Electrochemistry

2013-11-26 6:44 am

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If it cannot be calculated, state the further information you need.

回答 (2)

2013-11-29 1:25 am
✔ 最佳答案
Information should be enough.

The standard reduction potentials are measured under standard conditions: concentrations of all species are 1M, pressure of all gases are 1atm, with temperature at 298K.

For reaction mixtures with "non-standard" concentrations, one can use Nernst equation to calculate the reduction potential for a half-equation of reduction:

E = E0 - (RT/nF) ln ( [reduced form]^x / [oxidized form]^y )
that
E = new reduction potential
E0 = standard reduction potential
R = universal gas constant
T = temperature (in K)
n = number of electrons involved in that half equation
F = Faraday constant = 96485 Coulomb
= charge of 1 mole of electrons
= Avogadro's constant x elementary charge
ln(x) is the natural-log of x
[reduced form] is the concentration of the species on the reduced side of the half equation, and vice versa for [oxidized form]
(actually activity should be used, but let's assume activity = concentration for simplicity)
x, y = coefficients of the species in the half-equation

take the following equation as example
O2 + 4H(+) + 4e- ------> 2H2O
the new E = E0 - ((8.314 x 298) / (4 x 96485)) ln ( 1 / P(O2) [H(+)]^4 )

E0 = +1.23
number of electrons transferred in the half equation is 4, so n = 4
concentration of H2O is constant and can be ignored
pressure of O2 is kept at 1atm
coefficient for proton is 4, so the concentration term of proton has power of 4

pH = 3.60
[H(+)] = 10^(-3.60) = 2.512E-4 M

E = +1.23 - ((8.314 x 298) / (4 x 96485)) ln (1 / (2.512E-4)^4 )
= +1.23 - 0.213
= +1.02 V

for the overall cell reaction, E = (+1.02) - (+1.07)
= -0.05 V

With negative cell potential, probably the reaction is not spontaneous.

But well, while the reaction is energetically unfeasible, no information is given on its kinetic properties, so...
2013-12-01 11:43 pm


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