Oxidation and reduction

Example of how to add TikZ arrows to chemical reactions made with the mhchem package. TikZ coordinate nodes are put in the reaction equations and later used as reference points for drawing using TikZ's remember picture feature.

Edit and compile if you like:

% Oxidation and reduction
% Author: Arne Röhrs
% Note that two compilations are necessary. 
\documentclass{article}
\usepackage{tikz}
\usepackage[version=3,arrows=pgf]{mhchem}


\begin{document}

% Insert a named coordinate for later reference.
% Usage:
% \ncoord{name}
% \ncoord[1em,2em]{name}
\newcommand\ncoord[2][0,0]{%
    \tikz[remember picture,overlay]{\path (#1) coordinate (#2);}%
}

% Define a handful of special cases of \ncoord
\newcommand\ccoord[1]{\ncoord[0.5em,0.9em]{#1}}%
\newcommand\bcoord[1]{\ncoord[0.4em,-0.3em]{#1}}%
\newcommand\dcoord[1]{\ncoord[0.9em,1.5em]{#1}}%
\newcommand\ecoord[1]{\ncoord[0.9em,-0.3em]{#1}}%

\tikzset{
    oxidation/.style={thick,red!80!black},
    reduction/.style={thick,blue!80!black},
}


~

% First reaction
\[
\ce{{\ccoord{a}} Na + {\bcoord{c}} Cl -> {\ccoord{b}} Na+  + {\bcoord{d}} Cl-}
\]
\tikz[overlay,remember picture] {
    \draw[oxidation,->] (a) -- ++(0,0.7em) -| (b)
        node[above, near start] {Oxidation};
    \draw[reduction,->] (c) -- ++(0,-0.7em) -| (d)
        node[below, near start] {Reduktion};
}
\\[0.5cm] % add some extra vertical spacing

% Second reaction
\[
    \ce{%
    {\dcoord{a}} $\overset{\text{0}}{\text{2\,Na}}$
    +{\ecoord{c}}$\overset{\text{0}}{\text{Cl$_2$}}$ ->
    {\dcoord{b}} $\overset{\text{+I}}{\text{2\,Na$^+$}}$  +
    {\ecoord{d}} $\overset{\text{-I}}{\text{2\,Cl$^-$}}$}
\]
\tikz[overlay,remember picture] {
    \draw[oxidation,->] (a) -- ++(0,0.7em) -| (b)
        node[above, near start] {Oxidation};
    \draw[reduction,->] (c) -- ++(0,-0.7em) -| (d)
        node[below, near start] {Reduktion};
}



\end{document}