Some of the important types of chemical reactions are : 1. Combination reactions, 2. Decomposition reactions, 3. Displacement reactions, 4. Double displacement reactions, and 5. Oxidation and Reduction reactions.
Those reactions in which two or more substances combine to form a single substance are called combination reactions.
Example 1: Magnesium and oxygen combine, when heated, to form magnesium oxide.
2Mg (s) + O2 (g)→2MgO (s)
Example 2: Hydrogen burns in oxygen to form water :
2H2 (g) + O2 (g) → 2H2O (l)
Example 3: Carbon (coal) burns in air to form carbon dioxide : C (s) + O2 (g) → CO2 (g)
Take a small amount of calcium oxide or quick lime in a beaker.
Slowly add water to this.
What do you feel when touching the beaker?
Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.
CaO(s) + H2O(l) → Ca(OH)2(aq)
IMPORTANT QUESTIONS AND ANSWERS:
1.Which of the following is a combination :
(a) Cl2 + 2KI → 2KCl + I2 (b) 2K + Cl2 → 2KCl
ANS: (b) is the combination reaction because a combination reaction is in which two or more substances combine to form a single substance.
EXOTHERMIC REACTION AND ENDOTHERMIC REACTION
Reactions in whichheat is released along with the formation of products are called exothermicchemical reactions.
Examples of Exothermic Reaction
Burning of natural gas
Respiration reaction in human beings
The decomposition of vegetable matter into compost
Reactions in which heat is absorbed along with the formation of products are calledendothermic chemical reactions.