ons are characterised by absorp on or evolu on of heat, light or
any other radia on.
Depending upon the absorp on or evolu on of heat, Thirst for Knowledge
chemical reac ons are of two types.
Almost all reac ons are accompanied
Exothermic reac on: The chemical reac on in which
by energy changes.
heat energy is evolved is called exothermic reac on.
Examples:

• When water is added to quicklime, heat energy is evolved.
CaO(s) + H O(l) Ca(OH) (aq) + Heat
2
2

Quicklime Water Calcium hydroxide
• When carbon burns in oxygen, heat energy is evolved.
C(s) + O (g) CO (g)  + Heat
2
2
Carbon Oxygen Carbon dioxide
Endothermic reac on: The chemical reac on in which heat energy is absorbed is called endothermic
reac on.

Examples:
• When carbon and sulphur are heated, heat energy is absorbed.
C(s) + 2S(s) + Heat CS (l)
2
Carbon Sulphur Carbon disulphide
• When nitrogen and oxygen are heated together at a temperature of about 3000°C, nitric
oxide gas is formed.
3000°C
N (g) + O (g) + Heat 2NO(g) 
2
2
Nitrogen Oxygen Nitric oxide
Absorp on of light: Plants prepare food from carbon dioxide by absorp on of sunlight.
Sunlight
6CO (g) + 6H O(l) Chlorophyll C H O (aq) + 6O (g)
6 12
6
2
2
2
Carbon dioxide Water Glucose Oxygen
Conditions Necessary for Chemical Reactions
Chemical reac ons take place when necessary condi ons are met. Some chemical reac ons occur on
their own while some chemical reac ons take place in the presence of heat, light and electricity. Some
reac ons also take place when reactants are subjected to high pressure.

Let’s study certain condi ons required for chemical reac ons to take place.

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