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Substances whose chemical reaction is easily perceptiple

  1. Aug 18, 2013 #1
    I was trying to solve the following exercise:

    Choose the option that has the pair of substances whose chemical reaction is easily perceptiple
    a)Br2(aq) + NaCl(aq)
    b) Cl2(aq) + NaI(aq)
    c) H2(g) + MgSO4(aq)
    d) Ag(c) + ZnSO4(aq)
    e) HCl(aq) + Cu(c)

    I would say if the salt was an alkali salt, a would discolor the solution (Br2 is brown/red), as Br2 + H2O -> HBrO + HBr, and the equilibrum tends to form the products when the solution is basic. But the salt is NaCl, so nothing would happen
    The second one I would say no change would be noticed
    The third I would say some of the SO4-- could reduce to SO2 or some of the Mg++ to Mg, oxydating the H2 to H2O, but in a very low scale, so nothing would be noticed too
    For the third, as ZnSO4 forms a acid solution, I would say some of the silver would oxydate to Ag+, even silver being a noble metal. This will occur with a low velocity but I would say it could be noticed.
    For the last one, as Cu isn't a noble metal, I would say it will fastly oxidate to Cu2+, and the solution will color to blue, easily noticed

    I chose e as the answer, but by book is saying it is b. Why is that?
  2. jcsd
  3. Aug 18, 2013 #2
    The reaction in b) will make sodium chloride and iodine. The iodine would give the solution a very dark brown colour that would be hard to miss. The answer isn't e) because that reaction doesn't happen. Copper is one of the few metal which are actually less reactive than hydrogen, so it can't replace it in the HCl.
  4. Aug 19, 2013 #3
    Thanks, I saw where was my mistake now,i thought ag and cu had negative reducing potentials, not positive (even ag being a noble metal)
    Last question
    In another exercise i have the following reaction
    H2SO4(l) + NaCl(c)
    The answer is that it forms a gas
    But I searched the reducing potential of the reaction
    SO4 2- + 4H+ + 2e- -> H2SO3 + H2O
    This reaction is in atkins' book , and i presume H2SO4 should be H2O and SO2
    Anyway, the standard potential is +0.17v, assuming H+ = 1mol/l and so2 = 1 mol/l, which is not any absurd
    And this potential is not even closer to the potential required to oxidate cl- (1.36 v), or the one to oxidate oh- to H2O and O2 (0.4v), so in an aquous solution i would say nothing would occur. But it says H2SO4 is liquid (=pure), what happens in this case?
  5. Aug 19, 2013 #4


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