Standard Reduction (Electrode) Potentials at 25 °C Half-Cell Reaction E° (volts) F2(g) + 2 e→2 F(aq) 2.87 Ce4*(aq) + e –→ Ce"(aq) 3+ 1.61 2+ MnO4 (aq) + 8 H"(aq) + 5 e¯ –→ Mn-T(aq) + 4 H2O(1) 1.51 Cl2(g) + 2 e' —2 СГ (aq) 1.36 Cr2072"(aq) + 14 H*(aq) + 6 e¯ →2 Cr"(aq) + 7 H2O(1) 1.33 O2(g) + 4 H*(aq) + 4 e→2 H2O(1) 1.229 Br2(1) + 2 e¯ –→2 Br¯(aq) 1.08 NO3 (aq) + 4 H"(aq) +3 e¯ –→ NO(g) + 2 H2O(1) 0.96 2 Hg-*(aq) + 2 e¯ → Hg22 (aq) 0.920 Hg-"(aq) + 2 e →Hg(1) 0.855 Ag"(aq) + e¯ – Ag(s) 0.799 2+, Hg2"(aq) + 2 e¯ →2 Hg(1) 0.789 Fe*(aq) + e → Fe2*(aq) 0.771 I2(s) + 2 e → 2 1 (aq) 0.535 Fe(CN)6° (aq) + e→ Fe(CN)64 (aq) 0.48 Cu2*(aq) + 2 e → Cu(s) 0.337 Cu2*(aq) + e" – Cu"(aq) 0.153 S(s) + 2 H*(aq) + 2 e → H2S(aq) 0.14 2 H*(aq) + 2 e → H2(g) 0.0000 Pb2*(aq) + 2 e → Pb(s) -0.126 Sn-"(aq) + 2 e¯ → Sn(s) „2+ -0.14 Ni2+(ag) + 2 e →Ni(s) -0.25 Co2+(ag) + 2 e →Co(s) -0.28 Cd2*(aq) + 2 e →Cd(s) -0.403 Cr3+ C* (aq) (aq) + e -0.41 Fe2*(aq) + 2 e →Fe(s) -0.44 Cr3+ (aq) + 3 e → Cr(s) -0.74 Zn2*(aq) + 2 e →Zn(s) -0.763 2 H20(1) + 2 e → H2(g) + 2 OH (aq) -0.83 Mn-"(aq) + 2 e¯ → Mn(s) 2+ -1.18 Als*(aq) + 3 e → Al(s) -1.66 2+ Mg"(aq) + 2 e →Mg(s) -2.37 Na"(aq) + e –→ Na(s) -2.714 K(aq) + e→ K(s) -2.925 Li*(aq) + e Li(s) -3.045
When the Pb2+ concentration is 5.43×10-4 M, the observed cell potential at 298K for an
3Pb2+(aq) + 2Cr(s)3Pb(s) + 2Cr3+(aq)
Answer: __ M
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The Nernst Equation
A non-standard cell or half-cell potential can be calculated using the Nernst Equation:
E = Eo - RT/nF (ln Q)
where
E | = | potential under non-standard conditions |
Eo | = | standard potential |
R | = | ideal gas constant |
T | = | kelvin temperature |
n | = | number of moles of electrons for the reaction as written |
F | = | charge carried by 1 mol of electrons |
Q | = | reaction quotient |
It is customary to use the equation in a form where numerical values are substituted for R, T and F at a temperature of 25 °C.
For | |||
R | = | 8.314 J mol-1K-1 | |
T | = | 298.15 K | |
F | = | 96,485 J V-1 mol-1 |
RT/F = (8.314 J mol-1 K-1)(298.15 K)/((96,485 J V-1 mol-1)) = 0.0257 V
and the Nernst equation with the potentials in volts is:
E = Eo - (0.0257/n) ( ln Q)
nautral logarithm
Sometimes it is more convenient to use base-10 logarithms, and the substitution of 2.303 log for ln is made. (2.303 × 0.0257 = 0.0592) Then, the Nernst equation for base-10 logs at 25 °C is:
E = Eo - (0.0592/n)( log Q)
base-10 logarithm
A common student error is to use the wrong kind of logarithm. Be sure, when you choose an equation, to use the correct logarithm.
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