Homework 14.25

[Angela 1K]

The question asks to put the following metals in order of increasing strength as reducing agents for species in aqueous solutions.
Cu, Zn, Cr, Fe

According to the solutions manual, Zn is weaker than Cr. When looking at appendix 2B, the E^o of Cr³⁺ + 3e^- —> Cr is -0.74 V. However the E^o of Cr^2- + 2e^- —> Cr is -0.91 V. The only way that Zn can be weaker than Cr is if we refer to the second equation rather than the first for Cr. How would we know to refer to the second half reaction for Cr rather the first?

Thanks, and hopefully that makes sense.

[Payton Schwesinger 1J]

I was wondering this too. Maybe you have to compare the ones with the same oxidation state/number of electrons if there is more than one option?

[Hannah Chew 2A]

I asked the UA this, and he said it was definitely unclear, but you should always go with the half reaction that is most (-), even if there are multiple listed half reactions involving the same element.

[Diane Bui 2J]

This problem was also unclear to me, but I estimated the average between the two numbers and used that value to compare which standard potentials were greater.

[soniatripathy]

I think that in terms if application for this class, they will only give us one value for easier comparison (i was worried about this for the test but the sheet they gave us seemed to be simplified and eliminated this confusion)