What is an oxidation number? How is it used to identify redox reactions? Explain why, except for ionic compounds, oxidation number does not have any physical significance.

Understanding oxidation numbers is crucial for identifying redox reactions. These numbers, often conceptual rather than a reflection of factual charge, act as indicators to ascertain whether a chemical reaction involves the gain or loss of electrons. In simple terms, an oxidation number can be seen as a hypothetical charge an atom would have if all bonds to atoms of different elements were completely ionic.

Grasping the concept of oxidation numbers is akin to learning the alphabet before forming words; it's a fundamental tool necessary for the deeper understanding of redox chemistry. When analyzing a molecule, for instance, assigning oxidation numbers to each element facilitates the determination of which elements are being oxidized or reduced—critical for deciphering the underlying chemistry of redox processes.

A redox reaction is essentially an elegant dance of electrons between atoms. One atom will lose electrons, experiencing an increase in its oxidation number, while another atom gains those electrons, resulting in a decrease in its oxidation number. This interplay is the heartbeat of a redox reaction.

Consider the simple reaction of magnesium and oxygen to form magnesium oxide: \(2Mg + O_2 \rightarrow 2MgO\). Magnesium's oxidation number ascends from 0 to +2, surrendering electrons to oxygen, whose oxidation number descends from 0 to -2. This is a quintessential redox reaction, showcasing both oxidation and reduction.

While oxidation numbers provide a simplified view, the actual charge distribution within a molecule is more nuanced. It involves understanding how electrons are shared between atoms in covalent bonds. Electronegativity, a measure of an atom's ability to attract electrons in a bond, plays a key role in this distribution.

For example, in a water molecule \(H_2O\), the oxygen has a much higher electronegativity than the hydrogen atoms. As a result, it attracts more electron density towards itself, leading to a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms. This real charge distribution is different from the oxidation numbers of +1 for hydrogen and -2 for oxygen, which are based on the assumption of completely ionic interactions.

Balancing chemical equations is a key skill in chemistry, akin to ensuring both sides of the ledger balance in accounting. Oxidation numbers can be a valuable tool in this process, especially for redox reactions. By comparing the changes in oxidation numbers, one can deduce the number of electrons exchanged and balance the equation accordingly.

For instance, the reaction of iron (\text{III}) oxide with carbon monoxide to produce iron and carbon dioxide, \(Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2\), can be balanced by noting that the carbon in carbon monoxide increases from +2 to +4 while iron decreases from +3 to 0. By maintaining the electron inventory, a balanced and accurate chemical equation emerges, ensuring the law of conservation of mass is respected.