Classifying and Characterizing Chemical Reactions

Introduction to Oxidation - Reduction Reactions

Oxidation reductions—or, as they are commonly referred to, redox reactions—result in the transfer of electrons from one reactant to another. Compared to precipitation reactions, which are solubility reactions, and acid–base or proton-transfer reactions, oxidation–reduction reactions are typically significantly more energetic. These are the explosions and the fires of chemistry. In fact, some of the reactions we have encountered thus far—such as combustion reactions, first ionization reactions, and electron affinity reactions—fall into the classification of oxidation–reduction reactions. The equation for the first ionization energy for \(\text{Na}\) shows a neutral \(\text{Na}\) atom losing an electron to form the \(\text{Na}^+\) cation, and the equation for the electron affinity for \(\text{Cl}\) shows a neutral \(\text{Cl}\) atom gaining an electron to form a \(\text{Cl}^-\) anion. Each of these equations depicts half of a full oxidation–reduction reaction because, combined, they describe an electron transfer reaction from \(\text{Na}\) to \(\text{Cl}\). This is the hallmark of an oxidation–reduction reaction.

\(\text{Na} \rightarrow \text{Na}^+ + e^-\)

\(\text{Cl} + e^- \rightarrow \text{Cl}^-\)

\(\text{Na} + \text{Cl} \rightarrow \text{Na}^+ + \text{Cl}^-\)

In this Na and Cl electron-transfer reaction, Na is said to be oxidized because it lost an electron, and Cl is said to be reduced because it gained an electron. Because sodium (Na) is the means or agent by which chlorine (Cl) gains an electron and is reduced, Na is referred to as the reducing agent. Likewise, because Cl is the means or agent by which Na loses an electron and is oxidized, Cl is referred to as the oxidizing agent.

Electron Transfer Reactions Transcript

Oxidation Numbers

In simple cases, it is straightforward to tell if a reactant has been oxidized or reduced by looking to see if the charge on a reactant changes—for example, in the reaction of \(\text{Mg}\) going to \(\text{Mg}^{2+}\) or \(\text{O}\) going to \(\text{O}^{2-}\), which combine to form the ionic compound \(\text{MgO}\). However, it is not always simple. Consequently, oxidation numbers are used to determine if an atom has gained or lost electrons in a reaction. If the oxidation number assigned to an atom increases in the reaction, the atom has been oxidized in the reaction, and conversely, if the oxidation number assigned to an atom decreases in the reaction, the atom has been reduced in the reaction.

Oxidation numbers are assigned to individual atoms based on the following electron “bookkeeping” rules.

Rules for Assigning Oxidation Numbers

Rule 1: All atoms in their elemental form are assigned 0.

Rule 2: All single-element ions are assigned their charges. Examples: \(\text{Na}^+\) is +1, \(\text{Cl}^-\) is −1; in \(\text{PbCl}_2\), Pb is +2 and Cl is −1; in \(\text{Ca}_2\text{C}\), Ca is +2 and C is −4.

Rule 3: H is assigned +1. There are, however, some rare exceptions like \(\text{NaH}\) and \(\text{LiAlH}_4\), in which H is assigned −1.

Rule 4: O is assigned −2. The major exception to this rule is \(\text{H}_2\text{O}_2\) (peroxide), in which H is +1, so each O must be −1.

Rule 5: F is always −1; there are no exceptions.

Rule 6: In any other bonding situation, the bonding electrons are assigned to the more electronegative atom. Examples: In \(\text{ClBr}\), Cl is −1 and Br is +1 and in \(\text{CCl}_4\), C is +4 and Cl is −1.

When these rules are applied, any change in the oxidation number assigned to a reactant atom, as compared to its oxidation number assignment as a product atom, is an indication that the atom has been oxidized or reduced in the reaction. If the oxidation number assigned to an atom increases in the reaction and becomes more positive, the atom has lost electrons and is oxidized. If the oxidation number assigned to an atom decreases and becomes more negative, it indicates that the atom has been reduced in the reaction because it gained electrons.

Oxidation Numbers Transcript

Using Oxidation Numbers Transcript

Oxidation Practice