HW 14.55

[Ana Ordonez 1G]

A 1.0M NiSO4(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction; (b) the anode reaction. (c) With no over potential or passivity at the electrodes, what is the minimum potential that must be supplied to the cell for the onset of electrolysis?

Can someone help me understand how Ni2+(aq) + 2e- -> Ni(s) with E°= -0.23V is the cathode, and 2H20(l) -> O2(g) + 4 H+(aq) + 4e- with E°= +1.23V is the anode. Also, in the book the E°for the anode was +0.82V so how did it change? Lastly, how do I know whether to use 2H2O(l) + 2e- -> H2(g) + 2OH(aq) or O2(g) + 4 H+(aq) + 4e- ->2H20(l)?

Thank you!

[Belicia Tang 1B]

I have the same question. How did you know to use 2H20(l) -> O2(g) + 4 H+(aq) + 4e- as the anode reaction? Is it because the NiSO4 (aq) is in water?



Thanks!

[Lizzie Zhang 2C]

In the solution manual it says, because SO4 2- will not oxidize thus we have to use the water one. However, I'm so confused that how do you decide whether an ion will oxidize or not? There is an equation for the reduction of SO4 2- in appendix, why it cannot be oxidized?

Thanks for anyone who's willing to explain!

[Michelle Nguyen 2L]

Hi all,
You can tell which half reaction to use for water by the fact that water must always be the reactant! Therefore, to find which substance gets reduced at the cathode, compare the substance in question to the cell potential of water getting reduced, and at the cathode, compare to the cell potential of water being oxidized, always making sure water is the reactant and not a product. This is because we start off the reaction with water, not with the other compounds involved. At the cathode, the reduced substance is the one whose standard reduction potential is more positive, and at the anode, we pick the oxidized substance as the one with the more negative standard reduction potential, just like how we decide which is oxidized and which is reduced for galvanic cells. Hope that helps!

[ClaireHW]

How do we know to use H2O in part b?

(Claire Woolson Dis 1K)