Help with electrochemistry

[September24]

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This is problem 957 in EK 1001 chemistry

Consider the electrochemical cell that utilizes lithium and zinc


Li-+e-=Li E0= -3.05V
Zn(2+)+2e-=Zn E0= -0.76V

What is the standard voltage for this cell.


I understand that E0cell=Ecathode-Eanode

Here, zinc is a better oxidizing agent so it will be reduced at cathode. Therefore lithium will be oxidized at anode.

Oxidation potential for lithium is 3.05V while the reduction potential for zinc is -0.76.

If Ecell=Ecathode-Eanode
Ecell= -0.76-(+3.05)

Ecell= -3.81


However, the answer according to EK is 2.29V. I feel like I grossly have a concept messed up here.

 

[Ninblades]

This is problem 957 in EK 1001 chemistry

Consider the electrochemical cell that utilizes lithium and zinc


Li-+e-=Li E0= -3.05V
Zn(2+)+2e-=Zn E0= -0.76V

What is the standard voltage for this cell.


I understand that E0cell=Ecathode-Eanode

Here, zinc is a better oxidizing agent so it will be reduced at cathode. Therefore lithium will be oxidized at anode.

Oxidation potential for lithium is 3.05V while the reduction potential for zinc is -0.76.

If Ecell=Ecathode-Eanode
Ecell= -0.76-(+3.05)

Ecell= -3.81


However, the answer according to EK is 2.29V. I feel like I grossly have a concept messed up here.

Nope, you're exactly right, just be careful with the signs while doing cell potential calculations. It's better to remember the equation as:

Ecell = Ereduction + Eoxidation

Since zinc is reduced, and lithium is oxidized, you have to remember to flip the sign in the Ereduction of lithium to get Eoxidation of lithium

Ecell = -0.76 - (-3.05)
Ecell = 2.29V

 

[September24]

Ohhhhh. Okay. So since I already flipped the sign for lithium's potential, I can just add the two potentials?

I assume the cathode-anode equation already incorporates the flipped signs? Basically, if I recognize that lithium is the anode and the potential must become reversed, I can just add the two? Sorry if its getting confusing.

 

[FuzzyMemory]

Yep, that's right. Just remember when reading reduction potential tables to flip the signs for the opposite (oxidation) reaction, then it's just a simple matter of adding the two values. That's why I like to remember Ecell = Ereduction + Eoxidation. It makes it easier to keep track of signs.