A chemist wants to convert substance M to substance O in a single overall process whose standard free-energy change lies between −10 and −20kJ mol−1. She has access to the following reactions (all ΔG∘ values are in kJ mol−1):
(1) M → N +12
(2) N → O −25
(3) M → P +30
(4) P → O −35
(5) O → M +5
Assuming intermediates must cancel completely, which pair of reactions should she combine to meet her free-energy target for the overall transformation M → O?
a) Reactions (1) and (4)
b) Reactions (3) and (4)
c) Reactions (1) and (2)
d) Reactions (2) and (5)