Learning Objectives Identify substances which are oxidized or reduced in a chemical reaction. Earlier in this text we discussed the function of batteries in supplying a source of electrical energy. All batteries function on the basis of electron transfer reactions, commonly known as redox reactions. There are several other examples of important redox reactions: corrosion of a metal, combustion of a flammable substance, and digestion of our food all involve redox reactions. Some of these are reactions we want to have occur, while others we might want to prevent. The more we know about redox reactions, the better we will be able to design systems to make use of the ones we want and avoid the ones that we don't want. Historically, the terms oxidation and reduction had to do with how oxygen behaved in a chemical reaction. A chemical was oxidized if it reacted with oxygen, and reduced if it reacted in a way that lost oxygen. Eventually this process was generalized to include other types of reactions in which electrons are being transferred. An oxidation-reduction reaction is a reaction that involves the full or partial transfer of electrons from one reactant to another. Oxidation is the full or partial loss of electrons (gaining of oxygen is only one way to do this). Reduction is the full or partial gain of electrons (loss of oxygen is only one way to do this). A redox reaction is another term for an oxidation-reduction reaction. In the course of a chemical reaction between a metal and a nonmetal, electrons are transferred from the metal atoms to the nonmetal atoms. For example, when zinc metal is mixed with sulfur and heated, the compound zinc sulfide is produced. Two valence electrons from each zinc atom are transferred to each sulfur atom. Since the zinc is losing electrons in the reaction, it is being oxidized. The sulfur is gaining electrons and is thus being reduced. An oxidation-reduction reaction is a reaction that involves the full or partial transfer of electrons from one reactant to another. Oxidation is the full or partial loss of electrons or the gain of oxygen. Reduction is the full or partial gain of electrons or the loss of oxygen. A redox reaction is another term for an oxidation-reduction reaction. Each of these processes can be shown in a separate equation called a half-reaction. A half-reaction is an equation that shows either the oxidation or the reduction reaction that occurs during a redox reaction. \[\begin{align*} &\text{Oxidation:} \: \ce{Zn} \rightarrow \ce{Zn^{2+}} + 2 \ce{e^-} \\ &\text{Reduction:} \: \ce{S} + 2 \ce{e^-} \rightarrow \ce{S^{2-}} \end{align*}\nonumber \] It is important to remember that the two half-reactions occur simultaneously. The resulting ions that are formed are then attracted to one another in an ionic bond. In the reaction above, zinc is being oxidized by losing electrons. However, there must be another substance present that gains those electrons and in this case that is the sulfur. In other words, the sulfur is causing the zinc to be oxidized. Sulfur is called the oxidizing agent. The zinc causes the sulfur to gain electrons and become reduced and so the zinc is called the reducing agent. The oxidizing agent is a substance that causes oxidation by accepting electrons. The reducing agent is a substance that causes reduction by losing electrons. The simplest way to think of this is that the oxidizing agent is the substance that is reduced, while the reducing agent is the substance that is oxidized. The example below shows how to analyze a redox reaction. Example \(\PageIndex{1}\) When chlorine gas is bubbled into a solution of sodium bromide, a reaction occurs which produces aqueous sodium chloride and bromine. Determine what is being oxidized and what is being reduced. Identify the oxidizing and reducing agents. \[\ce{Cl_2} \left( g \right) + 2 \ce{NaBr} \left( aq \right) \rightarrow 2 \ce{NaCl} \left( aq \right) + \ce{Br_2} \left( l \right)\nonumber \] Solution: Step 1: Plan the problem. Break the reaction down into a net ionic equation and then into half-reactions. The substance that loses electrons is being oxidized and is the reducing agent. The substance that gains electrons is being reduced and is the oxidizing agent. Step 2: Solve. \[\begin{align*} \ce{Cl_2} \left( g \right) + \cancel{2 \ce{Na^+} \left( aq \right)} + 2 \ce{Br^-} \left( aq \right) &\rightarrow \cancel{2 \ce{Na^+} \left( aq \right)} + 2 \ce{Cl^-} \left( aq \right) + \ce{Br_2} \left( l \right) \\ \ce{Cl_2} \left( g \right) + 2 \ce{Br^-} \left( aq \right) &\rightarrow 2 \ce{Cl^-} \left( aq \right) + \ce{Br_2} \left(l \right) \: \: \: \: \: \left( \text{net ionic equation} \right) \end{align*}\nonumber \] \[\begin{align*} &\text{Oxidation:} \: \ce{Cl_2} \left( g \right) + 2 \ce{e^-} \rightarrow 2 \ce{Cl^-} \left( aq \right) \\ &\text{Reduction:} \: 2 \ce{Br^-} \left( aq \right) \rightarrow \ce{Br_2} \left( l \right) + 2 \ce{e^-} \end{align*}\nonumber \] The \(\ce{Cl_2}\) is being reduced and is the oxidizing agent. The \(\ce{Br^-}\) is being oxidized and is the reducing agent. Section Summary An oxidation-reduction reaction, or redox reaction, is a reaction that involves the full or partial transfer of electrons from one reactant to another. Oxidation is the full or partial loss of electrons or the gain of oxygen. Reduction is the full or partial gain of electrons or the loss of oxygen. Oxidation and reduction reactions must occur together; neither can happen alone in a reaction. A half-reaction is an equation that shows either the oxidation or the reduction reaction that occurs during a redox reaction. Oxidation and reduction reactions must occur together; neither can happen alone in a reaction. The oxidizing agent is a substance that causes oxidation by accepting electrons. The reducing agent is a substance that causes reduction by losing electrons. Glossary oxidation-reduction reaction (redox reaction) A reaction that involves the transfer of electrons from one reactant to another. oxidation The loss of electrons. reduction The gain of electrons. half reaction An equation that shows either the oxidation or the reduction reaction that occurs during a redox reaction. oxidizing agent A substance that causes oxidation by accepting electrons. reducing agent A substance that causes reduction by losing electrons.