Question

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?

Answer 1

The standard electrode potentials for the reactions,

SO₄^2- + 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