Profile
Settings
Refer your friends
Sign out
An oxidation-reduction (redox) reaction is the process of chemical reaction in which electrons are transferred between two species. Any chemical process in which the oxidation number of a chemical compound, atom, or ion changes by accepting or donating electrons is an oxidation-reduction reaction. Redox reactions are common and essential to some of life’s most basic functions, also including respiration, combustion, photosynthesis, and corrosion or rusting.
Redox reaction
The term oxidation was created to explain reactions wherein the metals react with oxygen in the air to produce metal oxides. When iron is exposed to the atmosphere in the presence of water, it rusts—a form of iron oxide. When aluminium metal is exposed to air, it forms a continuous, transparent layer of aluminium oxide on its surface. The metal acquires a positive charge in both cases by electron transfer to the neutral oxygen atoms of an oxygen molecule. As a consequence, the oxygen atoms gain a negative charge and combine to form oxide ions (O²-). Metals have been oxidised because they have lost electrons to oxygen; oxidation is thus the loss of electrons. In contrast, because the oxygen atoms have managed to gain electrons, they have been lowered, so reduction is electron gain. Every oxidation must be accompanied by a reduction. As a result, these reactions are referred to as oxidation-reduction reactions, or “redox” reactions.
Previously, the word reduction referred to the mass decrease observed when a metal oxide was subjected to heat with carbon monoxide, a reaction commonly used to extract metals from their ores. Solid copper(I) oxide loses mass when subjected to heat with hydrogen, for example, because the creation of pure copper is preceded by the loss of oxygen atoms as a high volatility product (water vapour).
Oxidation-reduction reactions are now described as reactions in which the oxidation states of one or more elements in the reactants change due to an electron transfer, which chooses to follow the mnemonic “oxidation is loss, reduction is gain,” or “oil rig.” The oxidation state of each atom in a compound is the charge that an atom would have if all of its bonding electrons were transferred to the atom with the highest electron attraction. Atoms in their elemental form, such as H2 or O2, have a zero oxidation state.
Every neutral oxygen atom acquires two electrons and then becomes negatively charged, starting to form an oxide ion; as a result, oxygen in the product has an oxidation state of 2 and has been reduced. Every neutral aluminium atom loses three electrons in the process, resulting in an aluminium ion with an oxidation state of +3 in the product, indicating that aluminium has been oxidised.
The Process of Assigning Oxidation States
In binary ionic compounds, assigning oxidation states to the elements is simple: the oxidation states of the elements are similar to the charges on the monatomic ions. You learnt how to estimate the formulas of simple ionic compounds based on the sign and magnitude of charge on monatomic ions generated by neutral elements in a previous lesson. Atoms in covalent compounds, on the other hand, share electrons. However, by considering the elements as if they were ionic, we can still assign oxidation states to them. Although oxidation states in covalent bonding are rather arbitrary, they are important accounting tools for understanding and predicting many reactions.
Redox Reaction Types
Many different types of reactions are categorised as redox reactions, and memorising them all would be impossible. However, there are a few major forms of redox reactions that you should be aware of. These are some examples:
- Synthesis reactions: A redox reaction in which the formation of any compound is carried out straightforwardly from the elements, such as water from hydrogen and oxygen.
- Decomposition reactions: Similarly, the breakdown of a compound into its constituent elements is a redox reaction, as in water electrolysis.
- Combustion reaction: Many chemicals combust (burn) when exposed to oxygen. Organic chemicals, in particular, hydrocarbons, burn in the existence of oxygen to form carbon dioxide and water as byproducts.
- Reactions to Single-Displacement: Aqueous acid oxidises some metals, while aqueous solutions of various metal salts oxidise others. Both types of reactions are known as single-displacement reactions because the ion in solution is displaced by metal oxidation.
Conclusion
The overall chemical equation is divided into an oxidation equation and a reduction equation to balance oxidation–reduction reactions. Electrons are passed from one compound or atom to another during oxidation–reduction reactions. The oxidation state method, which divides the overall reaction into an oxidation equation and a reduction equation, can be used to balance oxidation–reduction reactions in solution. There are many different kinds of redox reactions. Single-displacement reactions occur when metals react with acids or another metal salt, resulting in the dissolution of the first metal and precipitation of a second metal (or evolution of hydrogen gas). The activity series, which organises metals and H2 in order of decreasing oxidation tendency, can be used to predict the outcome of these reactions. In the activity series, any metal will reduce metal ions below it. The active metals are at the upper side of the activity series, while the inert metals are at the bottom.
