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Homework Help: Redox reactions chlorine water

  1. Oct 20, 2011 #1
    1. The problem statement, all variables and given/known data

    I am having trouble trying to find out if things react with each other or not. I don't really know how to read the reactivity series well :confused:

    2. Relevant equations
    Question 1: a student was given a beaker of an unknown ionic solution to identify. she conducted the following tests:

    1. add br2 (aq): mixture turns red-brown colour
    2. add Al(s): no observed change to solution or metal

    the unknown ionic solution could be:
    NaI (aq), AgCl, ZnI2 or CuBr2

    Question 2:
    when chlorine water is added to a sample of colourless ionic solution, the solution changed to a straw brown colour. The unknown solution is most likely to be:

    KI(aq) or KBr (aq) or Br2 (aq) or F2(aq)

    3. The attempt at a solution
    For question 1,I know that aluminium usually likes to give away electrons so the answer is likely to be NaI? because Na is a stronger reducing agent than Al I think.

    For question 2
    I can eliminate Br2 and F2 since they are not ionic but I am stumped on the other two possible answers.
  2. jcsd
  3. Oct 20, 2011 #2

    I like Serena

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    Homework Helper

    Hi paperdoll! :smile:

    I have a picture here that may help you:

    With Na to the far left, you know that it will ionize in water immediately.
    (Think of NaCl salt that easily dissolves.)

    What Al far into the direction of the arrow, you know that Al will bind to any negative ion easily.
    (Al will typically oxidize.)

    With Br slightly ahead of I, Br would bind more tightly than I.
  4. Oct 21, 2011 #3
    I understand the Na and the Al but I still don't understand Br and I. On my reduction potentials table Br is +1.07 and I is +0.54. But in this case, Br- would want to go to Br (l) which is a oxidation reaction. So wouldn't I be better at oxidation than Br? a bit confused :confused:
  5. Oct 21, 2011 #4

    I like Serena

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    I think you have it the wrong way around.

    A high reduction potential means a strong tendency to attract an electron (and thereby reduce the charge).
    In an oxidation reaction Br becomes Br- (just like in regular oxidation O becomes O2-).

    So Br is better at oxidation (becoming Br-) than I is.

    You'll see that Na has a strongly negative reduction potential (strong tendency Na → Na+).
    Al has a less negative reduction potential (Al → Al3+).
    I has a positive reduction potential (I → I-).
    And Br has a more strongly positive reduction potential (stronger tendency Br → Br-).
    Last edited: Oct 21, 2011
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