# Homework Help: Calculate Free Energy for Formation

1. Oct 14, 2007

### mrlucky0

I realize that this is a chem prob, but I'm stuck with what I thought was a straightforward problem while studying for my physical chemistry exam.

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

The standard molar enthalpy of combustion of solid phenol (C6H5OH) is -3054 kJ/mol at 298k and it's standard molar entropy is 144.0 J/(K*mol). Calculate the standard free energy of formation of phentol at 298k.

2. Relevant equations

G = H - TS

3. The attempt at a solution

G = (-3054.0 * 10^3 J/mol) - (298 K) (144.0 J/k/mol)

From what I know about the determination of G, I simply plugged in those numbers but my solution was way off. The solution should be -50.42 kJ/mol but help with deriving it from first principle is what I'm after.

2. Jan 26, 2008

### gaea

energy of formation of C6H5OH

you have to calculate the standard reaction entropy first and the reaction enthalpies.

G = rxn enthalpies - (Temp x rxn entropy)

The reaction entropy is: entropy of products - entropy of reactants
the reaction enthalpy is: enthalpy of products - entropy of reactants