Page 53 - Chemistry ICSE Class IX
P. 53
Chemical Changes and Reactions 41
Why do we observe heat changes in chemical reactions
/ƚ ŵĂLJ ďĞ ƉŽŝŶƚĞĚ ŽƵƚ ŚĞƌĞ ƚŚĂƚ ƚŚĞ
A chemical reaction involves breaking of bonds in the reactant molecules ƚĞƌŵ ͚heat oĨ reacƟon͛ ŝƐ ŽŌĞŶ ƵƐĞĚ͘
and formation of new bonds in the product molecules. /ƚ ŚĂƐ ŶŽǁ ďĞĞŶ ƌĞƉůĂĐĞĚ ďLJ Ă ŶĞǁĞƌ
ƚĞƌŵ ĐĂůůĞĚ enthalpy oĨ reacƟon͘
To break the bonds in a reactant molecule, energy is absorbed, i.e., the
bond breaking is an endothermic process. During the formation of new
bonds in the product molecules, energy is released, i.e., the bond formation
is an exothermic process.
Energy change in any reaction is equal to the difference between the
energy absorbed during bond breaking, and the energy released during
bond formation, i.e.,
⎛ Energy absorbed during the ⎞ ⎛ Energy evolved during ⎞
Energy change during a reaction = ⎜ ⎝ bond-breaking process ⎟ ⎠ − ⎜ ⎝ the bond-forming process ⎠ ⎟
Reactions in Which Heat is Either Absorbed or
Evolved
If the energies involved are considered to be heat energies, then the heat
change during the reaction is given by
⎛ Heat absorbed during the ⎞ ⎛ Heat evolved during the ⎞
Heat change during a reaction = ⎜ bond-breaking process ⎟ − ⎜ bond-forming process ⎟
⎝
⎝
⎠
⎠
Depending upon the heat absorbed and evolved, the following two
cases become possible:
When the heat evolved in the formation of new bonds is more than the
heat absorbed in the breaking of bonds, then heat change during the
reaction is negative (–ve).
When the heat evolved in the formation of new bonds is less than the
heat absorbed in the breaking of bonds, then heat change during the
reaction is positive (+ve).
What is an exothermic reaction
The term exothermic means giving out heat, (exo – out and thermic – heat).
So, the reactions in which heat is liberated (given out) are known as
exothermic reactions.
Following are some typical exothermic reactions:
Burning of carbon. Carbon (C) burns in oxygen (or in air) to form
carbon dioxide (CO ) gas and liberate a large amount of heat.
2
C(s) + O (g) o CO (g) + Heat
2
2
carbon oxygen carbon dioxide
Burning of methane. Methane burns in air to form carbon dioxide and dŚĞ ƌĞĂĐƟŽŶƐ ǁŚŝĐŚ ŚĂǀĞ ŶĞŐĂƟǀĞ
water, and liberate a large amount of heat. Thus, burning of methane ŚĞĂƚ ŽĨ ƌĞĂĐƟŽŶ ĂƌĞ ĐĂůůĞĚ
edžothermic reacƟons͘
(CH ) is an exothermic reaction. ͻ
4
CH (g) + 2O (g) o CO (g) + 2H O(g) + Heat dŚĞ ƌĞĂĐƟŽŶƐ ǁŚŝĐŚ ŚĂǀĞ ƉŽƐŝƟǀĞ
2
2
4
2
methane oxygen carbon dioxide water ŚĞĂƚ ŽĨ ƌĞĂĐƟŽŶ ĂƌĞ ĐĂůůĞĚ
endothermic reacƟons.
Some other exothermic reactions are:
Dissolution of concentrated sulphuric acid in water
Dissolution of caustic soda (sodium hydroxide, NaOH) in water
Combustion of LPG, kerosene, furnace oil, etc.
Reaction of lime (CaO) with water (H O)
2
