Calculate the delta G for each of the following reactions. State whether each reaction is spontaneous or non-spontaneous at standard conditions, and explain your reasoning.
a. 2 SO2(g) + O2(g)---> 2 SO3(g)
b. NO2(g) + NO(g) + O2(g)---> N2O5(g)
c. 6CL2(g) + 2 Fe2O3(s)--->4 FeCl3(s) + 3O2(g)