balancing redox half reaction P4-->H2PO2- (13.5d)

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AmirMahmoud_1J
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balancing redox half reaction P4-->H2PO2- (13.5d)

Postby AmirMahmoud_1J » Tue Feb 03, 2015 11:04 am

I am trying to balance the redox half reaction P4-->H2PO2-, and am getting 16OH-+P4-->4H2PO2-+8H20+12e- which doesn't match the answer key. Can somebody work this one out for me?

martha-1I
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Re: balancing redox half reaction P4-->H2PO2- (13.5d)

Postby martha-1I » Tue Feb 03, 2015 11:26 am

In order to balance this and any other half-reactions in a basic solution follow these steps:
1. Write down the half reactions
2. balance all elements except for hydrogen and oxygen
3. balance oxygen by adding H2O
4. Balance hydrogen by adding H+
5. Balance charges by adding e-
6. Multiply in order to get the same number of electrons on each side (product and reactant)
7. add as many OH- as there are H+ To both sides of each half reaction
8. Wherever there is an H+ and OH- on the same side combine them to form H2O
9. Add or subtract repeating molecules

If you follow these steps, you will end up with the correct answer.

Niharika Reddy 1D
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Re: balancing redox half reaction P4-->H2PO2- (13.5d)

Postby Niharika Reddy 1D » Tue Feb 03, 2015 2:10 pm

I have attached a picture of my work for this question.
Attachments
image.jpg
13.5d

AmirMahmoud_1J
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Re: balancing redox half reaction P4-->H2PO2- (13.5d)

Postby AmirMahmoud_1J » Tue Feb 03, 2015 4:41 pm

I thought when balancing out a basic solution you do not use H+.

Niharika Reddy 1D
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Re: balancing redox half reaction P4-->H2PO2- (13.5d)

Postby Niharika Reddy 1D » Tue Feb 03, 2015 4:44 pm

No H+ should be left when it's in a basic solution, which is why OH- is added to both sides to cancel out the H+. On the side with H+, the OH- combines with H+ to form water, but on the other side, it remains OH-, which is acceptable since it's a basic solution.

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Re: balancing redox half reaction P4-->H2PO2- (13.5d)

Postby Chem_Mod » Tue Feb 03, 2015 4:58 pm

When balancing a basic solution, you can balance as if acidic, then perform the extra steps to add OH- and cancel all the H+ as water.

You can also avoid the H+ altogether with the following:
Hydrogen balancing step: Add H2O to the side which is lacking in hydrogen, and add OH- to the other side.

Notice that this puts extra hydrogens on the side that needs them, as well as KEEPING oxygen balanced after you've already balanced it in the prior step.

martha-1I
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Re: balancing redox half reaction P4-->H2PO2- (13.5d)

Postby martha-1I » Wed Feb 04, 2015 9:19 am

Yes you do add the H+ even when it is in a basic solution because the OH- are added later, which will contribute to the formation of H2O. In the steps I wrote above, you would stop after step 6 if it were in an acidic solution. However, since it is in a basic solution, you keep going.


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