Determine the equilibrium constant, keq, at 25°c for the reaction 2br– (aq) + i2(s) br2(l) + 2i– (aq).

Electrochemical cell representation for above reaction is,

Br-/Br2//I2/I-

Reaction at Anode: Br2 + 2e-   →    2Br-               (1)
Reaction at Cathode: 2I-            →  I2   + 2e-          (2)

Standard reduction potential for Reaction 1 =  Ered(anode) = 1.066 v
Standard reduction potential for Reaction 2 = Ered(cathode) = 0.535 v

Eo cell = Ered(cathode)  -  Ered(anode)
            = 0.535 - 1.066
            = -0.531v

Now, we know that ΔGo = -nF (Eo cell)        (3)
Also, ΔGo = RTln(K)         (4)

Equation 3 and 4 we get,

ln (K)   = nF (Eo cell)  / RT
           = 2 X 96500 X (-0.531)/ (8.314 X 298)

∴ K = 1.085 X 10^-18.

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