Half-Reactions

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aristotelis1H
Posts: 16
Joined: Fri Sep 25, 2015 3:00 am

Half-Reactions

Postby aristotelis1H » Tue Feb 02, 2016 9:15 pm

When askedd to balance skeletal equations by using oxidation and reduction half-reactions, these half-reactions will be given to us? Because in some of the homework questions I could not find certain half-reactions.

For example, the following half-reaction is not in Appendex 2, A 18
C2H5OH(aq) -> C2H4O(aq) + 2H2+ +2e-

Are we expected to come up with certain half-reactions on our own?

chemgal4L
Posts: 19
Joined: Fri Sep 25, 2015 3:00 am

Re: Half-Reactions

Postby chemgal4L » Tue Feb 02, 2016 10:02 pm

I think you determine which atoms are being oxidized and which are being reduced, and based off that you can determine the separate half reactions.

Aya Ghoneum 1H
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: Half-Reactions

Postby Aya Ghoneum 1H » Wed Feb 03, 2016 3:04 pm

You will have to balance your own half reactions sometimes without being able to look them up in an Appendix. Here is a set of rules for balancing redox reactions.

For balancing in an acidic solution:

1. Divide the equation into two half reactions.
2. Balance any atom other than O or H first.
3. Balance O using H2O. Add as many H2O's as necessary to the side deficient in O.
4. Balance the H's by adding H+ to the side deficient in H.
5. The mass should now be balanced. To balance the charge, add electrons to the appropriate side.
6. Repeat steps 1 through 5 for both half reactions.
7. If the number of electrons lost does not equal the number of electrons gained, find the common multiple and multiply each half reaction by the necessary factor.
8. Sum both half reactions to obtain the balanced, net ionic reaction. In many cases, look for H+ ions and H2O molecules to cancel.
9. Check the final equation for mass and charge balance.


For balancing in a basic solution:

1. Divide the equation into two half reactions.
2. Balance any atom other than O or H first.
3. Balance O using H 2O. Add as many H2O's as necessary to the side deficient in O.
4. As if with acid solution, balance the H's by adding H+ to the side deficient in H.
5. Switch over to base by adding the same number of OH- as there are H+ to both sides of the half reaction.
6. One side of the half reaction will contain H+ and OH-. Convert the number of H+ and number of OH- to H2O's.
7. Cancel H2O molecules where possible.
8. Balance the charge by adding electrons to the appropriate side.
9. Repeat these steps for the second half reaction.
10. If the number of electrons lost does not equal the number of electrons gained, find the least common multiple and multiply each half reaction by the factor necessary to obtain the least common multiple.
11. Add the two half reactions together, canceling the electrons, and any OH- and/or H2O where possible.
12. Check the final equation for mass and charge balance.

I hope this helps!

mariellemateo2C
Posts: 7
Joined: Fri Sep 25, 2015 3:00 am

Re: Half-Reactions

Postby mariellemateo2C » Wed Feb 03, 2016 5:08 pm

Thank you for these directions, they are lifesavers!!


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