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An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.
Oxidation-reduction (redox) reactions are important because they are the world’s primary natural, biological, and man made energy sources. By eliminating hydrogen and replacing it with oxygen, oxidation of molecules usually releases a lot of energy.
Oxidation-Reduction Reactions
An oxidation-reduction (redox) reaction is a type of chemical reaction in which two species exchange electrons. Any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron is known as an oxidation-reduction reaction.
Photosynthesis, respiration, combustion, and corrosion or rusting are just a few of the essential operations of life that rely on redox reactions.
Rules for Assigning Oxidation States
When an atom joins with other atoms in a compound, its oxidation state (OS) corresponds to the number of electrons, e-, that it loses, gains, or appears to use. There are seven rules to follow when determining an atom’s oxidation state:
A particular atom’s oxidation state is 0.
In a neutral species, the total oxidation state of all atoms is 0, and in an ion, the total oxidation state of all atoms is equal to the ion charge.
Metals in Group 1 have an oxidation state of +1, while metals in Group 2 have an oxidation state of +2.
Fluorine has an oxidation state of -1 in compounds.
In most compounds, hydrogen has an oxidation state of +1.
In most compounds, oxygen has an oxidation state of -2.
The oxidation states of Group 17 elements are -1, Group 16 elements are -2, and Group 15 elements are -3 in binary metal complexes.
Oxidation-Reduction Reactions
Redox reactions are composed of two parts, a reduced half and an oxidised half, that always occur together.
The reduced half gains electrons and the oxidation number decreases, while the oxidised half loses electrons and the oxidation number increases.
Simple ways to remember this include the mnemonic devices OIL RIG, meaning “oxidation is loss” and “reduction is gain.” There is no net change in the number of electrons in a redox reaction.
Those given off in the oxidation half reaction are taken up by another species in the reduction half reaction.
The two species that exchange electrons in a redox reaction are given special names:
The ion or molecule that accepts electrons is called the oxidising agent – by accepting electrons it oxidises other species.
The ion or molecule that donates electrons is called the reducing agent – by giving electrons it reduces the other species.
Hence, what is oxidised is the reducing agent and what is reduced is the oxidising agent. (Note: the oxidising and reducing agents can be the same element or compound, as in disproportionation reactions discussed below).
A good example of a redox reaction is the thermite reaction, in which iron atoms in ferric oxide lose (or give up) O atoms to Al atoms, producing Al2O3 .
Fe2O3 (s) + 2Al (s) → Al2O3 (s) + 2Fe (l)
Combination Reactions
Combination reactions are among the simplest redox reactions and, as the name suggests, involves “combining” elements to form a chemical compound. As usual, oxidation and reduction occur together. The general equation for a combination reaction is given below:
A+B⟶AB
Consider the combination reaction of hydrogen and oxygen
H2+O2⟶H2O
Solution
0 + 0 → (2)(+1) + (-2) = 0
In this reaction both H2 and O2 are free elements; following Rule #1, their oxidation states are 0. The product is H2O, which has a total oxidation state of 0.
According to Rule #6, the oxidation state of oxygen is usually -2. Therefore, the oxidation state of H in H2O must be +1.
Conclusion
An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.
Oxidation-reduction (redox) reactions are important because they are the world’s primary natural, biological, and man made energy sources. By eliminating hydrogen and replacing it with oxygen, oxidation of molecules usually releases a lot of energy.
