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Electrode potentials (AQA A2 CHEM5)

Discussion in 'Science' started by mike ryan, Dec 29, 2010.

  1. I too found this difficult, partly because I was was first taught it using the wrong sign convention by a then rookie teacher (he has since retired so it show how long I have been in this game!!).

    Although I cannot offer inspirational practicals, my teaching method has always been to use a thought experiment assuming very litle prior knowledge but simply relying on common sense.
    Imagine dunking a piece of metal in a solution of its ions (eg copper in copper sulfate solution). Two things might happen: A... ions become metal or B... metal becomes ions. In each case some elecron transfer is required. ie either A... M+ + e- ---> M or B... M ---> M+ + e-
    Neither will actually happen because there is nowhere for the electrons to go / come from.
    So, let's stick another metal nearby conveniently linked by a bit of wire so that the electrons can go between the two metals. One of the metals is bound to be better at either A or B.
    Still nothing will actually happen because the solutions will end up being either positively or negatively charged.
    So, let's set up a way for extra ions of whatever charge is needed to be provided. If we use saturated potassium nitrate in an agar gel, the ions will only diffuse slowly and I happen to know that potassium ions and nitrate ions diffuse at about the same speed so they won't make one side more positive unfairly.
    Now something can actually happen. Chemical change occurs at the electrodes, electrons move along the wire and spare ions diffuse out of the agar gel to compensate.
    If we know something about the ability of each metal to do change A and B (look back to our old reactivity series work from GCSE), we can predict what will happen. We could measure the "voltage" between the two bits of metal by sticking a voltmeter in place. We might expect the bigger the difference in "reactivity" to give the bigger "voltage".
    How about we find a way for gases and their ions or even two sets of ions to exchange electrons. We could do these too and put them in a series. Since we cannot tell how much of each reacting pair is due to each of the component electrodes, we need a standard. This "sea-level" has all the usual suspects for standard conditions and we will choose hydrogen gas.

    Nothing too wonderful but it works for my classes.

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