% Fichier exo4.18.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
\begin{eqnarray}
MnO_4^- + 8H^+ + 5e^- & 
  \stackrel{\displaystyle\longrightarrow}{\longleftarrow} &
    Mn^{2+} + 4H_2O \\
H_2O_2 &
  \stackrel{\displaystyle\longrightarrow}{\longleftarrow} &
    O_2 + 2H^+ + 2e^- \\
\hline
2MnO_4^- + 5H_2O_2 + 6H^+ &
  \longrightarrow &
    2Mn^{2+} + 5O_2 + 8H_2O
\end{eqnarray}
\end{document}
