Question

What is the free energy change $\left(\Delta G\right)$ when $1.0$ mole of water at ${100}^{o}C$ and $1atm$ pressure is converted into steam at ${100}^{o}C$ and $2atm$ pressure?

• A. Zero cal
• B. $540cal$
• C. $517.13cal$
• D. $510cal$

Answer 1

The correct option is C $517.13cal$

Change in free energy id zero in a phase transition at same temperature and pressure.

But it is a function of pressure as:

$\Delta G=nRT\ln \left(\frac{p_2}{p_1}\right)$

$R=2cal/K/mol,n=1,T=373K,p_1=1atm,p_2=2atm$

on substitution

$\Delta G=1.987\times 373\ln \left(\frac{2}{1}\right)$

$\Delta G=517.1cal/mol$

Option C is correct