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THIRD LAW OF THERMODYNAMICS
The entropy of a pure crystalline substance increases with increase of temperature, because molecular motion increases with increase of temperature and vice - versa.
Or the entropy of a perfectly crystalline solid approaches zero as the absolute zero of temperature is approached. This is third law of thermodynamics.
The branch of chemistry which deals with energy changes involved in chemical reactions is called thermochemistry. The energy change that occurs in a chemical reactions is largely due to change of bond energy.
Change of internal energy in a chemical reaction
Let us consider a chemical reaction taking place at constant temperature and at constant volume. In such a case, w = 0 and hence from the first law
ΔU = qv
Where qv is the heat exchanged at constant volume, or heat or enthalpy of reaction at constant volume.
Change of Enthalpy in a chemical reaction
Let qP be the heat exchanged in the chemical reaction taking place at constant pressure, Then evidently,
ΔH = qP = Heat or Enthalpy of reaction at constant pressure.
Exothermic and Endothermic reaction
Reaction that give out heat, i.e. which are accompanied by evolution of heat, are called exothermic reaction. In such reactions ΔH is negative. On the other hand, reaction that intake heat, i.e. which are accompanied by absorption of heat are called endothermic reactions. In these reactions ΔH is positive.
Solved example. Fill in the blanks with appropriate word in following:
(i) Combustion of reactions are usually ……………………..
(ii) Combustion of F2 in oxygen is ……………………..
Solution: (ii) Exothermic