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### Half Reactions

Posted: Tue Mar 10, 2020 12:37 pm
Whenever we balance the half-reactions, how do we know whether we need to add an H+, OH-, and/or H2O on what side of the reduction or oxidation half-reaction?

### Re: Half Reactions

Posted: Tue Mar 10, 2020 12:51 pm
You first balance the oxygens by adding H2O on whatever side needs oxygen. Then you balance the extra hydrogens by adding H+ on the other side. If the reaction occurs in a basic solution, then you add to both sides the same amount of OH- as H+ you added.

### Re: Half Reactions

Posted: Tue Mar 10, 2020 3:15 pm
Yes, you can treat balancing half reactions as if they are in acid at first in order to balance the number of electrons and negative charges on one side with protons on the other. The only difference is that for a basic solution, you have to add OH- to balance out the H+.

### Re: Half Reactions

Posted: Tue Mar 10, 2020 5:30 pm
in acidic solutions, you balance oxygen by adding H2O to the side lacking oxygen and you balance H by adding H+ to the side needing hydrogen. in basic solutions, you add H2O to the side lacking oxygen to balance oxygen. Then to balance hydrogen, add H2O to the side needing hydrogen and add OH- to the opposite side of the equation

### Re: Half Reactions

Posted: Tue Mar 10, 2020 6:14 pm
First balance all elements expect H and O. Balance O next using H2O and balance H with H+. Then you balance out charges by adding e-.

### Re: Half Reactions

Posted: Wed Mar 11, 2020 10:48 am
Always add H2O first to balance out any O in the reactions. Then, depending on whether it's in acidic or basic solution, add H+ or OH- (and H2O on the other side to balance O) in order to balance out the H.

### Re: Half Reactions

Posted: Wed Mar 11, 2020 10:52 am
In an acidic solution, we use H+ and H2O to balance the half-reaction. First you balance the oxygen using H2O, then hydrogen with H+.
In a basic solution, we use OH- and H2O. I wish I had better advice than this but you kind of just mess with the ratios until all elements balance.

### Re: Half Reactions

Posted: Wed Mar 11, 2020 1:21 pm
I find that for basic solutions, It is easier to add H2O to the side that already has oxygen and then from there you can use OH- to balance it out from the other side.

### Re: Half Reactions

Posted: Wed Mar 11, 2020 2:32 pm
Dr. Lavelle has a great instructional sheet on his website on how to balance redox reactions. It explains all of this is detail. Helped me a lot for test 2

### Re: Half Reactions

Posted: Thu Mar 12, 2020 7:47 am
for half reactions you have to play attention on whether the solution is acidic or basic, this will tell you if you should use H+ and H2O (for acidic solutions) or OH- and H2O (for basic solutions). Start off with adding H2O to the side that needs it to balance out the O atoms. From there, depending on the solution, you use to OH- or H+ to balance out the H atoms.

### Re: Half Reactions

Posted: Thu Mar 12, 2020 8:57 am
You may find Toolbox 6K.1 on page 538 of the textbook helpful for this. First balance the species being either oxidized or reduced in the half reaction. Then, balance oxygen by adding liquid water molecules. If the solution is acidic, add aqueous H+ ions to balance hydrogen. If the solution is basic, balance hydrogen by determining which side of the equation needs more hydrogen and how many more hydrogen atoms it needs; add that number of liquid water molecules to that side of the equation and add the same number of aqueous OH- molecules to the other side. Since water contains two hydrogen molecules and one oxygen, and OH- contains one oxygen and one hydrogen, the net result of adding the same number of moles of these to either side of the equation is just adding hydrogen to the side you added water to. From here, add electrons to balance the charge of each side of the equation. Hope that made sense. Toolbox 6K.1 may clarify further.

### Re: Half Reactions

Posted: Thu Mar 12, 2020 9:31 am
Philip wrote:Whenever we balance the half-reactions, how do we know whether we need to add an H+, OH-, and/or H2O on what side of the reduction or oxidation half-reaction?

Remember it depends on whether it is being balanced in acidic or basic conditions!

### Re: Half Reactions

Posted: Thu Mar 12, 2020 4:10 pm
You add H+ in acidic reaction and OH- in basic reactions and then you can determine where to add it in each equation based on where the equation is unbalanced.

### Re: Half Reactions

Posted: Thu Mar 12, 2020 4:18 pm
It's based off of whether it's acidic or basic. The problem will tell you.

### Re: Half Reactions

Posted: Sat Mar 14, 2020 10:11 am
It depends on the solution. If it is acidic, you add H+ to balance H. If it is basic, you add h2o to the side that needs H and OH- to the other side.

### Re: Half Reactions

Posted: Sat Mar 14, 2020 10:18 am
The problem would probably tell you whether the solution is acidic or basic. But you always add H2O when balancing oxygen.