Without reduction and oxidation this world would not work. Because in every reaction in which electrons are transported, oxidation and reduction inevitably occur. These are opposite processes: oxidation is the loss of electrons and reduction is the acceptance of electrons. Here in this explanation you will learn everything about the reduction.
In the reduction one or more electrons are accepted. These are transferred from one atom or molecule to another.
You have to be aware that in an electron transfer, one reaction partner gives up electrons and another reaction partner takes up these given electrons. This acceptance of electrons is called reduction. Since an electron transfer consists of an electron donation and electron acceptance, this electron exchange is also called a redox reaction, composed of the words editorauction and Oxidentification
Reduction – meaning and basics
In a reduction, one of the reactants gains electrons. Reactants can be single atoms or whole molecules. A first prerequisite for this is, on the one hand, that there is another reaction partner who is willing to donate electrons.
In the Original on oxidation you will find more information about the requirements of oxidation.
As previously mentioned, electron transfer always consists of an oxidation and a reduction. You can illustrate this with an example in which magnesium and chlorine react with each other to form magnesium chloride:
Oxidation:
Reduction:
The numbers marked in red correspond to the oxidation numbers of the atoms. Oxidation numbers are to be distinguished from ionic charges. While ionic charges can only be specified for ions and express how many more or fewer electrons an atom has in its atomic shell compared to its protons, oxidation numbers are used for atoms that are involved in electron pair bonds.
In electron pair bonding, electrons are shared between the bonding partners, with two atoms each pulling on the electrons. Depending on which atom pulls harder, specifically has higher electronegativity, the electrons will be pulled more to the side of the stronger atom.
In order to understand why the electrons are then accepted at all, you should understand the concept of electronegativity know. Electronegativity is a number that indicates how much an atom attracts electrons or how much an atom is willing to lose or gain an electron.
Atoms with high electronegativity are able to grab the electrons from atoms with low electronegativity. In the course of electron acceptance, the more electronegative atoms are reduced – their oxidation number decreases.
Even if the electrons are not completely withdrawn from the weaker atom, it receives a so-called positive Partcharge, also called partial charge. This charge is not to be equated with an actual ionic charge, but merely expresses an uneven charge distribution. The stronger atom gets a partial negative charge because the electrons tend to be closer to it.
In order to characterize the strength of these partial charges, oxidation numbers are used, which indicate how many electrons are attracted to the atom compared to the normal state.
In the example of the molecule methanol, the carbon atom has an oxidation number of -2. Carbon is more electronegative than hydrogen – so it is formally assigned the three bonding electrons to the hydrogen atoms. At the same time, an electron is withdrawn from the carbon atom by the more electronegative oxygen atom.
Figure 1: Structural formula of methanolSource: chemistry.school
Motto: During reduction, the oxidation number is reduced!
reduction example
The reduction always occurs together with the oxidation: Both reactions never take place independently of each other and together they are also called redox reactions.
Oxidation and reduction can be summarized as follows:
- Electron loss = oxidation/increase in oxidation number: more positive value
- Donate electrons = reduction/decrease in oxidation number: more negative value
For example, if an iron nail is placed in a solution of copper sulphate, a red-brown coating of metallic copper forms on the nail. This happens because the iron atoms donate electrons to the copper ions. The copper is reduced and the iron oxidized. In this general definition, a reduction always occurs in parallel with an oxidation:
Oxidation: Fe → Fe²⁺ + 2e⁻
Reduction: Cu²⁺ + 2e⁻ → Cu
Redox reaction: Fe + Cu²⁺ → Cu + Fe²⁺
The iron acts here as a reducing agent, which in this context is itself oxidized during the redox reaction. Reducing agent because it reduces the reaction partner, the copper ions, and is not reduced itself! In order to reduce copper ions, the iron itself has to give up electrons, i.e. it has to be oxidized.
Conditions for reductions
But what is the «driving force» of the reduction? Why can some elements be reduced and why are there elements that can be reduced better than others?
driving force of reduction
The drive of every atom is the fulfillment of the octet rule. The octet rule states that every atom strives for a stable state. This stable state can be reached by an atom by having eight outer electrons. However, since only very few atoms naturally have eight outer electrons, atoms can give up, take up or share electrons when forming a molecule.
Chlorine is an example of reduction. Chlorine has seven outer electrons. You can tell by the fact that it is in the seventh main group in the periodic table. In order to obey the octet rule, chlorine must gain one more electron. Chlorine must therefore be reduced. This is then done, for example, with the help of sodium, which only has one outer electron and would like to give it up in order to achieve the noble gas configuration. In this state, the octet rule is also fulfilled for sodium.
If you want to learn more about outer electrons and atomic structure, have a look at the article on atomic structure.
reduction conditions
There are different reduction conditions for metals and non-metals. In the case of metals (e.g. magnesium and calcium), in the elementary state in which the number of protons is equal to the number of electrons, they cannot accept any further electrons. This is only in oxidized form or cationic form possible.
nonmetals, such as chlorine and iodine, however, can be further reduced in the elementary state. In the already reduced form, however, no further reduction is then possible.
With regard to the example reaction of magnesium with chlorine, magnesium represents the metal and chlorine the non-metal. Chlorine is reduced and must therefore be present in elementary form. Chloride ions, i.e. Cl–could not accept any more electrons and therefore could not react with magnesium.
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The redox series – it depends on the partner
Some substances can be reduced better than others. Using a table of specific voltage ratings, too redox series called, which has been determined experimentally, one can read how high the ability of a particle is to accept electrons. These values represent the previously mentioned electronegativity.
The higher/more positive the voltage values are, the higher the ability to be reduced (accept electrons), so the higher it is reducing power. At the same time, the lower the ability to be oxidized (donate electrons), so the lower it is oxidizing ability.
It also applies that the element/atom that emits electrons, i.e. is oxidized, represents the reducing agent. Conversely, the reduced element is called the oxidizing agent.
This means that reducing agents are the opposite of the so-called oxidizing agents. Oxidizing agents are substances that can accept electrons while reducing agents are substances that donate electrons. Therefore, oxidizing agents are also referred to as electron acceptors and reducing agents are also referred to as electron donors. In contrast to reducing agents, oxidizing agents (i.e. other substances) oxidize themselves and are reduced in the process.
Referring to the above example of magnesium reacting with chlorine, magnesium has a lower potential than chlorine and its reducing power or ability to retain electrons is lower. Therefore, it donates its electrons to chlorine, which has a higher reducing power.
Hydrogen as an electron donor
In certain reactions in organic chemistry, such as addition reactions, hydrogen or a hydrogen cation can bond to a carbon or oxygen atom.
Because hydrogen has a lower electronegativity than carbon and oxygen, the shared electrons are more strongly pulled toward the carbon and oxygen sides, respectively. The oxidation number of carbon/oxygen increases unless another atom that is weaker than carbon has been replaced, otherwise the oxidation number remains the same or if the hydrogen atom is newly added.
In this respect, one can expect an increase in the oxidation number of the attacked atom when hydrogen is added to oxygen, carbon and sulfur.
However, it cannot be said that the attacked atom is reduced across the board. You always have to compare the oxidation number of the attacked atom in the previous bond and in the new bond and pay attention to whether a weaker atom is splitting off from the attacked atom at the same time.
Oxygen as an electron acceptor
Oxygen, unlike hydrogen, has a significantly higher electronegativity.
For example, if elementary oxygen binds to a carbon atom or a sulfur atom, it is not the attacked atom that is reduced but the oxygen atom (see table above).
In elementary oxygen, oxygen formally has an oxidation number of ±0, since the partner with whom the electrons are shared (covalent bond) is also oxygen and thus pulls the electrons equally strongly.
However, if the oxygen atom binds to a carbon atom, it can experience a change in oxidation number of two units in the case of a double bond, for example.
However, as with hydrogen, it cannot be said that oxygen is reduced across the board. One must always compare the oxidation number of the oxygen atom in the previous bond and in the new bond.
However, it does occur in most cases and should therefore be kept in mind, ieEspecially in problems where you have to explain why a reaction is a redox reaction, you have to be able to show which atom has been reduced.
So it’s worth paying attention to whether an oxygen atom now has weaker partners or whether a carbon atom now has more hydrogen atoms…