Electrochem - about the usage of standard pontential

[crays]

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^- \rightarrow Cl^- (E = +1.36) --- 1
¹/₂ I₂ + e^- \rightarrow I^- (E =+0.54) --- 2
¹/₂ H₂O₂ + H⁺ + e^- \rightarrow H₂O (E = +1.77) --- 3
O₂ + 2H⁺ + 2e^- \rightarrow 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)

 

[Borek]

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.

--
methods

 

[crays]

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 ?

 

[Borek]

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.

--

 

[crays]

Thanks i think i got it now, so i take the value what was given at first :)