# Thermodynamics: Calculating work done in irreversible expansion

1. May 28, 2013

### Rocalvic

1. The problem statement, all variables and given/known data

Suppose 100g of ethane (C2H6) expands isothermally at 350C from 50ml to 2L. Calculate the heat and work done by the system and the change in entropy if:

i)The process is via a reversible path.
ii)The process is non-reversible.

I don't know how to answer ii)
2. Relevant equations
w=-nRTln(v2/v1)
3. The attempt at a solution

I have managed to answer i)

Work = -NRTln (v2/v1) = (0.30026)(8.315)(623.15)ln(2/0.050)
Work = -5738.79 J.

Entropy change = nRln (v2/v1) = (0.30026)(8.3145)ln(2/0.050)
Entropy Change = 9.209 J.K

Heat: Q=-w, so Heat = 5738.79.

ii) I am unsure how to answer this question, do I apply the same principle? Or is this a result of free expansion and therefore the work done is 0?
Thanks

2. May 28, 2013

### Staff: Mentor

Hi Rocalvic,

Welcome to Physics Forums!!!!

You analyzed part i correctly, and you are also correct to note that there is not enough information provided to solve part ii (except for the change in entropy, of course). To do part ii, you need to know the pressure imposed by the surroundings during the irreversible change.