Page 15 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

A,B,C,D: formulas for the participating species (atoms, molecules, or ions)
e– : represents electrons
a,b,c,d : number of moles of each species appearing in the half-reaction

E0 = Standard Electrode potential
R = gas constant, 8.3 J K–1 mol–1
T = temp, K
n = number of moles of electrons that appear in the half-reaction
F = Faraday constant = 96,485 Coulombs/mole

Letter in brackets represents activity

• E0 equals to half-cell potential WHEN the log term is zero

• WHEN all reactants and products are in their standard state
activities, the log term is zero

Typical half-cell reactions / Nernst Expressions

1) Zn2+ + 2e– Zn(s)

No term of elemental zinc is included because it is pure solid

0.059 1
[Zn2+]
E = E0 – log

2

2) 2H+ + 2e– H2(g)

PH2 Is the partial pressure of H2 in atm.

0.059 PH2

E = E0 – log

2 [H+] 2

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