Recap: Why O2 and H2 gases are oxidizing and reducing agents but their ions are not? Can O2 be oxidized?

The subject looks simple but sometimes can be very confusing.

Every chemical reaction involving electron transfer is an oxidation or reduction. Removal of an electron or electrons is an oxidation, addition a reduction. 

Real life applications of redox reactions are many like, human health, water treatment, photosynthesis, corrosion reactions, combustion reactions, explosives and explosions, fueling space shuttle etc, there could be many more.

When a substance becomes oxidizing agent?

A substance becomes oxidizing agent if it has high affinity for electrons or high electronegativity. The elements like F, Cl and O are typical examples of strong oxidizing agents with high electronegativity. Oxygen's electronegativity, in fact is next to fluorine. The fundamental of oxidation - reduction is based on electronegative / electropositive character of a substance.

In simple words, Electronegative implies having tendency to gain electron to form a negative ion. Electropositive implies having tendency to lose electron to form a positive ion. Non metals are electronegative and metals are electropositive. Generally, an oxidizing agent is electronegative while a reducing agent is electropositive.

What is Oxidation and reduction?

Oxidation is loss of electrons. Reduction is gain of electrons.

Let us take a simple reaction

2H2 + O2 = 2H2O

We can split this reaction into two half ionic equations.

2H2 = 4H+ + 4e

O2 + 4e = 2O - - 

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2H2 + O2 = 4H+ + 2O - - or,

2H2 + O2 = 2H2O

Hydrogen is losing 4 electrons which Oxygen is gaining.

Since oxidation is loss of electrons, H2 is oxidizing to H+ ions and as reduction is gain of electrons O2 is reducing to O- - ions. Both oxidation and reduction are taking place simultaneously, therefore these reactions are called redox reactions.

What are oxidizing / reducing agents ?

An oxidizing agent makes other to lose electrons and gain those electrons and get reduced. Opposite to this, a reducing agent loses electrons and get oxidized.

2H2 = 4H+ + 4e

O2 + 4e = 2O - - 

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2H2 + O2 = 2H2O

In this reaction, Oxygen is making Hydrogen to lose electrons, so O2 is oxidizing agent. Hydrogen is losing electrons so H2 is reducing agent.

Why O2and H2 gases are oxidising and reducing agents but their ions are not?

As explained, O2 when gas makes other like H2 gas to lose electrons, therefore, O2 gas is an oxidizing agent and H2 when gas loses electron in redox reaction, therefore H2 gas is a reducing agent. But their ions like H+ has already oxidized by losing maximum number of electrons it can lose as explained above so it cannot get further oxidized. Similarly, Oxygen ion is already reduced to O- - ions by gaining maximum number of electrons it can gain and therefore it cannot be further reduced. In some reactions H2 acts as oxidizing agent, to keep the discussion simple I will not go into those reactions.

Can O2 be further oxidized?

A stronger oxidizing agent like Fluorine can oxidize oxygen. Fluorine is more electronegative than oxygen.

4F + O2= 2F2O , Break this reaction to two half ionic equations.

4F + 4e = 4 F – [Fluorine is reducing by gaining electrons, ON is changing from zero to -1 ]

O2 = 2O ++ + 4e [Oxygen is oxidizing by losing electrons, ON is changing from zero to +2]

 Here fluorine is making oxygen to lose electrons. Fluorine is oxidizing agent. But it is a rare case, Oxygen, due to its high electronegativity, do not lose electrons easily. As explained above, O2 has to lose electron if it has to oxidize and acquire a positive charge, but its high electron affinity does not allow it to do so.

Can CO2 be oxidized?

Answer is no. Carbon has four valance electrons which it has lost to oxygen to get oxidized to CO2. Carbon does not have any more electron to lose. It cannot get further oxidized.

How to find out which reactant is oxidizing and which is reducing in a redox reaction?

Let us take a simple reaction, Mg + Cl2 = MgCl2

It is a redox reaction.

Write half ionic equations.

Mg = Mg++ + 2e , this is the only way Mg can ionize.

Cl2+ 2e = 2Cl- , this is the only way Cl2 can ionize.

Now, look at two half equations. Mg is losing electrons so it is oxidation reaction. Cl2 is gaining electrons so it is a reduction reaction. Cl2 is making Mg to lose electrons so Cl2 is oxidizing agent. Mg is losing electrons so it is a reducing agent.