[rafarock077]

I have to balance a redox (yes, i make 100 post about redox but even then im not able to make the correctly in basic medium). The thing is that im searching im a lot of pages but everyone have a different way to do it
 My theacher told me that in the half reactions you have to add first electrons then oh and after h2O, but in a post that i make asking this a guy told me that the correct way is  OH-/H2O to balance atoms and then electrons to balance charge. But also in some pages they and H+ instead of OH and idk why
 
The thing is that i have to balnce this and idk how

Ch3-Ch2-ch2-oh + MnO4K2 + SO4H2 = ch3-ch2-coh + So4Mn + SO4K2 + H20

[chenbeier]

The reaction what you describe is in acidic condition, because you add sulfuric acid.

But generally the law is, if oxygen is involved:

Basic condition : Oxidation add OH^- and get H₂O on product side.
Reduction add water and get OH^- on product side

Acidic condition: Oxidation add water and get H⁺ on product side
Reduction add H⁺ and get water on product side.

You can also say 2 OH^- = (O) + H₂O + 2 e^- Oxidation

and  (O) + H₂O + 2 e^- = 2 OH^- Reduction

For acidic condition

(O) + 2 H⁺  + 2 e^-= H₂O Oxidation

 H₂O  =  (O) + 2 H⁺  + 2 e^- Reduction

The (O) in brakets  is the oxygen what will add or removed of the molecule.

With this try the reaction.

[rafarock077]

in the basic condition when do you add the electrons at the beginning or at the end?
thx for the help btw

[chenbeier]

It is not the question when you add the electrons either in acidic or basic conditions.

The  receipe is the following

1. Find the redox pair , which ones are in your case?
2. Then you use the equations what I had given to you
3. Balance the charges with electrons.

Example:

Permangante get reduced in alcaline condition.

Redoxpair is MnO₄^- and MnO₂

The difference are 2 (O)

Use H₂O + (O) => 2 OH^- two times

MnO₄^- + 2H₂O => MnO₂ + 4OH^-

Balance the charge

MnO₄^- + 2H₂O + 3 e^- => MnO₂ + 4OH^-

[Borek]

in the basic condition when do you add the electrons at the beginning or at the end?

Always at the end - as we told you several times, first balance atoms, then (that is: last step, which not surprisingly is at the end) balance charges with electrons.

[chenbeier]

Well many ways guide to Rome.

It would be also possible to figure out the oxidation numbers first.

MnO₄ ^-  Mn is +7
MnO₂ Mn is +4
the difference is 3 what means add 3 electrons

MnO₄^- + 3 e^- => MnO₂

Then add water to remove two oxygens to get hydroxide

MnO₄^- + 3 e^- + 2H₂O  => MnO₂ + 4OH^-

But in my opinion it is the more complicated way.

[Borek]

It would be also possible to figure out the oxidation numbers first.

That's another method, you have just confused OP who already had problems understanding. Hardly helpful.

[chenbeier]

What do you want to tell me. The question was when the balancing of the charge should take place. The answer is it doesnt matter in the beginning or at the end. I showed the more simple way to do at the end, what you also suggested. But with your answer I had to react and to show the other way as well. I dont think it will confuses some one.

[Borek]

What do you want to tell me. The question was when the balancing of the charge should take place. The answer is it doesnt matter in the beginning or at the end.

Sure thing it matters. That's what started the whole discussion, please read the original question - OP was confused exactly about how it should be done, and they asked about the half reaction method. In their other thread they gave quite a good argument what the problem can be - if you go through the half reactions balancing charges first and then you try to balance atoms with OH^-, you end with reaction that is not balanced.

Now you drop in ON method which is out of the scope of the thread and you start to confuse things. Sure, when you balance with ON you balance main reagents first (avoiding electrons at all), but if balance with half reactions you balance charges (using electrons for each half reaction separately) at the end. If you do both methods at the same time and mix order, you risk ending up with garbage.

Once you are really fluent and know what you are doing you can often use some shortcuts, but that's not the way to show nor to teach to beginners, unless you want them to get completely lost. Have you ever studied pedagogy?

[chenbeier]

It is out of topic.

Have you ever studied pedagogy?

No. With 58 years I am to old. LOL. I started with basic job. Start as an electrician, i worked in company, after some years I went back to school and studied at the end chemical engineering and polymer chemistry, work in plating and electronics industry for R&D and work as trainer and coach to teach customers and coworkers at the end world wide. Now I am on the way of retirement.