Calculate Free Energy for Formation

  1. 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. jcsd
  3. 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
     
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