# Does the electrochemical series reverse in electrolysis?

Are these two statements true?

1. In a galvanic cell at standard lab conditions (SLC), the strongest reductant is $\ce{Li (s)}$ and the strongest oxidant is $\ce{F2 (g)}$.

2. In an electrolytic cell at SLC, the strongest oxidant is $\ce{Li+ (aq)}$ and the strongest reductant is $\ce{F- (aq)}$.

• Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. States of aggregation should not be subscripted, it is not wrong, but the recommendations (Sec. 2.1.) are different. – Martin - マーチン Sep 30 '15 at 10:51
• I thought so but thanks I'll keep that in mind – inspd Sep 30 '15 at 10:54

No, only the first is. The oxidising or reducing property of a species is tied to the species, not to the process in which the species is used. Therefore regardless of whether you are running a galvanic cell or an electrolytic cell, $\ce{F2}$ will always be a strong oxidant and $\ce{Li}$ a strong reductant.

The only difference about the two types of cells is that in the galvanic cell, you are letting the spontaneous cell reaction happen, which produces energy. In the electrolytic cell, you are using energy to force the non-spontaneous cell reaction to happen. The spontaneous cell reaction refers to the direction of the reaction with $\Delta G < 0$ (or equivalently, $E > 0$).

By this, I mean: you can use a galvanic cell to carry out the reaction $\ce{2 Li + F2 -> 2Li+ + 2F-}$. Since $\ce{Li}$ is a strong reductant and $\ce{F2}$ is a strong oxidant, this reaction will generate a lot of energy.

You can use an electrolytic cell to reverse that reaction (although you'd have to use molten and not aqueous $\ce{LiF}$). But since $\ce{Li+}$ is a weak oxidant and $\ce{F-}$ a weak reductant, you're going to need lots of energy to carry that out. Which makes perfect sense!

• SLC is an abbreviation for Standard Lab Conditions; i.e. 25 ºC temperature and 1 atm pressure. From what I've read, when you electrolyse molten NaCl, the oxidant is Na$^+$ and the reductant is Cl$^-$. Isn't that consistent with my second statement? – inspd Sep 30 '15 at 10:38
• @inspd I see, the more frequently used abbreviation is STP (standard temperature and pressure). You're right about the identities of the oxidant and reductant, but I think we might have different ideas about what the word "strong" means. If you refer to this page, "strong oxidant" means the LHS species for a half-reaction with a large positive $E^\circ$. "Strong reductant" means the RHS species for a half-reaction with a large negative $E^\circ$. – orthocresol Sep 30 '15 at 10:45
• then it should be "best in the LiF liquid" not "strongest in the chemical series" am I right ? – inspd Sep 30 '15 at 10:47
• @inspd Essentially that means a strong oxidant is a species that is easily reduced, and a strong reductant is a species that is easily oxidised. The fact that you would require a lot of energy for the electrolysis of NaCl means that the participating species (Na+ or Cl-) are hard to reduce/oxidise - which makes them weak oxidants/reductants. – orthocresol Sep 30 '15 at 10:47
• so when we electrolyse LiF(l), the best (but weakest) oxidant is: Li$^+$ and the best (but weakest) reductant is: F$^-$ – inspd Sep 30 '15 at 10:50