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Energy is one of the important factors which has significant contribution to our living standards. The economic prosperity of any nation depends on its industries which in tum are linked with the availability of energy. The three primary sources of energy are: solar energy, nuclear energy and chemical energy. The main focus of our discussion in this unit is on chemical energy. It is a well known fact that almost all the chemical reactions are associated with energy changes. Quite often, the energy change accompanying a chemical reaction is more significant than the reaction itself. For example, liberation of heat energy during the burning of coal is more significant to a common man than the production of carbon dioxide. The energy changes occurring during the chemical reactions may not always appear as heat energy; but also as electrical energy, work energy and radiant energy as well.

Some examples are:

• Burning of coal or cooking gas in air.

C + O2 àCO2 + heat energy

C3 H8 + 5O2 à 3CO2 + 4H2O + heat energy

• Burning of magnesium ribbon in oxygen.

2Mg + O2 2MgO + Heat + Light.

• Slow combustion of phosphorous in dark.

P4 + 5O2 2Pp5 +Light energy.

• Combustion of petrol in an internal combustion engine.

C10Hn + 15 +O2 + 10CO2 + 11H2O +Mechanical energy + Heat.

Indirect redox reaction occurring in galvanic cell.

Zn(s) + CuSOi4(aq) à Cu(s) + ZnSO4(aq) +Electric energy

Thus, we find that chemical reactions are accompanied by energy changes appearing in different forms. These energy changes take place because during chemical reactions certain bonds are cleaved and certain new bonds are formed. Energy is consumed during cleavage of bonds while energy is released during the formation of bonds. Since the bond energy varies from one bond to another, the chemical reactions are always accompanied by absorption or release of energy.