Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example: C (g) + O₂ (g) → CO₂ + Heat Energy

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

Example:
              heat
N₂ + O₂  -------> 2NO

Respiration is considered as an exothermic reaction because in respiration oxidation of glucose takes place which produces large amount of heat energy.

C₆H₁₂O₆ (aq) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (l) + Energy

Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.For Example:
Decomposition Reaction:
                heat
CaCO₃(s) --------> CaO(s) + CO₂(g)

Combination Reaction:
CaO (s) + H₂O (l) → Ca(OH)₂ (aq)

 

In a displacement reaction, a more reactive element replaces a less reactive element from a compound.
For Example: CuSo₄ (aq) + Zn (s) → ZnSO₄ (aq) + Cu (s)
In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

For Example: Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)

2AgNO₃ (aq) + Cu (s) → Cu(NO₃)₂ (aq) + 2Ag (s)
Silver Nitrate + Copper → Copper Nitrate + Silver

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.For Example:
Na₂CO₃ (aq) + CaCl₂ (aq) → CaCO₃ (s) + 2NaCl (aq)
Sodium Carbonate + Calcium Chloride → Calcium Carbonate + Sodium Chloride

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction

Oxidation Reaction: It is a chemical reaction in which gain of oxygen or loss of hydrogen takes place.
For example:

In equation (i), H2is oxidized to H2O and in equation (ii), Cu is oxidised to CuO.

Reduction Reaction: It is a chemical reaction in which loss of oxygen or gain of hydrogen takes place.

For example:

 

In equation (i), CO2is reduced to CO and in equation (ii), CuO is reduced to Cu.