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Any chemical reaction in which the oxidation number of a participating chemical species changes is known as an oxidation-reduction reaction. The word refers to a broad range of processes. Many oxidation-reduction reactions are as frequent and familiar as fire, metal corrosion and dissolving, fruit browning, and essential life functions like breathing and photosynthesis.
The most important classifications
Most oxidation-reduction (redox) reactions involve the transfer of oxygen atoms, hydrogen atoms, or electrons, with all three processes sharing two important characteristics:
(1) They are coupled—that is, a reciprocal reduction occurs in any oxidation reaction
(2) they involve a characteristic net chemical change—that is, an atom or electron moves from one unit of matter to another. In examples of the three most prevalent types of oxidation-reduction reactions, both reciprocity and net change are shown.
Transfer of oxygen atoms
Carbon produces carbon dioxide and mercury metal when it combines with mercury(II) oxide (a molecule in which mercury has a bonding capacity of +2; see below Oxidation-state transition).
Carbon is oxidised when it receives oxygen; mercury(II) oxide is reduced when it loses oxygen; and the net change is the transfer of two oxygen atoms from mercury(II) oxide units to a carbon atom.
Transfer of hydrogen atoms
In the following reaction, hydrogen atoms are transported from hydrazine, a nitrogen-hydrogen molecule, to oxygen.
Hydrazine is oxidised to molecular nitrogen when it loses hydrogen, while oxygen is reduced to water when it gains hydrogen.
Transfer of electrons
According to the equation, zinc metal and copper(II) ions react in water to produce copper metal and an aqueous (denoted by aq) zinc ion.
The zinc metal is oxidised, forming an aqueous zinc ion, by transferring two electrons, whereas the copper(II) ion is reduced to copper metal by acquiring electrons. The net change is the transfer of two electrons, which zinc loses and copper gains.
The oxidation and reduction processes are referred to as redox reactions because they are complementary. The oxidising agent is the reagent that causes the oxidation, while the reducing agent is the reagent that is reduced. Mercury(II) oxide, oxygen, and the copper(II) ion are oxidising agents in the examples above, while carbon, hydrazine, and zinc are reducing agents.
Changes in oxidation state
Modern molecular structure theory has enabled comprehensive definitions of oxidation and reduction.
Each element’s bonding characteristics are determined by the presence of a positive nucleus surrounded by negative electrons. Atoms donate, acquire, or share electrons when creating chemical bonds. This allows each atom to be assigned an oxidation number, which indicates how many of its electrons are capable of establishing bonds with other atoms.
The bonding pattern within a molecule is calculated based on the specific atoms in a molecule and their known bonding capacities, and each atom is viewed as being in a certain oxidation state, indicated by an oxidation number.
Theoretical framework
Stoichiometric foundation
The following description of redox processes provides no information on the method by which change occurs. The stoichiometry of the reaction, which offers the distinctive combining proportions of elements and compounds, is a thorough description of the net chemical change for a process.
Conclusion
Any chemical reaction in which the oxidation number of a participating chemical species changes is known as an oxidation-reduction reaction.
The word refers to a broad range of processes. Many oxidation-reduction reactions are as frequent and familiar as fire, metal corrosion and dissolving, fruit browning, and essential life functions like breathing and photosynthesis.
