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Electrochem - about the usage of standard pontential

  1. Nov 8, 2009 #1
    Hi guys, i've got this question where i am abit confused by which value should be used.

    Predict if a spontaneous reaction takes place between
    a) H2O2 and Cl2
    b) H2O2 and I2
    c) H2O2 and KI

    1/2 Cl2 + e- [tex]\rightarrow[/tex] Cl- (E = +1.36) --- 1
    1/2 I2 + e- [tex]\rightarrow[/tex] I- (E =+0.54) --- 2
    1/2 H2O2 + H+ + e- [tex]\rightarrow[/tex] H2O (E = +1.77) --- 3
    O2 + 2H+ + 2e- [tex]\rightarrow[/tex] H2O2 (E = +0.68) --- 4

    As i've seen the solution, my question is, why for a) and b) i couldn't use the value for 1 and 3?

    for example for a) using the 3 and 1 value, i get a positive value which means it is spontaneous, but in my book, it uses the value 1 and 4 which is spontaneous too.

    (sorry for the messy E value thing)
  2. jcsd
  3. Nov 9, 2009 #2


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    Staff: Mentor

    1&3 means you are going to reduce both Cl2 and H2O2 - where are you going to take electrons from? 1&4 means one acts as an oxidizer, the other as reducing agent, and electrons nicely balance.

  4. Nov 9, 2009 #3
    Thanks borek. But i wonder why i couldn't revert the equation for 1? is it because it stated that it has to be Cl2 ?
  5. Nov 9, 2009 #4


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    Staff: Mentor

    Initial mixture doesn't contain Cl-. If you were asked whether Cl- can be oxidized to Cl2 by hydrogen peroxide, reversed 1 might be used as a clue.

  6. Nov 9, 2009 #5
    Thanks i think i got it now, so i take the value what was given at first :)
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