strongest reducing agent EK question

[nai54]

Please help! On problem 921 in exam krackers it asks which is the strongest reducing agent?


My answer was F- because it has the most negative value, which I thought the higher the value the more it is oxidized therefore a better reducing agent.

But the solution says:
B is correct. Reducing agents want to be oxidized. Hg2+ and Au3+ can't be oxidized; their charges can't be increased anymore. F- and Cl- are already shown being oxidized, and the voltage is greater for the chloride, so choice B is correct. It is a fairly standard MCAT trick to write some half-reactions as oxidations rather than reductions, so watch out for it!

2F -> F2 +2e- -2.87V

2CI ->Cl2+2e -1.36 v

Hg2+ + 2e- ->Hg +0.85 v

Au3+ + 3e- -> Au +1.52 v

Can someone please explain why the answer is Cl- and not F??

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[Cawolf]

Between F- and Cl- you want to pick the halide that will most easily give up an electron.

The more positive oxidation potential (or less negative if you prefer) will be the "easiest" to do. It will require the least potential to accomplish - so chloride is the best choice.

So you are correct in saying that the highest value is the better reducing agent, but you picked the lower value. Sign matters, we aren't looking at magnitude.

 

[nai54]

Okay I think I was getting confused because they wrote cl- and f- oxidation potentials and I'm used to it being reduction potentials. I know I need to be able to distinguish it either way.

But just for my clarification, if we changed the signs of F- and Cl- oxidation potentials to + 2.87V and +1.36V so now these are their reduction potentials, F- has the greatest value , which usually means it is more likely to be reduced which shouldn't be the case, right?!

I don't know if I'm thinking too hard about this.

 

[Cawolf]

You mean if you wrote them as the reduction of the dihalides to halides?

F is more electronegative than Cl so it should have a higher reduction potential.

 

[nai54]

Yes ok that makes perfect sense. I see where my confusion came from. I was looking at these tables broadly and not realizing which species in the reaction the potentials actually pertained to. Thanks so much for straightening this out with me!

Let me just make sure I'm understanding everything clearly.
So the -2.87V value is the potential for the oxidation of F- and a +2.87V will be the reduction potential of F2?
On the other hand, the +0.85V is the reduction potential for Hg2+ and a -0.85V is the oxidation potential for Hg?

Annnd if we change everything to oxidation potentials Hg will be the most likely to undergo oxidation as opposed to Au, F2, and Cl2 right? Because it has the "most positive value?"

 

[Cawolf]

So the -2.87V value is the potential for the oxidation of F- and a +2.87V will be the reduction potential of F2?

Yes.

On the other hand, the +0.85V is the reduction potential for Hg2+ and a -0.85V is the oxidation potential for Hg?

Yes.

Annnd if we change everything to oxidation potentials Hg will be the most likely to undergo oxidation as opposed to Au, F2, and Cl2 right? Because it has the "most positive value?"

Yes.

 

[nai54]

Yay thanks again!