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Electrochemistry and liquid junctions

  1. Apr 23, 2012 #1
    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?
  2. jcsd
  3. Apr 24, 2012 #2


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    Staff: Mentor

    Write net ionic reaction equation.
  4. Apr 24, 2012 #3
    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...
  5. Apr 24, 2012 #4


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    Staff: Mentor

    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-.
  6. Apr 24, 2012 #5
    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?
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