# Electrochem - about the usage of standard pontential

1. Nov 8, 2009

### 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)

2. Nov 9, 2009

### 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.

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methods

3. Nov 9, 2009

### 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 ?

4. Nov 9, 2009

### 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.

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5. Nov 9, 2009

### crays

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