What is an oxidation-reduction or redox reaction?

At the core of redox reactions are two processes: oxidation and reduction. Oxidation describes the loss of electrons by a molecule, an atom, or an ion. It doesn't necessarily involve oxygen, despite the name suggesting so. Historically, the term came from reactions involving oxygen, which is a common oxidizing agent, but it has since expanded in meaning.

Reduction, conversely, refers to the gain of electrons. When a substance undergoes reduction, it gains electrons, typically becoming more negatively charged. In a redox reaction, these two processes occur simultaneously—a substance can't be oxidized unless another one gets reduced. This symmetry is the key aspect of why these reactions are paired as oxidation-reduction (or redox) reactions.

The substances that drive the process of oxidation and reduction are known as oxidizing and reducing agents. An oxidizing agent gains electrons from another substance, causing that substance to oxidize. Consequently, the oxidizing agent gets reduced in the process. Common oxidizing agents include substances like oxygen, chlorine, and hydrogen peroxide.

On the other side, a reducing agent loses electrons to another substance, leading to its own oxidation. This action causes the other substance to reduce. Examples of reducing agents include metals such as sodium and magnesium. Identifying these agents in a reaction can significantly enhance the understanding of the redox process.

The fundamental aspect of redox reactions is the transfer of electrons. Electrons being negatively charged subatomic particles, act as the currency in chemical reactions. As they move from one atom or molecule to another, they change the oxidation states of these species. This transfer can happen through various methods, including direct contact or through a conductor in electrochemical reactions.

Electron transfer is critical to numerous biological and geological processes—ranging from the metabolic energy production in cells to the corrosion of metals. Understanding electron transfer mechanisms is essential for mastering redox chemistry and its applications.

Oxidation states (also known as oxidation numbers) provide a way to keep track of electrons in redox reactions. They are hypothetical charges that an atom would have if all bonds to atoms of different elements were 100% ionic. An increase in oxidation state indicates oxidation, while a decrease indicates reduction.

For example, in the reaction between hydrogen and fluorine to form hydrogen fluoride, hydrogen's oxidation state increases from 0 to +1 (oxidation), and fluorine's decreases from 0 to -1 (reduction). Assigning and using oxidation states is helpful in balancing redox equations and identifying the roles of reactants and products in a redox reaction. It's important to remember that the total increase and decrease in oxidation numbers must balance for the reaction to obey the law of conservation of charge.