Oxidation and reduction are two fundamental processes that occur in chemical reactions. These processes are often referred to as redox reactions, which involve the transfer of electrons between species. Let’s look at the definitions and characteristics of oxidation and reduction:
First, an oxidation number is an imaginary number that gets assigned to an atom. The number represents the imaginary charge that the atom would have if all the bonds to the atom were completely ionic.
Oxidation numbers can be assigned to the atoms in a reaction using the following guidelines:
- Free elements: Atoms in their elemental form (uncombined) have an oxidation number of zero. For example, oxygen gas (O2) or hydrogen gas (H2) both have an oxidation number of zero.
- Monatomic ions: The oxidation number of a monatomic ion is equal to its charge. For example, the oxidation number of sodium ion (Na+) is +1, and the oxidation number of chloride ion (Cl–) is -1.
- Oxygen: In most compounds, oxygen has an oxidation number of -2. However, there are some exceptions. In peroxides (compounds containing the peroxide ion, O22-), such as hydrogen peroxide (H2O2), each oxygen atom has an oxidation number of -1.
- Hydrogen: In most compounds, hydrogen has an oxidation number of +1. However, in compounds such as metal hydrides (e.g., sodium hydride, NaH), hydrogen has an oxidation number of -1.
- Group 1 elements: Elements in Group 1 (alkali metals) have an oxidation number of +1 when combined with other elements. For example, sodium (Na) in NaCl has an oxidation number of +1.
- Group 2 elements: Elements in Group 2 (alkaline earth metals) have an oxidation number of +2 when combined with other elements. For example, calcium (Ca) in CaO has an oxidation number of +2.
- Fluorine: Fluorine (F) always has an oxidation number of -1 in its compounds. This is because it is the most electronegative element and tends to attract electrons towards itself.
- The other Halogens (Cl, Br, and I): These have an oxidation number of -1 in compounds, except when they are combined with oxygen or fluorine. For example, the oxidation number of Cl in the ion ClO4− is +7 (since O has an oxidation number of −2, and the overall charge on the ion is −1).
- Sum of oxidation numbers: The sum of oxidation numbers in a neutral compound is zero. In an ion or a polyatomic ion, the sum of oxidation numbers is equal to the charge of the ion.
It’s important to note that these guidelines provide general rules, but there can be exceptions in certain compounds or situations.
Oxidation is a process in which a species loses electrons, resulting in an increase in its oxidation state.
Key features of oxidation include:
- Loss of electrons: During oxidation, a species loses one or more electrons from its outermost energy level.
- Increase in oxidation state: As electrons are lost, the oxidation state of the species increases.
- Oxidizing agent: The species that causes the oxidation by accepting the electrons is referred to as the oxidizing agent. It gets reduced in the process.
Rusting of iron (Fe → Fe2O3), combustion of fuels, and the reaction of metals with oxygen.
With the reaction Fe2O3:
- Fe as a reactant has an oxidation number of 0.
- Fe as a product in the compound iron (III) oxide (Fe2O3) has an oxidation number of +3.
- Going from an oxidation number of 0 to +3 is the result of losing 3 electrons. So, Fe has been oxidized.
Reduction is the opposite process of oxidation. It involves the gain of electrons by a species, resulting in a decrease in its oxidation state.
Key features of reduction include:
- Gain of electrons: In reduction, a species accepts one or more electrons into its outermost energy level.
- Decrease in oxidation state: As electrons are gained, the oxidation state of the species decreases.
- Reducing agent: The species that causes the reduction by donating the electrons is known as the reducing agent. It gets oxidized in the process.
Hydrogen gas reducing copper (II) oxide to copper metal (CuO + H2 → Cu + H2O), the reduction of carbon dioxide to organic compounds during photosynthesis.
In this reaction CuO + H2 → Cu + H2O
- Copper (Cu) as a reactant combined with oxygen (CuO) has an oxidation number of +2
- Cu as a product has an oxidation number of 0
- Going from an oxidation number of +2 to 0 is the result of gaining 2 electrons. So, Cu has been reduced.
Here is a mnemonic to help you remember oxidation and reduction:
Redox reactions involve both oxidation and reduction processes occurring simultaneously. One species undergoes oxidation (loses electrons) while another species undergoes reduction (gains electrons).
Redox reactions are characterized by:
- Transfer of electrons: Electrons are transferred from the species being oxidized to the species being reduced.
- Conservation of charge: The overall charge of the system remains balanced, as the electrons lost by one species are gained by another.
- Electron donor and acceptor: One species acts as the reducing agent, donating electrons, while the other species acts as the oxidizing agent, accepting electrons.
In summary, oxidation involves the loss of electrons and an increase in oxidation state, while reduction involves the gain of electrons and a decrease in oxidation state. Together, these processes form redox reactions, which are integral to many chemical transformations in various fields, including chemistry, biology, and energy production.