# Homework Help: Phase change?

1. Oct 9, 2005

### doomed

phase change?????

How do I calculate the values of change in entropy (S) and change in Gibbs free enery (G) for the conversion of n=1 mol of liquid water at 100 C and 1 bar pressure in to vapor at the same temperatue and a pressure of 0.1 bar. Assume ideal behavior for the vapor. the molar enthalpy for vaporization of water at 100 C and 1 bar is 40.6 kJ/mol.

I know that delta S = delta H/T, but how the change in pressure play into this problem for delta S and delta G????

2. Oct 9, 2005

### clive

I'll give you some hints for the entropy:

You have 2 thermodynamic processes there:
1) vaporization
2) isothermal expansion form 1 bar to 0.1 bar.

Because the temperature is the same, the variation in entropy can easily be evaluated by
$$\Delta S=\frac{Q_1+Q_2}{T}$$
where Q1+Q2 represents the total transfered heat.

Now you have
$$Q_1=m\cdot \lambda_v$$
for the vaporization at 100 C
and
$$Q_2=\nu R T \ln \frac{V_2}{V_1}$$
for the isothermal expansion

and so on.......($$p_1 V_1=p_2 V_2$$ is the answer at your last question)

Last edited: Oct 9, 2005
3. Oct 9, 2005

### Tom Mattson

Staff Emeritus
doomed,

1.) Post only one thread per topic.

2.) Post homework questions in the Homework Help Forums, not the Physics Forum.