What is oxidation?

The oxidation it is a process whose effects we can observe around us every day. If we cut an apple, its white flesh turns brown in a few minutes. We know it has rusted.

But what has happened? The oxygen of the air has reacted with some substances of the apple producing new compounds with different characteristics.

The same thing happens with iron when it rusts and becomes more brittle. Oxygen reacts with metallic iron and iron oxide is formed.

However, the concept of oxidation from a chemical point of view is much broader and does not have to involve oxygen, but has to do with the gain or loss of electrons with respect to the normal state of the element.

In this article we will learn what exactly is the oxidationwe will define the Oxidation state of an element and we will see some basic notions about the redox reactions or oxidation-reduction.

Definition and concept of oxidation

For a long time the oxidation was defined as the oxygen gainthat is, the combination of an element or molecule with oxygen atoms to form one or more substances different from the starting ones.

For example, him iron it oxidizes producing iron trioxide (FaithtwoEITHER3), a brown substance more brittle than metallic iron. This is a slow reaction between the metal and the oxygen in the air.

Other oxidation reactions are rapid and sometimes violent, for example, combustion of coal is a exothermic oxidation reaction in which heat is generated and gaseous hydrogen (Htwo) and carbon dioxide (COtwo).

With the discovery of electrons and the study of chemical reactions at a deeper level, it was observed that Substances other than oxygen reacted in a similar way..

For example, him iron also oxidizes in the presence of fluorine and sodium or its own oxygen burn in a chlorine atmosphere. Although it continued to be called oxidation, it was understood that what was really happening was not an exclusive property of oxygen.

What actually happens is that the oxygen atoms have a high electronegativity. This means that they exert a high attraction for electrons (negative charge).

When approaching other atoms of lower electronegativity, the outermost electrons are more attracted to the oxygen atom than to their own atom. This makes it easier for the element with the lower electronegativity to lose its electrons. and react with other substances.

The same thing happens when any two elements of different electronegativity are brought together without necessarily involving oxygen.

Thus, the oxidation of an element can be defined in a general way as the loss of one or more electrons. The opposite, the gain of electrons is the process of reduction.

This definition, however, can mislead one to think that ions are formed in oxidation or reduction reactions, but this does not have to be the case. An element is also oxidized when it shares its electrons in a covalent bond. In this case the element has been oxidized but has not lost its electrons (an ion has not been formed).

Oxidation state

A more precise definition of oxidation, and more used today, is done through the Oxidation state (EO), also known as oxidation number.

The oxidation state is hypothetical load What would the atom or molecule have if all its available bonds were of the ionic type? It is designated by zero and positive or negative numbers that would be the value of this hypothetical load.

For example, the Fe2+ would be iron in an oxidation state of +2 and indicates that it can accept 2 electrons. The Cl represents chlorine in oxidation state -1 and indicates that it can donate an electron.

An atom is oxidized when it experiences a increase in its oxidation state. For example, if the Fe2+ (ferrous state) becomes Fe3+ (ferric state), has gone from oxidation state +2 to +3, so it has oxidized.

This does not mean that it has necessarily lost an electron as it may also indicate that it previously shared 2 electrons in covalent bonds and now shares 3.

For example, in FeO (ferrous oxide or iron(II) oxide), iron has an EO of +2. in the FaithtwoEITHER3 (ferric oxide or iron(III) oxide) iron has an EO of +3. In both cases the oxidation state of oxygen is -2. The number of electrons in iron does not change, only its oxidation state.

Some general rules of oxidation state:

  1. The oxidation state of a free element is zero.
  2. For a monatomic ion (single ion), the oxidation state is equal to the net charge of the ion
  3. Hydrogen has an oxidation state of +1 and oxygen -2 in most compounds except hydrides, in which hydrogen has an oxidation state of -1 (for example LiH, lithium hydride), and peroxides, in which oxygen has an oxidation state of -1 (for example Htwo0twohydrogen peroxide).
  4. The sum of the oxidation states of the atoms that make up a neutral molecule is equal to zero. In a non-neutral molecule this sum is equal to the net charge of the molecule.

You can find more detailed information about the oxidation state in the IUPAC Gold Book: oxidation state.

Redox or oxidation-reduction reaction

As we have seen, the oxidation and the reduction are two reverse processes. A reaction of oxidation-reduction either redox reaction is a chemical reaction in which one of the reactants is oxidized (undergoes an increase in oxidation state) and another reactant is reduced (undergoes a reduction in oxidation state).

It can be given that:

  1. There is a change in the electronic structure: there is real transfer of electrons and ions are usually formed.
  2. No apparent change in electronic structure: there is no actual transfer of electrons and is usually associated with covalent or other non-ionic bonds.

For example, in the following reaction there is an actual transfer of electrons between iodine and chlorine. Chlorine is reduced, going from oxidation state zero to oxidation state -1. Iodine is oxidized from oxidation state -1 to 0. Sodium does not change its oxidation state:

2 NaI + Cl2 → I2 + 2 NaCl

It could be broken down into two steps:

  • 2I → Itwo + 2e: iodide ions are covalently bonded and lose one electron each. The sodium that was bound to the iodide ion remains free.
  • Cltwo + 2e → 2Cl: each atom of the dichloro molecule accepts an electron and chloride ions are formed that form an ionic bond with the sodium ion Na+ which released the iodine.

In the formation of hydrogen chloride (ClH) from dichloro gas and dihydrogen there is a redox reaction but no actual transfer of electrons:

Cl2 + H2 → 2 ClH

redox potential

The ability of one substance to oxidize anotherthat is, to increase its oxidation state, is known as redox potential. The concept of redox potential is related to pH. The pH measures the hydrogen or proton potential (ability to reduce) while the redox potential measures the electron potential.

In organic chemistry we often speak of the gain of hydrogen (or proton H+) as a reduction process and hydrogen loss as an oxidation process. The concept, as far as oxidation state is concerned, is exactly the same.

Go up