Cell Voltage Equation??

[amalgamgrillz]

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Hey everyone,

I was just wondering which equation is correct (or if they're both correct, but used in different scenarios, or something..) for calculating Cell Voltage (Celll Potential)?

Kaplan AP Chem Review and Some internet sites say:
E*cell = E*reduction - E*oxidation

Barrons DAT and Some internet sites say:
Emf = Eoxidation + Ereduction

Do we add the half reactions or subtract them?

 

[wizi]

Hey everyone,

I was just wondering which equation is correct (or if they're both correct, but used in different scenarios, or something..) for calculating Cell Voltage (Celll Potential)?

Kaplan AP Chem Review and Some internet sites say:
E*cell = E*reduction - E*oxidation

Barrons DAT and Some internet sites say:
Emf = Eoxidation + Ereduction

Do we add the half reactions or subtract them?

This is more like Gen Chem problem.
They are both correct.

Usually, when you want to calculate Ecell, the question will give you a reduction table
For example:
The question gives you this
Zn(2+) + 2e --> Zn (-0.76V)
Cu (2+) + 2e --> Cu (+0.34V)

Then you have to calculate Ecell of this equation

Zn + Cu(2+) --> Cu + Zn(2+)


To use this equation:
Emf = Eoxidation + Ereduction

Zn --> Zn(2+) = +++++++++0.76: oxidated (you have to change the SIGN yourself here).
Cu(2+) --> Cu = +0.34 V: reduced ( You dont have to change the sign)
--> Emf = +0.76 + 0.34 = 1.1 V


To use this equation:
E*cell = E*reduction - E*oxidation (this equation already change the sign for you, you dont have to change the sign)
Ecell = +0.34 - (-0.76) = + 1.1 V


You see, both equations give you the same result but the different is that Kaplan's equation changed the sign for you while Barron's doesnt.

 

[amalgamgrillz]

This is more like Gen Chem problem.
They are both correct.

Usually, when you want to calculate Ecell, the question will give you a reduction table
For example:
The question gives you this
Zn(2+) + 2e --> Zn (-0.76V)
Cu (2+) + 2e --> Cu (+0.34V)

Then you have to calculate Ecell of this equation

Zn + Cu(2+) --> Cu + Zn(2+)


To use this equation:
Emf = Eoxidation + Ereduction

Zn --> Zn(2+) = +++++++++0.76: oxidated (you have to change the SIGN yourself here).
Cu(2+) --> Cu = +0.34 V: reduced ( You dont have to change the sign)
--> Emf = +0.76 + 0.34 = 1.1 V


To use this equation:
E*cell = E*reduction - E*oxidation (this equation already change the sign for you, you dont have to change the sign)
Ecell = +0.34 - (-0.76) = + 1.1 V


You see, both equation gives you the same result but the different is that Kaplan's equation changed the sign for you while Barron's doesnt.

ahhh! ok thank you so much!🙂