## Adding H2O and OH- to Balance

Ryan_K_1K
Posts: 51
Joined: Wed Nov 14, 2018 12:18 am

### Adding H2O and OH- to Balance

Can someone explain how adding H2O and OH- to balance a half-reaction works?

Ryan_K_1K
Posts: 51
Joined: Wed Nov 14, 2018 12:18 am

### Re: Adding H2O and OH- to Balance

Additional Q: Are you trying to balance the charges on both sides of the half-reaction?

JonathanS 1H
Posts: 101
Joined: Thu Jul 11, 2019 12:17 am

### Re: Adding H2O and OH- to Balance

When we balance a redox reaction with alkaline conditions, we initially balance it with H+ ions, but to make the reaction alkaline we add H20 and OH-, balancing the charge of the H+ since there are no H+ ions in an alkaline solution. If it is under acidic conditions, we use the H+ ions.

Ashley Nguyen 2L
Posts: 103
Joined: Sat Aug 17, 2019 12:18 am
Been upvoted: 1 time

### Re: Adding H2O and OH- to Balance

If you're using OH- to balance an equation, I'm going to assume the reaction is occurring under basic conditions. First, balance all elements except for hydrogen and oxygen. Then, add H2O to balance the oxygen on the opposing side. Then, balance the hydrogen by using H+, and then the charges by using electrons. Then convert all H+ to H2O by adding equal amounts of OH- to both sides. To answer your additional question, the charges on both sides of the equation should always be balanced in the end reaction.

Matt F
Posts: 100
Joined: Sat Aug 17, 2019 12:17 am

### Re: Adding H2O and OH- to Balance

Ashley Nguyen 2L wrote:If you're using OH- to balance an equation, I'm going to assume the reaction is occurring under basic conditions. First, balance all elements except for hydrogen and oxygen. Then, add H2O to balance the oxygen on the opposing side. Then, balance the hydrogen by using H+, and then the charges by using electrons. Then convert all H+ to H2O by adding equal amounts of OH- to both sides. To answer your additional question, the charges on both sides of the equation should always be balanced in the end reaction.

Wouldn't you want to cancel H+ ions with OH- ions before balancing charges with electrons? Or does it not make a difference

Harry Zhang 1B
Posts: 101
Joined: Sat Sep 14, 2019 12:16 am

### Re: Adding H2O and OH- to Balance

In balancing half reactions, the first step is always to balance the number of oxygen atoms by adding H2O. The next step is to balance the H atoms, and this is what's different. In an acidic solution, after you balance the O atoms, you will balance the H atoms by adding H+ on the side that needs it, and then you will proceed to balance the charge using electrons and you are done. In a basic solution, however, you will balance the H atoms by adding the same number of H2O molecules on the side that needs it. For example, if you need 4 H atoms on one side, you will add 4 H2O instead of 2. This is so because in the next step, we will balance out the discrepancy by adding the same number of OH- molecules on the other side, which is 4 if you added 4 H2O on one side. Then you will proceed to balance the charge using electrons. In summary, the first step is always to balance O atoms by adding H2O in any solution. The second step is to balance the H atoms by adding H+ in acidic solution, or by adding H2O and then OH- in a basic solution. The last step is always to balance the charge by using electrons.