Chemistry What is the purpose of redox reactions?

[barryj]

Homework Statement

Explain why I care about redox reactions

Relevant Equations

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I am trying to understand redox reactions. I have read, and learned about oxidation numbers and even viewed examples of balancing redox equations. However, nowhere have I seen an explanation of why I care about redox equations. In contrast, I understand that a chemical equation must be balanced in order to perform stochiomery problems but how do I use what I have (or have not) learned about redox equations? So far I have been manipulating charges and oxidation numbers without knowing why I am doing so.

 

[TeethWhitener]

There are a zillion reasons why redox reactions are important. I’ll give you a few.

1) The very first chemical reaction humans ever mastered—building a fire—is a redox reaction between carbon (oxidized to CO₂) and oxygen (reduced to H₂O).

2) Redox reactions fundamentally involve electron transfer, which can be separated from the ion/molecule portion of the reaction. If you send the electrons through a metal wire (which doesn’t conduct ions) and the ions through a semi-permeable membrane (which doesn’t conduct electrons) you can control the reaction. This is what happens in a battery.

3) When two dissimilar metals are in contact, or when a metal is in contact with the air and an ionic conductor (like seawater), redox reactions happen in the form of corrosion. This is a problem that costs the world billions of dollars a year to combat.

4) Many of the reactions that keep you (and everything else) alive are redox reactions. In particular, the oxygen you breathe reacts with the fats, sugars, and proteins in your body to generate energy. And plants pull carbon dioxide from the air and convert it to sugar using light-mediated (and non-light-mediated) redox reactions.

 

[barryj]

But can't you solve and work with these types of reactions without having to consider oxidation numbers and etc? Why not just a combustion equation.

 

[TeethWhitener]

But can't you solve and work with these types of reactions without having to consider oxidation numbers and etc?

To quote Barney Gumbel: “I could do a lot of things if I had the money.”

I’m not sure you’re trying to meet me (or your teachers) halfway here. A combustion reaction is a redox reaction.

Knowing that redox involves electron transfer opens up a number of avenues for further examination. For instance, how hard is it to go from a +2 oxidation state to a +3 oxidation state? Questions like these make possible concepts such as flow batteries and other advanced electrochemical ideas. But you have to walk before you run, so learning the basics of electrochemistry and redox reactions is important.

 

[Borek]

You can definitely balance every reaction without using oxidation numbers, but you can't explain how the battery works without considering charge transfer between reactants - which is exactly what redox reactions are about.

 

[barryj]

I get it. I assume that when I learn about batteries that i will see that redox reactions become important.
Thanks. I will keep studying.

 

[symbolipoint]

But can't you solve and work with these types of reactions without having to consider oxidation numbers and etc? Why not just a combustion equation.

NO! The purpose of these written reactions is to use changes in reduction state to account for the chemical changes among compounds or reactants.

 

[Borek]

NO! The purpose of these written reactions is to use changes in reduction state to account for the chemical changes among compounds or reactants.

As I wrote earlier, mass and charge balance are perfectly enough to balance any reaction equation without taking reduction/oxidation state of compounds into account _directly_ - all that is needed are correctly written formulas (and charges in the case of ions). Yes, these formulas/charges do reflect the oxidation state already, but you don't need to worry about it if balancing is the only thing you are interested in.

 

[symbolipoint]

As I wrote earlier, mass and charge balance are perfectly enough to balance any reaction equation without taking reduction/oxidation state of compounds into account _directly_ - all that is needed are correctly written formulas (and charges in the case of ions). Yes, these formulas/charges do reflect the oxidation state already, but you don't need to worry about it if balancing is the only thing you are interested in.

I have trouble, very much trouble, understanding your explanation about avoiding using charge and electron balance in dealing with quantities of reactants and products for redox reactions. Looking at transfers of electrons and at charge changes while learning/studying chemistry was a big increase in understanding.

 

[Borek]

I have trouble, very much trouble, understanding your explanation about avoiding using charge and electron balance in dealing with quantities of reactants and products for redox reactions. Looking at transfers of electrons and at charge changes while learning/studying chemistry was a big increase in understanding.

2Mg + O₂ → 2MgO

is definitely a redox reaction, yet it can be easily balanced by inspection method without even knowing what electrons are.

For reaction like

10FeSO₄ + 2KMnO₄ + 8H₂SO₄ → 5Fe₂(SO₄)₃ + 2MnSO₄ + 8H₂O + K₂SO₄

things get more complicated, and balancing by inspection is still possible, although tedious. But if you use algebraic method it becomes reasonably easy (plus, as opposed to the inspection method, it is a systematic approach that doesn't require guessing not trial and error, it gets trivial to implement, which makes it possible to write computer program for balancing reaction equations - it simply solves the set of equations). The same algebraic method will also work perfectly well for things like

2Mn²⁺ + 5BiO₃^- + 14H⁺ → 2MnO₄^- + 5Bi³⁺ + 7H₂O

you just need to add an additional equation for charge balance.

Sure, by hand it is easier to balance these reactions following the electron transfer, either by using ON numbers or by splitting them into half reactions, but neither of these methods is required. Sure, when speaking about batteries the charge transfer is the most important part of the process and can't be ignored. But as long as we are talking about just _balancing_, charge transfer as a separate process doesn't matter.

 

[barryj]

What bothers me is that the "rules" for determining the oxidation numbers. I read that H is +1 unless it isn't, O is -2 unless it isn't and a few things like that. It is easy for me to understand how to balance equations and how the electrons move to form ions, it seems logical but these oxidation numbers do not seem logical.

 

[Borek]

What bothers me is that the "rules" for determining the oxidation numbers.

As I wrote somewhere above - ON are just an accounting device, not reflecting any real property of atoms in compounds. No wonder they seem artificial and the rules arbitrary.

 

[barryj]

I would really like to learn and be convinced there is a purpose for the ONs but so far I have not seen a good reason. There is a lot of pages in my chem book about ONs so they must be important somehow.

 

[symbolipoint]

I would really like to learn and be convinced there is a purpose for the ONs but so far I have not seen a good reason. There is a lot of pages in my chem book about ONs so they must be important somehow.

Oxidation Numbers are a way of accounting for electric charge so that the two half-reactions can be easily balanced according to change in numbers of electrons.

 

[barryj]

Can you show me an example of an equation where I need to use ON to balance it?

 

[Borek]

Can you show me an example of an equation where I need to use ON to balance it?

No, as every reaction can be balanced without using ON.

But try to balance reaction of oxidation of FeSO₄ by KMnO₄ in sulfuric acid using oxidation numbers and not using them - and you will see that they are quite convenient.

 

[Borek]

Oxidation Numbers are a way of accounting for electric charge so that the two half-reactions can be easily balanced according to change in numbers of electrons.

Half reactions don't require use of oxidation numbers (it is a shortcut that many students use, but it is not how half reaction methods is intended to be used) and they are much better approach whenever assigning oxidation numbers is tricky.

 

[symbolipoint]

Half reactions don't require use of oxidation numbers (it is a shortcut that many students use, but it is not how half reaction methods is intended to be used) and they are much better approach whenever assigning oxidation numbers is tricky.

I cannot imagine what you say there. The use of half reactions and balancing for electrons first had always been something I relied on and doing without that , just unimaginable!

 

[Borek]

I cannot imagine what you say there. The use of half reactions and balancing for electrons first had always been something I relied on and doing without that , just unimaginable!

Yes, using half reactions and balancing for electrons is the basis of the process, but it doesn't require use of oxidation numbers.

If the half reaction is

Fe → Fe²⁺

you balance it not by checking that the oxidation number changes from 0 to 2, so there are two electrons involved, but just by adding electrons till the total charge on each side of the reaction doesn't change. Charge conservation, pure and simple. Yes, the result is the same in this case:

Fe → Fe²⁺ + 2e^-

However, it is not always easy/possible to assign oxidation numbers, unless you want to do it in a very arbitrary way. For example in the case of oxalate oxidation oxidation numbers are rather inconvenient:

(COO^-)₂ → 2CO₂

You can assume ON for C changes from 3 to 4 but there is no basis for that. However, charge conservation means the balanced half reaction is

(COO^-)₂ → 2CO₂ + 2e^-

I believe much better cases would be related to more complicated complexes of transition metals with ligands that can be reduced or oxidized - using ON would be a complete mess, balancing half reactions just to keep charge unchanged is trivial.