Question

Define enthalpy of vaporization?

Answer 1

Hint: Firstly to understand the concept behind this question we should be aware of the drive behind the change of state from one state to another, definition of enthalpy and the process of vaporization.

Complete answer:
-The operation by which a system changes from one state to another state is known as process. This change in state is accompanied by change in energy. The change in energy may appear in the form of heat, work, light etc. The total energy stored in the system is known as enthalpy of the system which is given by :
H = U + PV
The change in enthalpy is given by:
$$\mathrm{\Delta }H=\mathrm{\Delta }U+P\mathrm{\Delta }V$$
-The process of conversion of liquid to gas phase is known as vaporization. Now, let’s understand the enthalpy of vaporization: We know that the liquid is heated to convert it to gas. When liquid evaporates it absorbs some amount of heat from the surroundings. From the above discussion we that enthalpy means the total energy stored in a system. So, we can say that evaporation of liquid is accomplished by an increase in enthalpy as this process absorbs heat where heat is also a kind of energy.
Case 1: The evaporation of one mole of water at ${25}^{\circ }C$ is 20.25KJ.
 $$H_2{O}_{(l)}\to H_2{O}_{(g)}$$
So, $\mathrm{\Delta }{H}_{vap}$= +20.25KJ
-When the vapour condenses to liquid, the heat is given out. Hence condensation of vapour to liquid decreases the enthalpy of the system.
Case 2: The condensation of one mole of water vapour at ${25}^{\circ }C$ is -20.25KJ.
$$H_2{O}_{(g)}\to H_2{O}_{(l)}$$
So, $\mathrm{\Delta }{H}_{vap}$= -20.25KJ
-We know the definition of enthalpy. In both cases we absorb $\mathrm{\Delta }{H}_{vap}$ is used. $\mathrm{\Delta }{H}_{vap}$ is the enthalpy of vaporization. Above two cases signify it.

Enthalpy of vaporization can be defined as the change in enthalpy when a liquid enters vapour state or when the vapour changes into liquid state.

Note: In case the sign of enthalpy of vaporization is positive because there absorption of heat takes place which adds energy to the system but in the second case the heat of vaporization is negative because there heat is given out, basically there is loss of heat. Understanding enthalpy well helps us define the enthalpy of vaporization still more easily.