TBR Gen Chem Chp 10 #65

[Psychodramandy]

Hi all -

I had a question regarding TBR's Electrolysis passage from their old books:

My question is this: The table suggests that Cl2(g) has a stronger reduction potential than Ni(2+) so it makes sense that Cl(2) would be the one that is becoming reduced and Ni(2+) will have to be reversed (oxidized) - thereby making its emf + 0.23 and adding it with 1.36. That's basically what the solution to this Q suggests.

However - from the question that was given - Cl2(g) is shown to be the product. Does that not mean that we have to reverse the Cl2(g) + 2e- rxn instead and keep Ni2+ the way that it is?

Many thanks for your help!

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[aldol16]

Well, the cell potential in this case would be -1.36 V + -0.23 V, giving you -1.59 V. If you apply a potential of +1.75 V, that should be enough to drive the the reaction forward...

 

[Psychodramandy]

Well, the cell potential in this case would be -1.36 V + -0.23 V, giving you -1.59 V. If you apply a potential of +1.75 V, that should be enough to drive the the reaction forward...

Yo! Yeah thanks. I think TBR's reasoning for the answer was pretty whack.

 

[BerkReviewTeach]

Wow, I had to open a box from storage to find that version of the book.

The reasoning was basically as follows. For an unfavorable reaction to proceed, it must be coupled with a more favorable reaction. Each of the answer choices involves an unfavorable redox reaction (an electrolytic reaction), so the question is whether the applied voltage is large enough to overcome the reaction's negative voltage.

Choice A
-1.36 for oxidation of chloride to chlorine gas and -0.23 for the reduction of nickel II cation into nickel:

Ecell = -1.36 + (-0.23) = -1.59 V so applying 1.75 V will be enough to power the reaction.
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Choice B
-1.36 for oxidation of chloride to chlorine gas and -0.76 for the reduction of zinc II cation into nickel:

Ecell = -1.36 + (-0.76) = -2.12 V so applying 2.25 V will be enough to power the reaction.
​

Choice C
-1.09 for oxidation of bromide to bromine liquid and -0.23 for the reduction of nickel II cation into nickel:

Ecell = -1.09 + (-0.23) = -1.32 V so applying 1.60 V will be enough to power the reaction.
​

Choice D
-1.09 for oxidation of bromide to bromine liquid and-0.76 for the reduction of zinc II cation into nickel:

Ecell = -1.09 + (-0.76) = -1.85 V so applying 1.50 V will NOT be enough to power the reaction.
​

The reasoning associated with coupling reactions is pretty common in both general chemistry and biochemistry, so hopefully that concept seems okay.