Enthalpy of Vaporizations:
During evaporation of liquid, some absorption of heat from surroundings takes place. Hence, Liquid evaporation is fulfilled by increase in enthalpy.
The increase in enthalpy while evaporation of one mole of water at 25oC is 43.93 kJ.
This result can be expressed in the form of thermo-chemical equation which is given below:
H2O (l) ———–> H2O (g);
∆H = + 43.93 kJ
When vapors are condensed into liquid sate, evolution of heat take place. Hence condensation of liquid takes place by decrease in enthalpy.
During condensation of one mole of water vapor at 25oC, the decrease in enthalpy is 43.93 kJ. Therefore,
H2O (g) ———–> H2O (l);
∆H = – 43.93 kJ
The change in enthalpy i.e. ∆H when liquid changes into vapor state and when vapor changes into liquid state is known as Enthalpy of Vaporization.
Enthalpy of Fusion:
Liquid is formed when melting of solid take place. During this the absorption of heat takes place.
In this enthalpy of system increases. The increase in enthalpy while Fusion of one mole of water at 0oC is 6.02 kJ.
It is shown below:
H2O (s) ———–> H2O (l);
∆H = + 6.02 kJ
On the other hand, when liquid freezes and changes into solid state. Heat is given out. Hence in this, the enthalpy of the decreases i.e.:
H2O (l) ———–> H2O (s);
∆H = – 6.02 kJ
The change in enthalpy i.e. ∆H when solid changes into liquid state and when liquid changes into solid state is known as Enthalpy of Fusion.
