Electrochemistry - Standard Cell Potentials

[nothing123]

Hi,

Just having a little trouble understanding the concept of standard cell potential...what is it exactly? I understand that it is compared to a reference potential of 0 for the reaction 2H+ + 2e- -> H2(g). But what does it mean when a given reaction has positive potential (other than the fact that it's spontaneous)? For example, if a given reaction (A -> A2+ + 2e-) has a cell potential of 2V, does that mean the electrons and positive ions in the reaction actually generate a potential of 2 volts (at least relative to the reference potential)?

One of the questions I encountered asked something like this: "Given the following 2 reactions, which one generates more electrons for the circuit when both reactions run to completion (assume standard conditions and same starting concentrations)"?

A --> A2+ + 2e-, E* = 1.00
B --> B2+ + 2e-. E* = 2.00

Apparently, they generate the same number of electrons...how exactly? If one has a higher voltage, wouldn't it give more charges and thus electrons? Further, I tried to look at this mathematically:
deltaG* = -nfE* and
deltaG = deltaG* + RTlnK.
Then at equilibrium,
0 = -nFE* + RTlnK so
nfE* = RTlnK

Well certainly if E* is larger, then K would be larger meaning more products are favored at equilibrium and in turn more electrons, no?

Thanks for any help, much appreciated.

 

[Kushal]

the standard cell potential is more the tendency of an element to form its hydrated ion in a standard half cell.

the standard half cell would consist of the element in contact with a solution of its own hydrated ion. in a standard cell, the aqueous solutions are 1 molar, the gases are at 1 atm pressure and the temperature is 25 degrees.

the more positive the potential value is, the higher the tendency of the element to form its hydrated ion in the half cell.

like in your first half cell equation, A ----> A²⁺ + 2e^-no, if the potential is positive, the reaction is not necessarily spontaneous. the potential value gives no indication of the reaction rate(kinetic). it says only whether the reaction is energetically feasible or not.

if you have hydrogen gas reacting with Cu²⁺

H₂ <------> 2H⁺ + 2e- E* = 0 V

Cu²⁺ + 2e- <-------> Cu E* = +0.34 V

overall:

H₂ + Cu²⁺ --------> 2H⁺ + Cu

emf = +0.34 V

but the reaction does not occur at rtp!

i don't if this is correct but, for your question, you should the equillibria aspect.
if the electrode potential is higher, then the equillibrium should be more to the right, and hence produce more electrons.

 

[Borek]

does that mean the electrons and positive ions in the reaction actually generate a potential of 2 volts (at least relative to the reference potential)?

Yes, assuming all substances taking place in the reaction are in their standard state (ie they all have activities equal to 1), measured potential of the system afainst hydrogen electrode will be 2.

Apparently, they generate the same number of electrons...how exactly? If one has a higher voltage, wouldn't it give more charges and thus electrons?

Same charge, but different amount of energy. Larger K doesn't necesarilly mean higher charge trasnferred. I mean - it does mean higher charge, but charge difference between K=10¹⁰ and K=10²⁰ is neglectable, in both cases reaction for all practical purposes simply goes to completion.