What are oxidation states? How can oxidation states be used to identify redox reactions?

The term 'redox' is a portmanteau of reduction and oxidation. It describes a specific type of chemical reaction where the oxidation states of atoms change due to the transfer of electrons.

Reduction refers to the gain of electrons by a molecule, atom, or ion, which leads to a decrease in its oxidation state. Conversely, oxidation involves an atom losing electrons, resulting in an increase in its oxidation state. These two processes always occur together; when one species is oxidized, another is reduced in the process.

In essence, identifying a redox reaction means spotting these changes in oxidation states. For instance, in the reaction between hydrogen and oxygen to form water, hydrogen is oxidized from an oxidation state of 0 to +1, while oxygen is reduced from 0 to -2.

Assigning oxidation numbers to the atoms within molecules or ions is like balancing an accounting sheet for electrons; it helps us to keep track of the electron gain or loss. To ensure accuracy while assigning these numbers, follow the established rules:

• The oxidation state of a pure element is always zero. For example, O₂ or H₂ have oxidation states of 0 because they are not combined with other elements.
• The oxidation state for any simple (monatomic) ion is the charge it carries. For example, Na⁺ has an oxidation state of +1, Cl^- is -1.
• Hydrogen typically has an oxidation state of +1, except when it's bonded to metals where it can be -1, reflecting its behavior in hydrides.
• Oxygen almost always has an oxidation state of -2, though there are some exceptions, such as in peroxides.
• The sum of oxidation states in a neutral compound must be zero, while in a polyatomic ion, it must equal the ion's charge.

These guiding principles can be applied to complex molecules and aid in understanding the underlying rules governing electron transfer.

At the heart of a redox process is the transfer of electrons between atoms or molecules. This electron exchange is fundamental to many chemical reactions and is often visualized as one species donating electrons and another accepting them.

The results of these transfers are observable in everyday chemical processes, such as the rusting of iron or the browning of an apple slice exposed to air. In a more technical context, the movement of electrons is the basis for electrochemical cells and batteries, where electron flow generates electric current.

Electron transfer is not merely the movement of negative charges; it also involves the accompanying energy change. This energy change is crucial to biological processes and industrial chemical reactions, driving the transformation of matter and the flow of energy in and out of systems.