[Big-Daddy]

Solutions of two weak polyprotic acids , H_xA and H_yB, are each placed in contact with standard hydrogen electrode at 298 K. When a cell is constructed by interconnecting them through a salt bridge find the emf of the cell.

Since E° will be 0 as both electrodes establish the SHE equilibrium, can we go on to say that

[tex]E = \frac{RT}{2F} \cdot ln(\frac{c_{H^+,A}^2}{c_{H^+,B}^2})[/tex]

Where I have set A as the right-hand electrode and B as the left hand electrode, and now we just need to work out the concentrations of H⁺_A and H⁺_B, i.e. the proton concentrations in each compartment, to find the value of the emf E?

[Borek]

E° doesn't mater, it always cancels out for concentration electrodes.

[Big-Daddy]

Ok, and then all I need to do is calculate the concentrations of H⁺ in each compartment? Using the standard equilibrium calculations, if the system is known to be at equilibrium, and then I find the EMF?

[Big-Daddy]

Hi, I just had a thought - surely at equilibrium the EMF will be 0, so there actually is no way of estimating what the H+ concentrations will be (except for at equilibrium when they will have to end up summing to EMF = 0 anyway), so what does this question really mean?

Originally I had paraphrased it - it's actually "two weak acids HA and HB (initial concentrations and Ka values given) are each placed in contact with standard hydrogen electrode at 298 K. When a cell is constructed by interconnecting them through a salt bridge find the emf of the cell." I managed to attain a potential solution by using equilibrium calculations to find [H+]_A and [H+]_B, but neither does this work out to EMF = 0, as it should at equilibrium, nor does it apply if EMF does not = 0. So what could I do? This is a textbook problem, must be some way of solving it?

[Borek]

You have separate acid dissociation equilibria in each half cell, but total system is not at equilibrium.

[Big-Daddy]

So the HA -> H+ + A- and HB -> H+ + B- reversible reactions are at equilibrium, hence my equilibrium calculations are satisfactory to find the concentrations of H+ required for the original equation in the first post - but the SHE reversible reaction (1/2 H+ + e- -> H2) established in each solution has not reached equilibrium? But how can this be, as this means [H+] must still be changing for the SHE reaction to be nearing equilibrium still, whereas according to the acid dissociation equilibria, the equilibrium point has already been reached so [H+] should now be constant?

[Borek]

[H⁺] would be changing only if you would allow current to flow, no current, no reaction, system doesn't go to equilibrium.

[Big-Daddy]

Ok, got it, thanks.