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14.5 Half Reactions

Posted: Tue Feb 07, 2017 2:33 pm
by Josh Ku 3H
I'm a little confused on writing the oxidation and reduction half reactions.
In 14.5 they are looking at the unbalanced equation: O3 + Br- --> O2 + BrO3-
It makes sense for how bromine is being oxidized and oxygen is reduced but for the half reaction of oxygen the solution manual uses the equation: O2 --> O3

In this equation isn't the oxygen's oxidation number not changing? Doesn't the oxygen get reduced when combining with the bromine?

Re: 14.5 Half Reactions

Posted: Tue Feb 07, 2017 3:44 pm
by Chem_Mod
This problem is a little tricky.

First, the solutions uses the equation O3O2 so that is not incorrect. You are also correct in that the oxidation number of ozone and oxygen gas are not changing, however, they do donate electrons to BrO3- as that is where the oxygen atom goes after it splits off from ozone.

You have to look at it from that perspective and then balance accordingly. Remember, since the reaction is in basic solution, you must balance oxygen molecules with water then balance the hydrogen. After that, cancel the waters and then make sure the equation is electrically balanced as well. Do that fr both half reactions then combine them to get the balanced equation.

Re: 14.5 Half Reactions

Posted: Tue Feb 07, 2017 3:51 pm
by Chem_Mod
Yes, ozone O3 is reduced when it becomes O2 and yes the oxidation numbers of ozone and oxygen gas are zero.

However, they participate in charge transfer with H2O.

When solving this problem, it is clear that Br- becomes oxidized. When the reaction O3 (g) --> O2 (g) is balanced as a reduction, the answer should present itself. Remember to balance as in basic solution.

Re: 14.5 Half Reactions

Posted: Thu Feb 09, 2017 4:51 pm
by Josh_Zhong_1F
But for this problem, the solution says that O3 is the oxidizing agent and Br- is the reducing agent.
I thought both O3 and Br- are oxidizing agent..

When both O3 and O2 have oxidation number "0", how do we determine whether it is oxidizing or reducing?


Re: 14.5 Half Reactions

Posted: Thu Feb 09, 2017 4:57 pm
by Josh_Zhong_1F
OHHHH..I think I understand what you mean Professor.
Please correct me if I am wrong,

O3 reacts with BrO3-; therefore, O3 is an oxidizing agent because O3 oxidation number is 0 while Br oxidation number is +5
Br- reacts with O2; therefore, Br- is a reducing agent because Br- oxidation number is -1 while O2 oxidation number is -4

We only use O3 to balance O2 because we cannot use O3 to balance BrO3- right??