\documentclass[12pt,oneside]{memoir}
\usepackage{epsfig}
\usepackage{ulem}

\begin{document}

\begin{equation}
\label{rxn}
2Ag(s) + Hg_{2}Cl_{2}(s) = 2AgCl(s) + 2Hg(l)
\end{equation}

\begin{equation}
\label{fhalf}
AgCl + e^{-} = Ag + Cl^{-} \hspace{2cc} E^{o} \rightleftharpoons 0.22233V
\end{equation}
and
\begin{equation}
\label{lhalf}
Hg_{2}Cl_{2} + 2e^{-} \rightleftharpoons 2Hg + 2Cl^{-} \hspace{2cc} E^{o} = 0.26808V
\end{equation}
Below, we shall take
\begin{equation}
\label{grosseqn}
Hg_{2}Cl_{2} + \xout{2e^{-}} + 2Ag + \xout{2Cl^{-}} \rightleftharpoons 2AgCl + \xout{2e^{-}} + 2Hg + \xout{2Cl^{-}}
\end{equation}




\end{document}