Vaporization Heat and the Heat Capacity of H20 comparison

[StarChem]

According to the Vaporization Heat table, the heat needed for 1 mol of H2O to evaporate at 100°C is 40.7KJ and 44.0KJ/mol is needed to evaporate H2O at 25°C. Thus 44.0-40.7=3.7KJ is the energy needed to heat H2O to 100°C from 25°C. However, according to the heat capacity of H2O, 3.7KJ will only warm the water by ~+48.6°C, which is not enough to reach 100°C starting from 25°C!

 

[Borek]

Thus 44.0-40.7=3.7KJ is the energy needed to heat H2O to 100°C from 25°C

Nope, doesn't follow. To convert liquid water at 25°C to vapor at 100°C you have to either heat water and evaporate it, or evaporate water and heat the vapor - plus you will need to do work compressing the vapor, as saturated vapor pressure at 25°C is much lower than 1 atm that you will get at 100°C.

 

[StarChem]

Thanks Borek for your reply.
I'm still confused though; how water can be evaporated at 25°C then heated, Is it by lowering the pressure? And what the compression work is needed for?

 

[Borek]

how water can be evaporated at 25°C then heated

Water evaporates at all temperatures, till the vapor gets saturated. Then you heat up the vapor - not much different from heating up air.

To convert water vapor saturated at 25 °C (about 3.17 kPa) to water vapor saturated at 100 °C (101 kPa) you need to both heat the vapor up and compress it.