How much energy do we require during a distillation

Performing a simple distillation experiment is every chemist s delight. We gently warm a mixture of liquids, allowing each component to boil off at its own characteristic temperature (the boiling temperature r(boii)). Each gaseous component cools and condenses to allow collection. Purification and separation are thereby effected. 

Although we have looked already at boiling and condensation, until now we have always assumed that no work was done. We now see how invalid this assumption was. A heater located within the distillation apparatus, such as an isomantle, supplies heat energy q to molecules of the liquid. Heating the flask increases the internal energy U of the liquids sufficiently for it to vaporize and thence become a gas. 

But not all of the heater s energy q goes into raising U. We need some of it to perform pressure-volume work, since the vapour formed on boiling works to push back the external atmosphere. The difference between the internal energy U and the available energy (the enthalpy) is given by 

H is a state function since p, V and U are each state functions. As a state function, the enthalpy is convenient for dealing with systems in which the pressure is constant but the volume is free to change. This way, an enthalpy can be equated with the energy supplied as heat, so q = AH. 

Worked Example 3.8 A mole of water vaporizes. What is the change in enthalpy, A HI 

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