## Electrochemistry and reverse reactions

Viridiana G [3M]
Posts: 6
Joined: Fri Sep 26, 2014 2:02 pm

### Electrochemistry and reverse reactions

When we work with reverse reactions in Electrochemistry, does the sign of $\Delta E_{cell}^{\circ}$ flip (positive/negative) as well when we look up its value for the reduction half-reaction?

For example, I wrote out $Ni \to Ni^{2+} + 2e^{-}$ for exercise 13.11
When we look up the $\Delta E_{cell}^{\circ}$ value for this, we find $Ni^{2+} + 2e^{-} \to Ni$ and $\Delta E_{cell}^{\circ}$ is equal to +.80V. Since we had to use the reverse reaction to find the value, to we flip the sign of the $\Delta E_{cell}^{\circ}$ value?

Thanks!

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Electrochemistry and reverse reactions

Yes, you change the sign of E° when you flip the reduction half reaction, but if you multiply the half reaction by a constant, you don't multiply E° by the same constant as we did for enthalpy, for example, since E° is an intensive property and is always the same.

If you use the equation E°cell=E°cathode-E°anode, however, you don't change the sign since this equation accounts for the sign change and uses the standard reduction potentials for both the oxidation and reduction half reactions.
Last edited by Niharika Reddy 1D on Thu Feb 05, 2015 7:36 pm, edited 1 time in total.

Megan White 2C
Posts: 18
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Electrochemistry and reverse reactions

Hi Viridiana!

Upon looking up the standard REDUCTION potentials in the index when dealing with an oxidation half reaction, you have two options:

#1: You can keep the value in the back of the book (without switching the sign)
#2: Or, you can switch the sign.

In using the formula for the cell potential (E cell = E cathode (reduction) - E anode (oxidation)), this is referring to the option #1 where the value for the oxidation reaction (written as a reduction reaction in the index) does not change sign in your calculations. You would simply subtract the value in the index for the oxidation reaction from the value in the index for the reduction reaction. No switching of signs.

If using option #2 (switching the sign for the value of the oxidation reaction written as a reduction reaction in the index), you will ADD the cathode (reduction) and the anode (oxidation) to find the final cell potential. You would be adding a positive + a negative value, which is essentially a subtraction problem.

I hope this helps!!

AKatukota
Posts: 100
Joined: Thu Jul 25, 2019 12:18 am

### Re: Electrochemistry and reverse reactions

This makes sense. I was wondering why in the equation we not change the sign. Thank you!