Kaplan Standard Reduction Potential

[betterfuture]

According to the following standard reduction potentials

So I always get confused as to why they switched the sign for the standard reduction potential for Zn from -0.76 to +0.76.

The way I look at it, from the reaction given, Zn is oxidized and Ag is reduced. As for the standard reduction potentials table given, it shows that the more + the standard reduction potential is, the more likely it is for that species to be reduced. Table shows that Ag is +0.8 and Zn is -0.76, so therefore, Ag is the species that is likely to be reduced while the less positive one gets oxidized, the Zn. So from the reaction, it shows exactly that - Ag is reduced and Zn is oxidized, so why the need to reverse only the sign for Zn?

I have always encountered these problems where sometimes they would switch the signs, sometimes they won't. Can someone who legit understands this help me out? This really irks me cause this always seems to happen. Thanks again!

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[aldol16]

I'm not sure what question you have. You say that Ag is reduced and Zn is oxidized. Therefore, the reaction Ag+ + e- ----> Ag is relevant and the redox potential is +0.8 V, as written. You acknowledge that Zn is oxidized. This is Zn ---> Zn2+ + 2e-. This is the formal reverse of the reaction shown above, so you have to reverse the redox potential if you're going to combine the equations and simply add the potentials.

The other way of doing it, which is much less intuitive, is to memorize the equation E cell = E(cathode) - E(anode). These are the standard reduction potentials and so it would be the potential associated with reduction minus the potential associated with oxidation. Here, the switching of the sign is built into the equation so that you just have to plug in the relevant numbers.

 

[betterfuture]

I'm not sure what question you have. You say that Ag is reduced and Zn is oxidized. Therefore, the reaction Ag+ + e- ----> Ag is relevant and the redox potential is +0.8 V, as written. You acknowledge that Zn is oxidized. This is Zn ---> Zn2+ + 2e-. This is the formal reverse of the reaction shown above, so you have to reverse the redox potential if you're going to combine the equations and simply add the potentials.

The other way of doing it, which is much less intuitive, is to memorize the equation E cell = E(cathode) - E(anode). These are the standard reduction potentials and so it would be the potential associated with reduction minus the potential associated with oxidation. Here, the switching of the sign is built into the equation so that you just have to plug in the relevant numbers.

I don't know if I experience the Mandela effect or not but I always remembered it as them just giving the tables for standard reduction and then using them to figure out cell potential for whatever reaction they give. So if something is oxidized you ALWAYS reverse the sign?

 

[aldol16]

I don't know if I experience the Mandela effect or not but I always remembered it as them just giving the tables for standard reduction and then using them to figure out cell potential for whatever reaction they give. So if something is oxidized you ALWAYS reverse the sign?

Just think about it like a Hess's law problem. If you're given some delta G and you have to flip the reaction, you must reverse the sign. E(cell) is just a manifestation of delta G, and they're related by a constant. What you probably did was just apply the equation E(cell) = E(cathode) - E(anode), which works if you have a table of standard reduction potentials. But now you have to spend computational space remembering the formula when you can just know Hess's law and figure it out that way.

 

[betterfuture]

There is a question in my book that does it differently.

 

[aldol16]

There is a question in my book that does it differently.

Do it whatever way you want. The important thing is getting the right answer. If the book answer doesn't give you the same answer as using Hess's law or the equation I posted above, then it's not correct or you're not interpreting the question correctly.

 

[betterfuture]

I don't think I am reading it wrong. It's a worked out problem but they used the reduction potential without reversing the sign. Thanks for your help anyways.

 

[aldol16]

I don't think I am reading it wrong. It's a worked out problem but they used the reduction potential without reversing the sign. Thanks for your help anyways.

The only case where you can use given standard reduction potentials without reversing the signs is in the equation E(cell) = E(cathode) - E(anode).