Standard Free Energies
Standard free energies of formation can be used to obtain standard free energies of reaction.
 
When G is determined at 25oC and 1 atm, we call it the standard free energy change, ΔGo. There are a number of ways of obtaining Go for a reaction. One of them is to computer ΔGo from ΔHo and ΔSo using:  
ΔG = ΔHo - TΔSo
 
Sample Problem
Compute Go for the hydrolysis of urea, CO(NH2)2,
 
CO(NH2)2(aq) + H2O(l) ----> CO2(g) + 2 NH3(g)
Solution
We can calculate Ho from standard heats of formation from Hess's Law.
 
                      ΔHo = [CO2 + (2)NH3] - [CO(NH2)2 + H2O]
                             = [ -393.5 + (2)-46.19 ] - [ -319.2 - 285.9 ]
                             = (-485.9 kJ) - (-605.1)
                             = +119.2 kJ
 
We have already calculated the So in a previous example.
But we must be careful to express Ho and So in the same units.
 
                   ΔGo = -119.2 kJ - (298.15 K)(0.3548 kJ/K)
                           = +119.2 kJ - 105.8 kJ
                           = +13.4 kJ
 
The value obtained for Go means that at room temperature 25oC urea will not spontaneously decompose in water.
 
It is also useful to calculate the Standard Free Energies of Formation, Gfo. The equation to use is a variation of the Hfo and Sfo equations you've already used.
 
                 Δ Gfo = (sum of Gfo of products) - (sun of Gfo of reactants)
 
Sample Problem
What is ΔGo for the combustion of ethyl alcohol (C2H5OH) to give CO2(g) and H2O(g)?
 
Solution
First we need a balanced equation
 
                  C2H5OH(l) + 3 O2(g) ----> 2 CO2(g) + 3 H2O(g)
 
Using the information in the databook found next to the enthalpy and entropy data you will find the Gibbs Free energy data.
 
                  ΔGo = [(2)CO2(g) + (3)H2O(g)] - [C2H5OH(l) + (3)O2(g)]
 
As with Hfo, Gfo is zero for an element in its standard state.
 
                 ΔGo = [2 mol(-394.4 kJ/mol) + 3 mol(-228.6 kJ/mol)]
                        = [1 mol(-174.8 kJ/mol) + 3 mol(0 kJ/mol)]
                        = (-1474.6 kJ) - (-174.8 kJ)
                        = -1299.8 kJ

Go to the Gibb's Free Energy Worksheet
Thermochemistry Unit Review
Entropy and Gibbs Free Energy Review