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.
COMBINATION REACTION
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)
CaO reacts with water releases heat
ACTIVITY 1.4
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:
Combination reaction
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
difference between 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.
The substance which we use for white-washing our house is lime (or quicklime) which is calcium oxide (CaO).
We put calcium oxide in a drum and add water to it slowly.
Calcium oxide reacts with water vigorously to form a white solid called calcium hydroxide (or slaked lime) with the evolution of heat.
More water is then added to get calcium hydroxide solution.
This calcium hydroxide solution is then applied to the walls of the house with a brush.
The calcium hydroxide solution, when applied to the walls, reacts slowly with the carbon dioxide gas present in air to form a thin, shining layer of calcium carbonate on the walls of the house :
Ca(OH)2 (aq) + CO2 (g) →CaCO3 (s) + H2O (l)
Since this process gives a white, shiny appearance to the walls of a house, it is called white-washing.
The calcium carbonate is actually formed after two to three days of white-washing and gives a shiny finish to the walls