Question

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

Short answer

Oxidation states indicate the degree of oxidation of an atom; oxidation is when the state increases (electron loss) and reduction is when it decreases (electron gain). Redox reactions result in changes in oxidation states, identifying which substance is oxidized and which is reduced.

Step-by-step solution

Definition of Oxidation States

Oxidation state (also called oxidation number) is a number assigned to an element in a compound or to a monoatomic ion that represents its degree of oxidation or reduction. The oxidation state is positive if the atom has lost electrons, and it is negative if the atom has gained electrons.

Rules for Assigning Oxidation States

There are a set of rules to assign the oxidation states to atoms: (1) The oxidation state of an atom in its elemental form is zero. (2) The oxidation state for a monoatomic ion is equal to its charge. (3) In compounds, Group 1 elements have an oxidation state of +1, Group 2 elements of +2, oxygen usually has an oxidation state of -2, and hydrogen usually has +1 (except when bonded to metals where it's -1). (4) The sum of the oxidation states in a neutral compound is zero, while in a polyatomic ion is equal to the charge of the ion.

Using Oxidation States to Identify Redox Reactions

Redox reactions involve the transfer of electrons between two species. In a redox reaction, the oxidation state of one or more elements changes. The substance whose oxidation number increases is being oxidized (loss of electrons), and the one whose oxidation number decreases is being reduced (gain of electrons). To identify redox reactions, compare the oxidation states of each element in the reactants and products. If there has been a change in the oxidation states, the reaction is a redox reaction.

Key concepts

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

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.

Electron Transfer in Chemistry

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.