- What is the reduction?
- Oxygen number
- Electronegative atom
- Examples
- Reduction in organic chemistry
- Metal extraction
- References
The reduction is all that chemical reaction where the atoms of one of the reactants ends up gaining electrons; What can also be seen this way: your vacancy or electronic “discomfort” is reduced. An atom gains electrons when a species donates them; that is, it rusts.
This type of reaction cannot happen by themselves: if one species accepts the electrons, another must necessarily give them up. Otherwise, matter would be being created from nothing, reducing the atoms after gaining electrons from the vacuum. Therefore, it is a redox (reduction / oxidation) half reaction.
Reduction of the silver tree. Source: Gabriel Bolívar.
An illustrative example of the reduction that can be shown in classrooms is the reaction between a metallic surface of copper, and an aqueous solution of silver nitrate, AgNO 3.
In solution, silver is found as positively charged Ag + cations. These, by interacting with the surface of the copper, shaped like a Christmas tree, snatch the electrons from the copper atoms. When this occurs, copper replaces silver in the nitrate salt; and as a result, copper nitrate, Cu (NO 3) 2, is formed.
Cu 2+ cations together with NO 3 - turn the solution bluish; and silver, whitens the Christmas tree as if covered with snow.
What is the reduction?
In reduction it has already been said that a species gains electrons. How can this be verified in a chemical equation? For example, in the equation for the reaction between Cu and AgNO 3, how do you know when the reduction occurs? To verify this, it is necessary to determine the oxidation number or state.
The elements in their natural states, have by definition an oxidation state equal to zero, since it is assumed that they have neither lost nor gained electrons. Thus, solid metals have zero oxidation state. Thus, silver goes from +1 (Ag +) to 0 (Ag). The charge of a metal ion is equal to its oxidation state.
On the other hand, the electrons came from copper: going from 0 (Cu) to +2 (Cu 2+). The nitrate anion, NO 3 - remains unchanged while both metals exchange electrons; therefore, the equation can be written as:
2Ag + + Cu => 2Ag + Cu 2+
Notice that both the charges and the atoms are balanced.
This is what chemical reduction consists of: in the gain of electrons that make the oxidation states of the atoms less positive than that of the electrons.
Oxygen number
Oxygens are very electronegative and oxidizing atoms, so when an atom forms compounds with them (like oxides), they have positive oxidation states. The greater the number of oxygens interacting with the atom, the more positive its oxidation state; or what is the same, is more rusty.
Therefore, when a compound has fewer oxygen atoms, it is said to be less oxidized; that is, the atom loses fewer electrons.
A classic example can be seen in carbon monoxide and carbon dioxide. For CO, carbon has an oxidation state of +2; while for CO 2, its oxidation state is +4.
So, if in a reaction CO 2 is transformed to CO, a reduction is said to be occurring; since, carbon now interacts with one oxygen and not two. For the opposite reaction, CO transforming into CO 2, we speak of an oxidation of carbon.
This applies to all atoms, especially metals in their metal oxides; for example, CrO 2 (Cr 4+) and CrO 3 (Cr 6+).
In chemical equations where one species loses oxygen while the other species gains it, an oxygen transfer is said to occur.
Electronegative atom
You can always determine if there was a reduction by changing the oxidation state to a less positive value. As just explained, a quick way to tell without even doing the math is by observing if there is a decrease in oxygen atoms in a compound.
The same can happen with any other atom that is more electronegative than the atom that gains or loses electrons.
For example, if CF 4 reacts in such a way that it becomes CH 4, then a reduction is said to have occurred; since fluorine is much more electronegative than the hydrogen atom. As a result, carbon is less oxidized in CH 4 than in CF 4, which is the same as saying that it has been reduced.
Examples
Reduction in organic chemistry
The example of CF 4 and CH 4 reflects what happens in organic reactions, where the reduction of the partial charge of an atom is considered as an electronic gain. This applies a lot when considering the reduction of oxygenated functional groups.
For example, consider the groups ROH, RCHO, and COOH. The first corresponds to alcohols, where carbon binds with oxygen (C-OH); the second is the aldehyde group, where carbon forms a double bond with oxygen and is also bonded to hydrogen (C = OH); and the third is the carboxyl group.
In the carboxyl group, carbon forms a double bond with one O, and a single bond with another O (HO-C = O).
Therefore, a reduction occurs if a carboxylic acid is transformed into an alcohol:
RCOOH => ROH
Metal extraction
Chemical reduction is extremely important in the processes of extracting metals from their minerals. Some of the reactions are:
HgS + O 2 => Hg + SO 2
Mercury sulfide is reduced to metallic mercury.
Cu 2 S + O 2 => 2 Cu + SO 2
Copper sulfide is reduced to metallic copper.
2ZnS + 3O 2 => 2ZnO + 2SO 2
ZnO + C => Zn + CO (note the transfer of O)
Zinc sulfide is first reduced to its monoxide and then to its metallic form.
Fe 2 O 3 + 3CO => 2Fe + 3CO 2
Ferric oxide is reduced to metallic iron.
WO 3 + 3H 2 => W + 3H 2 O
And tungsten trioxide is reduced to metallic tungsten.
As an exercise, the oxidation number of the metal can be determined before it is reduced.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Chemistry LibreTexts. (December 9, 2018). Oxidation-Reduction Reactions. Recovered from: chem.libretexts.org
- Helmenstine, Anne Marie, Ph.D. (July 04, 2018). The Definition of Reduction in Chemistry. Recovered from: thoughtco.com
- Hultzman R. (2019). Reduction in Chemistry: Definition & Overview. Study. Recovered from: study.com
- Clark J. (2013). Definitions of oxidation and reduction (redox). Recovered from: chemguide.co.uk
- Tutor View. (sf). Reduction reaction. Recovered from: chemistry.tutorvista.com