dat gchem ques...

[faerielynx]

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Consider the following electrode potentials:

Cu2+ + 2e- --> Cu(s) Eo = +0.34 V
2H20 --> O2 + 4H+ + 4e- Eo = -1.23 V

What is the Eo(cell) for the reaction shown in the following equation?

2Cu2+ + 2H20 --> 2Cu(s) + O2 + 4H+

A) -0.89 V
B) +0.55 V
C) +1.57 V
D) + 1.91 V

is it A?

 

[grami001]

the answer is A

 

[sweetpea2]

A

 

[happyasaclam88]

definitely not c. it's a.

 

[whoaaitzkyle]

can someone explain the rules of how to do this? I understand about the whole reduction potential is the Eo with the highest value but when you multiply the coefficients is it a rule that you don't multiply the Eo?

 

[sweetpea2]

the answer is A, sorry i thought they were both reduction potentials. There are a few ways to calculate this but heres the way i would do it:

You can do E reduced-E oxidized or E ox + E red, but make sure your signs are right. In the net equation the Cu is being reduced and the H20 is being oxidized. Also, coefficients dont matter with reduction potentials. So you have a reduction potential of .34 and an oxidation potential of -1.23. So you can use my 2nd equation E ox + E red so just keep the signs and add to get -.89 V. Usually, in chemistry both equations are given as reduction potentials, so i just immediately changed the sign.

 

[nixon13]

i always get confused on these because H2O is being oxidized and its value is -1.24. So wouldnt you flip the sign if you wrote the equation backwards as a reduction rxn?

 

[smile101]

i always get confused on these because H2O is being oxidized and its value is -1.24. So wouldnt you flip the sign if you wrote the equation backwards as a reduction rxn?

H2O is actually being oxidized, so you wouldn't flip the reaction. If you look at both the rxns as reduction potentials (i.e. with negative signs) then you see that -0.34 is higher than -1.24, which means Cu2 will have potentials to reduce. This question is easy because the rxn is written out, but in any case if you are given only reduction potentials to work with, then one with higher reduction potential will reduce and other will be oxidized.

 

[nixon13]

how do you know if your signs are right is my question? I always thought to flip the sign of the Eo on the oxidation equation, but this seems incorrect.

 

[nixon13]

I was being lazy and didnt want to look in my book, but then i got impatient waiting for a reply so i looked in the book. Come to find out, this **** is easy. Thanks for the help anyways.

 

[sweetpea2]

yea no need to flip the sign if you are give a reduction potential and oxidation potential. Thats an easy problem. If you are give 2 reduction potentials then switch the oxidation potential to the other sign and then add.