Half-Reaction Method: Answers to Questions

[Jules18]

I'm just learning the half-rxn method, so I really hope there's somebody else on this forum who's familiar with it. If anyone needs a refresher, go to the link at the bottom of the post.

One of the "half-rxns" I'm doing is this:

HNO2 --> NO3(-1)

So, I'm confused because
1) There's oxygens on both sides, so I'm not sure if I should still add H2O to compensate for them.
2) What do I do about the hydrogen on the left?

http://www.sciencemadness.org/talk/viewthread.php?tid=12384

 

[Borek]

1) There's oxygens on both sides, so I'm not sure if I should still add H2O to compensate for them.

Yes - you will be not able to balance nitrogen and oxygen at the same time without adding water.

2) What do I do about the hydrogen on the left?

It ends in the water (or OH^-) on the right.

Note: if it happens in water you can always use H⁺, OH^- and H₂O on both sides of the reaction. In the end they will either cancel out, or you can add something to both sides of the reaction to get the same result and be consistent with the pH of teh solution.

For example, if you hane an excess hydrogen on the left, you can assume it end as H⁺ in the products. Later, if you don't want H⁺ in the products, you can add OH^- to both sides - that will mean H⁺+OH^- on the right. That's obviously just water molecule.

balancing using half reaction method

 

[quicknote]

Hello there,

I am familiar with the half-reaction method and I would be happy to help answer your questions. In regards to your first question, you are correct in thinking that you need to add H2O to balance the oxygen atoms. However, in this case, you do not need to add any additional water molecules because there are already the same number of oxygen atoms on both sides of the equation. This is because HNO2 already has three oxygen atoms, and NO3(-1) also has three oxygen atoms.

For your second question, the hydrogen on the left side of the equation can be balanced by adding H+ ions to the right side. This will also help balance the charge on both sides of the equation. The complete balanced half-reaction would look like this:

HNO2 + H+ --> NO3(-1) + H+

I hope this helps clarify any confusion you may have had. If you need further assistance, please do not hesitate to ask. Keep practicing and you will become more comfortable with the half-reaction method. Good luck!