# Homework Help: Thermodynamics Entropy

1. Feb 6, 2014

### ruiwp13

1. The problem statement, all variables and given/known data
Calculate the variation of entropy in the following processes:
a) Heating of 18 kg of water from 15 to 40ºC at ambient pressure.
b) Compression of 9 kg of water from ambient pressure to 7atm at the temperature of 15ºC.

2. Relevant equations
ΔS=Cp*ln(T_final/T_initial)

3. The attempt at a solution

So for a) I checked the Cp of water getting 4.18KJ/kgºK and multiplied it by the 18 kg. Then I tried to apply the formula with the Cp and the Tfinal=313.15K and TInitial=288.15K and I think it is correct.

For b) I think I have to use ΔS=R ln (P1 / P2) but I'm not sure. And if it is that expression, the Pressure must be in kPa?

Last edited: Feb 6, 2014
2. Feb 6, 2014

### Staff: Mentor

In part (a) the equation you used applies to liquid water or to an ideal gas at constant volume. I assume you knew you were doing it for liquid water. The equation you used in part (b) is for an ideal gas, but not a liquid. The question must be referring to liquid water, because water can't exist as a vapor at these pressures and 15C. If the temperature doesn't change, what is the change in the internal energy of the liquid water? How much work is done on the water to compress it from 1 atm to 7 atm? How much heat is involved? What is the change in entropy?

Chet