Example 2
Here is a little more complex reaction:
2ZnO(s)+2C(g)→2Zn(s)+2CO(g)
2ZnO(s)+2C(g)→2Zn(s)+2CO(g)
If this reaction occurs at room temperature (25º C) and the enthalpy, ΔHΔH, and standard free energy, ΔGΔG, is given at -957.8 kJ and -935.3 kJ, respectively. One must work backwards somewhat using the same equation from Example 1 for the free energy is given.
-935.3 kJ = -957.8 kJ + (298.15 K) (ΔSΔS)
22.47 kJ = (298.15 K) (ΔSΔS) (Add -957.8 kJ to both sides)
0.07538 kJ/K = ΔSΔS (Divide by 298.15 K to both sides)
Multiply the entropy by 1000 to convert the answer to Joules, and the new answer is 75.38 J/K.