Question

Place liquid water (system) at 25°C in a vacuum insulated space. What is the value of each thermodynamic function when evaporated and saturated?

deltaH(water),

deltaS(water) ,

deltaT(water) ,

deltaS(surrounding) ,

deltaS(universe)

delta is a triangel sign

Answer 1

Liquid water system in a vaccum insulated space will boil.

In vaccum as there is no loss of energy there is tendency for water to boil at lower pressure and freeze at lower temperature. The boiling point of water decreases at lower pressure so in vaccum, as the pressure is lower the liquid water tends to boil, but if we keep the pressure at vaccum constant and decrease the temperature then it freezes.

Thats what happens in space also , first the liquid water boils and then freezes.

In normal conditions,

At 25 C ,

When water is evaporated its delta H will be positive as we are supplying energy to it so that it boils . Water in its liquid state is bounded together by hydrogen bonding and it needs to absorb energy to break all that bonds. Without absorbing that energy it cannot break its bonds and change its state from liquid to water vapour.

Its an endothermic reaction.

So

And when we provide energy ( here in the form of heat) the molecules of the water in liquid state absorbs that energy and gain momentum and kinetic energy and starts to move faster , the freedom of movement increases as they move from liquid to vapour state and hence the disorderness of the system increases.

So

And its pretty clear that the temperature increases as we supply energy so

now as of delta S surrounding and delta S universe is positive as the delta S total must always be positive.

And when its saturated in a closed container its pretty much at equilibrium between its liquid and gaseous states

Now for a liquid water system in vaccum at a constant temperature of 25C.

Here during evaporation we donot need to supply energy for the water to boil at vaccum. The reaction is spontaneous as it is .

So for  

as for delta S surroundings and universe is also positive.

When the container in vaccum is saturated with water vapour then with the limitation of space freedom of movement is lower than before and the molecules collide with each other which produces energy

But as we know in vaccum as there is no loss of energy to the surroundings , the energy produced in the container has to used up there itself . The energy produced due to collision is the energy lost from the water molecules due to which they starts condensing to liquid.

So enthalpy change is negetive

And entropy decreses due to restriction of motion and hence the disorderness also decreases . So entropy is negetive

And temperature doesnt decrease so it means positive.

Therefore,

Delta S surrounding will be positive with a value higher than delta S (system) or delta S (water) so that the total entropy change stays positive because the delta S total always needs to be positive .

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