Electrochemistry and liquid junctions

[badbrain103]

I have Pt(s)/H2(g)/HBr(aq)/AgBr(s)/Ag(s)

The question asks why a liquid junction is not necessary. So I know AgBr is insoluble in water, and AgBr coats the silver metal. I just don't understand why it does not matter that the Br- anions from AgBr mixing with the H+ cations doesn't necessitate a liquid junction. Any help?

 

[Borek]

Write net ionic reaction equation.

 

[badbrain103]

well the two half reactions are:
H2 --> 2H+ + 2e-
2AgBr(s) + 2e- --> Ag(s) + Br-

giving the net ionic equation:

H2 + 2AgBr --> 2H+ + 2Ag(s) + 2Br-

there are no spectator ions...
so...?

 

[Borek]

H2 + 2AgBr --> 2H+ + 2Ag(s) + 2Br-

This is not exactly true. That is - what is really happening is that the solution is saturated with AgBr - so it contains some minute amounts of Ag⁺. This Ag⁺ gets reduced directly on the electrode. In theory it can also get reduced by hydrogen in the bulk of the solution, but as concentration of Ag⁺ is very small, this reaction would be pretty slow. Besides, once the Ag⁺ is removed from the solution, it has to saturate it again - this dissolution is another pretty slow process. Then, electrode is covered with wet AgBr in which transport of H₂ is very slow (no mixing). Thus we can safely assume there is no hydrogen "below" AgBr, just solution containing Ag⁺ and Br^-.

 

[badbrain103]

Ok sorry I haven't done electrochemistry in a long time and am a bit confused. So the solution is of AgBr mostly though it contains some small amount of H₂ and Ag⁺ - this is because most of the Ag⁺ is reduced quickly, right?

Because the reaction between H₂ and Ag⁺ is so slow, it is basically negligible and does not affect the cell potential at all, is this right?

What do you mean when you said the Ag⁺ is removed from the solution - that the majority of it is reduced to become Ag_(s)?

But finally, because there is no mixing between H2 and the Ag⁺ and Br^-, it is safe to assume that this does not affect the cell potential, thus why there is no need for a liquid junction?