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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 + DG2
This 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.
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