Redox is an abbreviation for reduction / oxidation, which tells us that one reactant is reduced and another is oxidised:
- Oxidation: Gain of oxygen and/or loss of hydrogen and/or loss of electrons and/or increase in oxidation state
- Reduction: Gain of hydrogen and/or gain of electrons and/or loss of oxygen and/or decrease in oxidation state
Formally, whether an atom is oxidised or reduced in a reaction depends on the change in oxidation state. Informally, the gain/loss of oxygen/hydrogen/electrons is usually quite easy to see. The oxidation state can be though of as the charge on the atom resulting from electrons being added or removed by the other atoms around it. We can work out the degree of electron addition/removal by considering electronegativity – an atomic property that measures how much an atom will attract electrons to itself when part of a molecule.
The oxidation state for any atom may be calculated by following a set of rules (and exceptions) based on electronegativity – they look quite daunting, but are quite simple to apply with a little practice and some familiarity with commonly-encountered species. It may also help to consult an electronegativity table.
- Atoms in their elemental states have oxidation states of 0.
- The sum of all oxidation states must match the charge on the molecule.
- Fluorine is the most electronegative element, and always has an oxidation state of -1.
- Hydrogen usually has an oxidation state of +1.
- Oxygen usually has an oxidation state of -2.
- Alkali metals (Li, Na etc.) usually have an oxidation state of +1.
- Alkaline earth metals (Mg, Ca etc.) usually have an oxidation state of +2.
- Halogens (Cl, Br etc.) usually have an oxidation state of -1.
- H2O: H is +1 (x2) and O is -2 for a neutral molecule overall.
- CH5+: H is +1 (x4) and the charge is +1, so C is -4.
- NO3–: O is -2 (x3) and the charge is -1, so N is -5.
- H3CCH2OH: H is +1 (x6) and the O is -2, so the -OH is -1 overall, the -CH3 carbon is -3 (from 3 x +1) and the -CH2OH carbon is -1 (from 2 x +1 and 1 x -1).
Oxidation and reduction
Consider the combustion of methane:
CH4 + 2O2 → CO2 + 2H2O
Carbon both loses hydrogen and gains oxygen, so it is being oxidised. Oxygen gains hydrogen (and carbon), so it is being reduced. Hydrogen gains oxygen (and loses carbon), so it is being oxidised. We can also look at it in terms of oxidation states:
- Carbon in CH4 has an oxidation state of -4. Carbon in CO2 also has an oxidation state of -4, so there is no change overall.
- Oxygen in its elemental form (O2) has an oxidation state of 0. Oxygen in H2O and CO2 has an oxidation state of -2, so it is being reduced.
- Hydrogen has an oxidation state of +1 throughout, so there is no change.
Combining all of this information, we can conclude:
- Methane is oxidised by oxygen.
- Oxygen is reduced by methane.
Therefore, we can describe oxygen as an oxidising agent and methane as a reducing agent. Strictly, these labels only apply in this reaction – we could conceivably have different reactions that reduced methane or oxidised oxygen.