At 70.0 ∘C , what is the maximum value of the reaction quotient, Q, needed to produce a non-negative E value for the reaction
SO42−(aq)+4H+(aq)+2Br−(aq)⇌Br2(aq)+SO2(g)+2H2O(l)
In other words, what is Q when E=0 at this temperature?
The standard electrode potentials for the reactions,
SO42- + 4H+ +
2e- SO2 +
2H2O E° = +0.20V
2Br- Br2
(aq) + 2e-
E°
= -1.07V
Therefore,
E° = 0.20 – 1.07 = -0.87V
Here, number of electrons transferred during the reaction (n) = 2
Temperature = 70 oC = 343.15 K
Nernst’s equation,
E = E° - RT/nF lnQ
0 = -0.87V - (8.314 J/molK x 343.15K/(2 x 96485 C/mol) lnQ
0 = -0.87 V – (2894.52 J/mol) lnQ / 192970 C/mol
0 = -0.87 V – 0.0147844 lnQ
0.87 V= – 0.0147844 lnQ
lnQ = - 0.87 /0.0147844
lnQ = -58.85
Q = e-58.85
Q = 2.7x 10-26
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