Worksheet question

Feb 2020
"Jonas is on the beach and bathing. When he comes out of the water is his body
covered with 2.5 dl of water. As long as the water remains, the heat of the sun and body will
be absorbed by the water so that it evaporates. How much energy is spent on it
to evaporate the water if it is from the beginning is 23 degrees warm.?"

The answer is 650 kJ. But I cannot arrive at this conclusion?
Apr 2015
Somerset, England
I presume you have calculated this by calculating the heat to raise the water from 23 to 100 degress then added the latent heat of water ?

Since the mass of 2.5 decilitres of water is 250g, I make this

250 (4.2(100-23) + 2.26) Joules = 81,415 J = 81.4 kJ.

But your water also expands on evaporation and the energy for the expansion work against the atmosphere also comes from the absorbed heat.

So the total energy input is higher than this.

Some call this total the enthalpy of vapourisation, some call it the heat of vapourisation.

This is 2.4465 kJ /g

so 250 * 2.4465 = 611.5 kJ

There is a vapourisation calculator, starting from any temp in degrees C here

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