Themes > Science > Chemistry > Miscellenous > Help file Index > Spontaneity and Entropy > Combining equations and DG If you combine two chemical equations, the DG for the combined reaction is the sum of the DGs for the reactions you added. Reaction 3 = Reaction 1 + Reaction 2 DG3 = DG1 + DG2This fact allows chemists to find ways to make non-spontaneous reactions occur: simply find another reaction that when added to the non-spontaneous one gives a total DG that is negative Example: The reaction to extract zinc metal from zinc sulfide shown below is not spontaneous at room temperature ZnS(s) -> Zn(s) + S(s)since it has a DG of +201.3 kJ/mol. What happens if you add some oxygen to the reaction vessel: the oxygen can react with the sulfur in the following reaction S(s) + O2(g) -> SO2(g)DG for this reaction is -300.2 kJ/mol. Solution: If we add the two reactions above, we get a third reaction: ZnS(s) -> Zn(s) + S(s) +        DG1 = +201.3 kJ/mol S(s) + O2(g) -> SO2(g) =        DG2 = -300.2 kJ/mol ZnS(s) + O2(g) -> Zn(s) + SO2(g)        DG3 = ? DG for the third reaction is equal to the sum of the first two: DG3 = 201.3 + -300.2 = -98.9 kJ/mol, and the reaction of zinc sulfide with oxygen to form zinc metal is spontaneous. Information provided by: http://learn.chem.vt.edu