4.2.1: Oxidation and Reduction
Oxidation-Reduction Reactions
When a transfer of electrons occurs between a metal and a nonmetal, we can view this as an example of oxidation-reduction reaction (or redox reaction).
Redox reactions are a broad class of chemical reactions, but some redox reactions involve the transfer of electrons between reactant species to yield ionic compounds, such as the reaction between sodium and chlorine to yield sodium chloride:
\[\ce{2Na}(s)+\ce{Cl2}(g)\rightarrow \ce{2NaCl}(s) \nonumber \]
It is helpful to view the process with regard to each individual reactant, that is, to represent the fate of each reactant in the form of an equation called a half-reaction :
\[ \begin{align*} \ce{2Na}(s) &\rightarrow \ce{2Na+}(s)+\ce{2e-} \\[4pt] \ce{Cl2}(g)+\ce{2e-} &\rightarrow \ce{2Cl-}(s) \end{align*} \nonumber \]
These equations show that Na atoms lose electrons while Cl atoms (in the Cl 2 molecule) gain electrons , the “ s ” subscripts for the resulting ions signifying they are present in the form of a solid ionic compound. For redox reactions of this sort, the loss and gain of electrons define the complementary processes that occur:
\(\begin{align}
\textbf{oxidation}&=\textrm{loss of electrons}\\
\textbf{reduction}&=\textrm{gain of electrons}
\end{align}\)
In this reaction, then, sodium is oxidized and chlorine undergoes reduction . Viewed from a more active perspective, sodium functions as a reducing agent (reductant) , since it provides electrons to (or reduces) chlorine. Likewise, chlorine functions as an oxidizing agent (oxidant) , as it effectively removes electrons from (oxidizes) sodium.
\(\begin{align}
\textbf{reducing agent}&=\textrm{species that is oxidized}\\
\textbf{oxidizing agent}&=\textrm{species that is reduced}
\end{align}\)