# Is oxidation always thermodynamically favorable?

In biological systems, energy is often extracted from molecules with reduced carbon backbones — the oxidation of those molecules has $\Delta G < 0$ and that energy can be captured, often in the reduction of other molecules. I'm wondering if it is always the case that oxidation is thermodynamically favorable and reduction thermodynamically unfavorable.

I suppose I'm not asking if the oxidation of any molecule is favorable (I'm pretty sure that's not the case), but for redox reactions that do occur, is it ever the case the oxidation half reaction is thermodynamically unfavorable and the reduction half reaction thermodynamically favorable?

• You can't measure the thermodynamics of a half-reaction. – Ivan Neretin Dec 8 '16 at 15:06
• You need two half reactions and the value of $\Delta G$ will depend where they are situated relative to one another in redox potential. Wikipedia has a large table of redox potentials, you can choose some reactions for yourself. – porphyrin Dec 8 '16 at 15:11
• @IvanNeretin - Is it therefore meaningless to talk about the ΔG for the oxidation of a particular molecule? The free energy of succinate in cells is higher than the free energy of fumarate, which is a result of the oxidation of succinate. Should I not say that the oxidation of succinate has a ΔG<0? – kevbonham Dec 8 '16 at 15:25
• Why, that's a full reaction, which is another story. It certainly has some $\Delta G$ and everything. – Ivan Neretin Dec 8 '16 at 15:37
• That is, if we write it as a full reaction: succinate plus oxygen $\to$ fumarate plus water. – Ivan Neretin Dec 8 '16 at 15:39

1. You can easily convert between the energy and potential difference of a redox reaction by multiplying with the exchanged charges: $$\Delta G= Uzq$$ $U$ is the potential (usually measured in Volt), $z$ the number of exchanged elementary charges and $q$ the elementary charge (usually measured in Coulomb).
2. Energy differences are transitive. That means: $$\Delta G (A\rightarrow C) = \Delta G (A \rightarrow B) + \Delta G (B \rightarrow C)$$