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@book{iupac:greenbook,
  author    = {E. Richard Cohan and Tomislav Cvita\v{s} and Jeremy G. Frey and
    Bertil Holmström and Kozo Kuchitsu and Roberto Marquardt and Ian Mills and
    Franco Pavese and Martin Quack and Jürgen Stohner and Herbert L. Strauss and
    Michio Takami and Anders J Thor},
  title     = {“Quantities, Symbols and Units in Physical Chemistry”, \acs{iupac}
    Green Book},
  edition   = {3rd Edition. 2nd Printing},
  year      = {2008},
  publisher = {\acs{iupac} \&\ RSC Publishing, Cambridge}
}
@book{iupac:redbook,
  author    = {Neil G. Connelly and Ture Damhus and Richard M. Hartshorn and
    Alan T. Hutton},
  title     = {“Nomenclature of Inorganic Chemistry”, \acs{iupac} Red Book},
  year      = {2005},
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  isbn      = {0-85404-438-8}
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@misc{eu:ghsystem_regulation,
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  title    = {Regulation (EC) No 1272/2008 of the European Parliament and of the Council},
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    amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending
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  journal  = {Official Journal of the European Union},
  date     = {2008-12-16}
}
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  title    = {GHS Implementation},
  url      = {http://www.unece.org/trans/danger/publi/ghs/implementation_e.html},
  urldate  = {2012-03-20},
  date     = {2012-03-20}
}
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\begin{document}

\section{Introduction}
Probably every chemist using \LaTeXe\ is aware of the great \paket{mhchem}
package by Martin Hensel.  There have always been some difficulties
intertwining it with \chemmacros, though.  Also, some other minor points in
\paket{mhchem} always bothered me, but they hardly seemed enough for a new
package.  They weren't even enough for a feature request to the \paket{mhchem}
author.  The challenge and the fun of creating a new package and the wish for
a highly customizable alternative led to \chemformula\ after all.

\chemformula\ works very similar to \paket{mhchem} but is more strict as to how
compounds, stoichiometric factors and arrows are input.  In the same time
\chemformula\ offers \emph{many} possibilities to customize the output.

\sinceversion{4.0}Since version 4.0, the \chemformula\ package can be used
independently from \chemmacros.  This means that if you say
\begin{beispiel}[code only]
  \usepackage{chemformula}
\end{beispiel}
then \chemmacros\ will not be loaded.  The \chemmacros\ package, however,
\emph{will} load \chemformula.

\section{Licence and Requirements}
Permission is granted to copy, distribute and/or modify this software under
the terms of the \LaTeX\ project public license (lppl) version 1.3 or later
(\url{http://www.latex-project.org/lppl.txt}).  The package has the status
``maintained.''

The \chemformula\ package needs and thus loads the packages \paket*{expl3}
(from the \paket*{l3kernel}\footnote{\CTANurl{l3kernel}}), \paket*{xparse},
\paket*{l3keys2e} and \paket*{xfrac} (from the
\paket*{l3packages}\footnote{\CTANurl{l3packages}}),
\paket*{tikz}\footnote{\CTANurl[graphics]{pgf}}, \paket{amsmath},
\paket*{nicefrac} and \paket*{scrlfile} (from the
\KOMAScript\footnote{\CTANurl{koma-script}} bundle).

\section{Setup}
All of \chemformula's options belong to \chemmacros' module
\textcolor{module}{\code{chemformula}}.  This means they can be setup with
\begin{beispiel}[code only]
 \chemsetup[chemformula]{<options>} or
 \chemsetup{chemformula/<option1>,chemformula/<option2>}
\end{beispiel}
However, if you're using \chemformula\ as a standalone package the command
\cmd{chemsetup} is not available.  This is why \chemformula\ also has its own
setup command:
\begin{beispiel}[code only]
  \setchemformula{<options>}
\end{beispiel}

\section{The Basic Principle}
\chemformula\ offers one main command.
\begin{beschreibung}
 \Befehl{ch}[<options>]{<input>}
\end{beschreibung}
The usage will seem very familiar to you if you're familiar with \paket{mhchem}:
\begin{beispiel}
 \ch{H2O} \\
 \ch{Sb2O3} \\
 \ch{H+} \\
 \ch{CrO4^2-} \\
 \ch{AgCl2-} \\
 \ch{[AgCl2]-} \\
 \ch{Y^{99}+} \\
 \ch{Y^{99+}} \\
 \ch{H2_{(aq)}} \\
 \ch{NO3-} \\
 \ch{(NH4)2S} \\
 \ch{^{227}_{90}Th+} \\
 $V_{\ch{H2O}}$ \\
 \ch{Ce^{IV}} \\
 \ch{KCr(SO4)2 * 12 H2O}
\end{beispiel}

However, there are differences.  The most notable one: \chemformula\
distinguishes between different types of input.  These different parts
\emph{have} to be separated with blanks:
\begin{beschreibung}
 \Befehl{ch}{part1 part2 part3 part4}
\end{beschreibung}
A blank in the input \emph{never} is a blank in the output.  This role of the
blank strictly holds and disregarding it can have unexpected results and even
lead to errors.

Another notable difference: \chemformula\ tries to avoid math mode whenever
possible:
\begin{beispiel}
 \ch{A + B ->[a] C} \\
 \ce{A + B ->[a] C}
\end{beispiel}

This means that \cmd{ch}{2H2O} is recognized as a \emph{single} part, which in
this case is recognized as a compound.
\begin{beispiel}
 \ch{2H2O} \\
 \ch{2 H2O}
\end{beispiel}
This also means, that a part cannot contain a blank since this will
automatically divide it into two parts.  If you need an extra blank in the
output you need to use \lstinline+~+.  However, since commands in most cases
gobble a space after them a input like \cmd{ch}{\textbackslash command ABC}
will be treated as a single part.  If you want or need to divide them you need
to add an empty group: \cmd{ch}{\textbackslash command\{\} ABC}.  The
different input types are described in the following sections.

% TODO: Module überdenken, Beschreibung anpassen
There are some options to customize the output of the \cmd{ch} command.  They
can either be applied locally using the optional argument or can be set
globally using the setup command.  All options of \chemformula\ belong to the
module \textcolor{module}{\code{chemformula}} and can be set in different
ways:
\begin{beschreibung}
 \Befehl{chemsetup}[chemformula]{<options>} (when loaded via \chemmacros)
 \Befehl{chemsetup}{chemformula/<options>} (when loaded via \chemmacros)
 \Befehl{setchemformula}{<options>} (independent from \chemmacros)
\end{beschreibung}

\section{Stoichiometric Factors}\secidx{Stoichiometric Factors}
A stoichiometric factor may only contain of numbers and the signs
\lstinline+.,_/()+
\begin{beispiel}
 \ch{2} \\
 \ch{12}

 % decimals:
 \ch{.5} \\
 \ch{5,75}

 % fractions:
 \ch{3/2} \\
 \ch{1_1/2}
 
 % ``iupac'':
 \ch{(1/2)}
\end{beispiel}

As you can see if you input decimal numbers a missing leading zero is added.

You have to be a little bit careful with the right syntax but I believe it is
rather intuitive.
\begin{beispiel}[code only]
 this won't work but will result in an error: \ch{1/1_1}
\end{beispiel}

If stoichiometric factors are enclosed with parentheses the fractions are not
recognized and missing leading zeros are not added.  What's inside the
parentheses is typeset as is.
\begin{beispiel}
 \ch{(1/2) H2O} \ch{1/2 H2O} \ch{0.5 H2O}
\end{beispiel}
You can find many examples like the following for stoichiometric factors in
parentheses in the \acs{iupac} Green Book~\cite{iupac:greenbook}:
\begin{reaction*}
 (1/5) K "\ox*{7,Mn}" O4 + (8/5) HCl == (1/5) "\ox*{2,Mn}" Cl2 + (1/2) Cl2 + (1/5) KCl + (4/5) H2O
\end{reaction*}

There are a few possibilities to customize the output.
\begin{beschreibung}
 \Option{decimal-marker}{<marker>}\Default{.}
   The symbol to indicate the decimal.
 \Option{frac-style}{math|xfrac|nicefrac}\Default{math}
   Determines how fractions are displayed.
 \Option{frac-math-cmd}{<command sequence>}\Default{\cmd*{frac}}
   \sinceversion{4.1}Allows you to choose which command is used with
   \key{frac-style}{math}.  This needs to be a command sequence that takes two
   arguments that are set in math mode.
 \Option{stoich-space}{<skip>}\Default{.1667em plus .0333em minus .0117em}
   The space that is placed after the stoichiometric factor.  A rubber
   length.
 \Option{stoich-paren-parse}{\default{true}|false}\Default{false}
   If set to true stoichiometric factors enclosed by parentheses also are
   parsed.
\end{beschreibung}
\begin{beispiel}
 \ch[decimal-marker={,}]{3.5} \ch[decimal-marker={$\cdot$}]{3,5} 
\end{beispiel}

The option \key{frac-style}{xfrac} uses the \lstinline+\sfrac+ command of the
\paket{xfrac} package.  The output strongly depends on the font you
use.\secidx[xfrac]{Stoichiometric Factors}
\begin{beispiel}
 \ch[frac-style=xfrac]{3/2} \ch[frac-style=xfrac]{1_1/2}
\end{beispiel}
\chemformula\ defines the instance \lstinline=formula-text-frac= which you can
redefine to your needs.  See the \paket{xfrac} documentation for further
information.  The default definition is this:
\begin{beispiel}[code only]
 \DeclareInstance{xfrac}{chemformula-text-frac}{text}
  {
    slash-left-kern  = -.15em ,
    slash-right-kern = -.15em
  }
\end{beispiel}
This document uses the font Linux Libertine~O and the following definition:
\begin{beispiel}[code only]
 \DeclareInstance{xfrac}{chemformula-text-frac}{text}
  {
    scale-factor        = 1 ,
    denominator-bot-sep = -.2ex ,
    denominator-format  = \scriptsize #1 ,
    numerator-top-sep   = -.2ex ,
    numerator-format    = \scriptsize #1 ,
    slash-right-kern    = .05em ,
    slash-left-kern     = .05em
  }
\end{beispiel}

The option \key{frac-style}{nicefrac} uses the \lstinline+\nicefrac+ command
of the \paket{nicefrac} package.\secidx[nicefrac]{Stoichiometric Factors}
\begin{beispiel}
 \ch[frac-style=nicefrac]{3/2} \ch[frac-style=nicefrac]{1_1/2}
\end{beispiel}

The option \key{stoich-space} allows you to customize the space between
stoichiometric factor and the group following after
it.\secidx[space]{Stoichiometric Factors}
\begin{beispiel}
 \ch{2 H2O} \\
 \ch[stoich-space=.3em]{2 H2O}
\end{beispiel}
\secidx*{Stoichiometric Factors}

\section{Compounds}\label{ssec:compounds}\secidx{Compounds}
\chemformula\ determines compounds as the type that ``doesn't fit in anywhere
else.''  This point will become more clear when you know what the other types
are.
\begin{beispiel}
 \ch{H2SO4} \\
 \ch{[Cu(NH3)4]^2+}
\end{beispiel}

\subsection{Adducts}\secidx[adducts]{Compounds}
\chemformula\ has two identifiers which will create adducts.
\begin{beschreibung}
 \Befehl{ch}{A.B}\newline \ch{A.B}
 \Befehl{ch}{A*B}\newline \ch{A*B}
\end{beschreibung}
\begin{beispiel}
 \ch{CaSO4.H2O} \\
 \ch{CaSO4*H2O}
\end{beispiel}
Since numbers in a compound always are treated as subscripts (see
section~\ref{ssec:subscripts}) you sometimes need to introduce stoichiometric
factors for the right output:
\begin{beispiel}
 \ch{Na3PO4*12H2O} \\
 \ch{Na3PO4* 12 H2O} \\
 \ch{Na3PO4 * 12 H2O}
\end{beispiel}

\subsection{Subscripts}\label{ssec:subscripts}\secidx[subscripts]{Compounds}
\emph{All} numbers in a compound are treated as subscripts.
\begin{beispiel}
 \ch{H2SO4}
\end{beispiel}
If you want a letter to be a subscript you can use the math syntax:
\begin{beispiel}
 \ch{A_nB_m}
\end{beispiel}
The subscript recognizes groups. You can also use math inside it.
\begin{beispiel}
 \ch{A_{$n$}B_{$m$}} \\
 \ch{NaCl_{(aq)}}
\end{beispiel}

\subsection{Commands}\secidx[commands]{Compounds}
Commands are allowed in a compound:
\begin{beispiel}
 \ch{\textbf{A2}B3} \ch{A2\color{red}B3}
\end{beispiel}

However, if the commands demand numbers as argument, \eg, space commands or
\chemmacros' \lstinline+\ox+ command the direct use will fail.  This is
because the numbers are treated as subscripts \emph{before} the command
expands.
\begin{beispiel}[code only]
  \ch{A\hspace{2mm}B} will raise an error because \hspace sees something like
  this: \hspace{$_2$mm}. Actually not at all like it but equally bad\ldots
\end{beispiel}
See section~\ref{ssec:text} for a way around this.

\subsection{Charges and Other Superscripts}\secidx[charges]{Compounds}\secidx[superscripts]{Compounds}
\paragraph{Basics}
If a compound \emph{ends} with a plus or minus sign it will be treated as
charge sign and typeset as superscript.  In other places a plus is treated as
a triple bond and a dash will be used as a single bond, see
section~\ref{ssec:bonds}.
\begin{beispiel}
 \ch{A+B} \ch{AB+} \\
 \ch{A-B} \ch{AB-}
\end{beispiel}

For longer charge groups or other superscripts you can use the math syntax.
It recognizes groups and you can use math inside them.  Inside these groups
neither \code{+} nor \code{-} are treated as bonds.  If a dot \code{.} is
inside a superscript it is treated as indicator for a radical.  A \code{*}
gives the excited state.
\begin{beispiel}
 \ch{A^{x-}} \\
 \ch{A^x-} \\
 \ch{A^{x}-} \\
 \ch{A^{$x-$}} \\
 \ch{RNO2^{-.}} \\
 \ch{^31H} \\
 \ch{^{14}6C} \\
 \ch{^{58}_{26}Fe} \\
 \ch{NO^*}
\end{beispiel}

Ions and ion composites with more than one charge can be typeset quite as
easy:
\begin{beispiel}
 \ch{SO4^2-} \ch{Ca^2+ SO4^2-}
\end{beispiel}

\paragraph{Charge Commands}\secidx[superscripts!charge commands]{Compounds}
You don't need to use \cmd{mch} and related commands inside \cmd{ch}.  Indeed,
you \emph{shouldn't} use them as they might mess with the subscript and
superscript alignment.  The \chemmacros\ option \code{circled} is obeyed by
\cmd{ch}.
\begin{beispiel}
 \chemsetup[option]{circled=all}
 \ch{H+ + OH- <=> H2O}
\end{beispiel}

\paragraph{Behaviour}\secidx[superscripts!behaviour]{Compounds}
The supercripts behave differently depending on their position in a compound,
if there are super- and subscripts following each other directly.
\begin{beispiel}
 \ch{^33B} \ch{{}^33B} \ch{3^3B} \ch{B^3} \ch{B3^3} \\
 \ch{^{23}_{123}B} \ch{{}^{23}_{123}B}  \ch{_{123}^{23}B} \ch{B^{23}} \ch{B_{123}^{23}} \\
 \ch{^{123}_{23}B} \ch{{}^{123}_{23}B}  \ch{_{23}^{123}B} \ch{B^{123}} \ch{B23^{123}}
\end{beispiel}
\begin{itemize}
\item If a compound \emph{starts} with a sub- or superscript both sub- and
  superscript are aligned to the \emph{right} else to the \emph{left}.
\item If a compound \emph{does not start} with a sub- or superscript and there
  is both a sub- and a superscript, the superscript is shifted additionally by
  a length determined from the option \key{charge-hshift}{<dim>}, also see
  page~\pageref{desc:charge-hshift}f.
\end{itemize}
The second point follows \ac{iupac}'s recommendations:
\begin{zitat}[{\acs{iupac} Green Book {\cite[][p.\,51]{iupac:greenbook}}}]
  In writing the formula for a complex ion, spacing for charge number can be
  added (staggered arrangement), as well as parentheses:
  \ch[charge-hshift=full]{SO4^2-}, \ch{(SO4)^2-} The staggered arrangement is
  now recommended.
\end{zitat}

\subsection{Bonds}\label{ssec:bonds}\secidx[bonds]{Compounds}
\subsubsection{Native Bonds}
There are three kinds of what I will call ``native bonds'':
\begin{beispiel}
 single: \ch{CH3-CH3} \\
 double: \ch{CH2=CH2} \\
 triple: \ch{CH+CH}
\end{beispiel}

\subsubsection{Flexible Bonds}
\paragraph{Predefined Bonds}
In addition to the three native bonds there are a few more which can be called
by
\begin{beschreibung}
 \Befehl{bond}{<bond name>}
\end{beschreibung}
The predefined bond types are shown in table~\ref{tab:bond_types}.

\begin{table}
 \centering
 \caption{Bonds available with \protect\cmd{bond}.}
 \label{tab:bond_types}
 \begin{tabular}{lcl}
  \toprule
   \bfseries name & \bfseries appearance & \bfseries aliases \\
  \midrule
   \code{single}  & \bond{single}        & \code{normal}, \code{sb} \\
   \code{double}  & \bond{double}        & \code{db} \\
   \code{triple}  & \bond{triple}        & \code{tp} \\
   \code{dotted}  & \bond{dotted}        & \code{semisingle} \\
   \code{deloc}   & \bond{deloc}         & \code{semidouble} \\
   \code{tdeloc}  & \bond{tdeloc}        & \code{semitriple} \\
   \code{co>}     & \bond{co>}           & \code{coordright} \\
   \code{<co}     & \bond{<co}           & \code{coordleft} \\
  \bottomrule
 \end{tabular}
\end{table}

\begin{beispiel}
 \ch{C\bond{sb}C\bond{db}C\bond{tp}C\bond{deloc}C\bond{tdeloc}C\bond{co>}C\bond{<co}C}
\end{beispiel}

\paragraph{Own Bonds}
\chemformula\ offers commands to define own bond types:
\begin{beschreibung}
 \Befehl{DeclareChemBond}{<name>}\ma{<code>}
 \Befehl{RenewChemBond}{<name>}\ma{<code>}
 \Befehl{DeclareChemBondAlias}{<new name>}\ma{<old name>}
 \Befehl{ShowChemBond}{<name>}
\end{beschreibung}
The usage is best described with an example.  So let's see how the
\code{single} bond and the \code{co>} bond are defined:
\begin{beispiel}[code only]
 \DeclareChemBond{single}
   { \draw[chembond] (chemformula-bond-start) -- (chemformula-bond-end) ; }
 \DeclareChemBond{coordright}
   { \draw[chembond,butt cap->] (chemformula-bond-start) -- (chemformula-bond-end) ; }
 \DeclareChemBondAlias{co>}{coordright}
\end{beispiel}
Two points are important: the names of the starting and the ending
coordinates, \code{chemformula-bond-start} and \code{chemformula-bond-end},
and the \TikZ style of the bonds \code{chembond}.

So, let's say you want to define a special kind of dashed bond.  You could do
this:
\begin{beispiel}
 \usetikzlibrary{decorations.pathreplacing}
 \makeatletter
 \DeclareChemBond{dashed}
   {
     \draw[
       chembond,
       decorate,
       decoration={ticks,segment length=\chemformula@bondlength/10,amplitude=1.5pt}]
       (chemformula-bond-start) -- (chemformula-bond-end) ;
   }
 \makeatother
 \chemsetup[chemformula]{bond-length=2ex}
 \ch{C\bond{dashed}C}
\end{beispiel}

The last example showed you another macro:
\makeatletter\cmd{chemformula@bondlength}\makeatother.  It only exists so you
can use it to access the bond length as set with \key{bond-length}
directly.

\subsection{Customization}\secidx[customization]{Compounds}\label{ssec:compounds:customization}
These options allow you to customize the ouptut of the compounds:
\begin{beschreibung}
 \Option{subscript-vshift}{<dim>}\Default{0pt}
   Extra vertical shift of the subscripts.
 \Option{subscript-style}{text|math}\Default{text}
   Style that is used to typeset the subscripts.
 \Option{charge-hshift}{<dim>}\Default{.25em}
   Shift of superscripts when following a subscript.\label{desc:charge-hshift}
 \Option{charge-vshift}{<dim>}\Default{0pt}
   Extra vertical shift of the superscripts.
 \Option{charge-style}{text|math}\Default{text}
   Style that is used to typeset the superscripts.
 \Option{adduct-space}{<dim>}\Default{.1333em}
   Space to the left and the right of the adduct point.
 \Option{bond-length}{<dim>}\Default{.5833em}
   The length of the bonds.
 \Option{bond-offset}{<dim>}\Default{.07em}
   Space between bond and atoms.
 \Option{bond-style}{<tikz>}\Default{}
   \TikZ\ options for the bonds.
 \Option{bond-penalty}{<num>}\Default{10000}
   \sinceversion{4.0a}The penalty that is inserted after a bond for
   (dis-)allowing line breaks.
 \Option{radical-style}{<tikz>}\Default{}
   \TikZ\ options for the radical point.
 \Option{radical-radius}{<dim>}\Default{.2ex}
   The radius of the radical point.
 \Option{radical-hshift}{<dim>}\Default{.15em}
   Horizontal shift before the radical point is drawn.
 \Option{radical-vshift}{<dim>}\Default{.5ex}
   Vertical shift relative to the current baseline.
 \Option{radical-space}{<dim>}\Default{.15em}
   Horizontal shift after the radical point is drawn.
\end{beschreibung}

Maybe you have noticed that charges of certain ions are shifted to the
right.\secidx[charges!shift]{Compounds}
\begin{beispiel}
 \ch{SO4^2-} \ch{NH4+} \ch{Na+}
\end{beispiel}
They are shifted if they \emph{follow} a subscript which follows \ac{iupac}
recommendations~\cite[][p.\,51]{iupac:greenbook}.  The amount of the shift can
be set with the option \key{charge-hshift}.
\begin{beispiel}
 \ch{SO4^2-} \ch{NH4+} \ch{Na+} \\
 \chemsetup[chemformula]{charge-hshift=.5ex}
 \ch{SO4^2-} \ch{NH4+} \ch{Na+} \\
 \chemsetup[chemformula]{charge-hshift=.5pt}
 \ch{SO4^2-} \ch{NH4+} \ch{Na+}
\end{beispiel}

Despite \ac{iupac}'s recommendation \chemformula\ does not make fully staggered
arrangements in the default setting as I find it hard to read in some cases
and ugly in others.  Since this is a subjective decision \chemformula\ not only
let's you define the absolute amount of the shift but also provides a
possibility for full staggered arrangements.  For this you have to use
\key{charge-hshift}{full}.
\begin{beispiel}
 \ch[charge-hshift=0pt]{C5H11+} \ch[charge-hshift=0pt]{SO4^2-} \\
 \ch{C5H11+} \ch{SO4^2-} \\
 \ch[charge-hshift=1ex]{C5H11+} \ch[charge-hshift=1ex]{SO4^2-} \\
 \ch[charge-hshift=full]{C5H11+} \ch[charge-hshift=full]{SO4^2-}
\end{beispiel}

If you don't want the charges to be typeset in text mode you can switch to
math mode:
\begin{beispiel}
 \ch{M^x+} \ch{SO4^2-} \\
 \chemsetup[chemformula]{charge-style = math}
 \ch{M^x+} \ch{SO4^2-}
\end{beispiel}

The option \key{subscript-vshift} can be used to adjust the vertical shift of
the subscripts:\secidx[subscripts!shift]{Compounds}
\begin{beispiel}
 \ch{H2SO4} \ch{Na3PO4} \\
 \chemsetup[chemformula]{subscript-vshift=.5ex}
 \ch{H2SO4} \ch{Na3PO4} \\
 \chemsetup[chemformula]{subscript-vshift=-.2ex}
 \ch{H2SO4} \ch{Na3PO4}
\end{beispiel}

You can choose the mode subscripts are typeset in the same way as it is
possible for the charges:
\begin{beispiel}
 \ch{A_nB_m} \ch{H2SO4} \\
 \chemsetup[chemformula]{subscript-style = math}
 \ch{A_nB_m} \ch{H2SO4}
\end{beispiel}

The option \key{adduct-space} sets the space left and right to the adduct
symbol $\cdot$.
\begin{beispiel}
 \ch{Na3PO3*H2O} \\
 \chemsetup[chemformula]{adduct-space=.2em}
 \ch{Na3PO3*H2O}
\end{beispiel}

Changing the length of the bonds:\secidx[bonds!length]{Compounds}
\begin{beispiel}
 \chemsetup[chemformula]{bond-length=4mm}%
 single: \ch{CH3-CH3} \\
 double: \ch{CH2=CH2} \\
 triple: \ch{CH+CH}
\end{beispiel}

You can change the distance between bond and atom, too:
\begin{beispiel}
 \ch{H-H + N+N + O=O} \\
 \ch[bond-offset=1pt]{H-H + N+N + O=O}
\end{beispiel}

\subsection{Standalone Formulae}\secidx[standalone]{Compounds}
\noindent\sinceversion{4.0}\chemformula\ offers a command that \emph{only
  accepts} the \enquote{compound} input type:
\begin{beschreibung}
  \Befehl{chcpd}[<options>]{<compound>}
\end{beschreibung}
\secidx*{Compounds}

\section{Special Input Types}\secidx{Special Types}
There are some \enquote{special type} input groups.

\subsection{Single Token Groups}\secidx[single token groups]{Special Types}
The first kind are groups which consist of only one token, namely of the
following ones:
\begin{beschreibung}
 \Befehl{ch}{ + } \ch{ + }\newline
   Creates the plus sign between compounds with space around it:\\
   \cmd{ch}{2 Na + Cl2} \ch{2 Na + Cl2}
 \Befehl{ch}{ v } \ch{ v }\newline
   Sign for precipitate: \cmd{ch}{BaSO4 v} \ch{BaSO4 v}
 \Befehl{ch}{ \lstinline+^+ } \ch{ ^ }\newline
   Sign for escaping gas\footnotemark: \cmd{ch}{\lstinline=H2 ^=}
   \ch{H2 ^}
\end{beschreibung}
\footnotetext{Is this the correct English term? Please correct me if it isn't.}

The space left and right of the plus sign can be set with this option:
\begin{beschreibung}
 \Option{plus-space}{<skip>}\Default{.3em plus .1em minus .1em}
   A rubber length.
 \Option{plus-penalty}{<num>}\Default{700}
   \sinceversion{4.0a}The penalty that is inserted after the plus sign for
   (dis-)allowing line breaks.
\end{beschreibung}
\begin{beispiel}
 \ch{A + B}\\
 \ch[plus-space=4pt]{A + B}
\end{beispiel}

\subsection{Option Input}\secidx[option input]{Special Types}
Sometimes you might want to apply an option only to a part of a, say,
reaction.  Of course you have the possibility to use \cmd{ch} several times.
\begin{beispiel}
 \ch{H2O +}\textcolor{red}{\ch{H2SO4}}\ch{-> H3O+ + HSO4-} \\
 \ch{H2O +}\ch[subscript-vshift=2pt]{H2SO4}\ch{-> H3O+ + HSO4-}
\end{beispiel}
This, however, interrupts the input in your source and \emph{may} mess with
the spacing. That's why there is an alternative:
\begin{beschreibung}\makeatletter
 \Befehl{ch}{ @\{<options>\} }\newline
   The options specified this way will be valid \emph{only} until the next
   compound is set.
\end{beschreibung}
\begin{beispiel}
 \ch{H2O +}\textcolor{red}{\ch{H2SO4}}\ch{-> H3O+ + HSO4-} \\
 \ch{H2O + @{format=\color{red}} H2SO4 -> H3O+ + HSO4-} \\
 or of course:\\
 \ch{H2O + \textcolor{red}{H2SO4} -> H3O+ + HSO4-}\\[1em]
 \ch{H2O +}\ch[subscript-vshift=2pt]{H2SO4}\ch{-> H3O+ + HSO4-} \\
 \ch{H2O + @{subscript-vshift=2pt} H2SO4 -> H3O+ + HSO4-}
\end{beispiel}
\secidx*{Special Types}

\section{Escaped Input}\secidx{Escaped Input}
In some cases it may be desirable to prevent \chemformula\ from parsing the
input.  This can be done in two ways.

\subsection{Text}\label{ssec:text}\secidx[text]{Escaped Input}
If you put something between \lstinline+" "+ or \lstinline+' '+ then the input
will be treated as normal text, except that spaces are not allowed and have to
be input with \lstinline+~+.
\begin{beschreibung}
 \Befehl{ch}{ "<escaped text>" }
 \Befehl{ch}{ '<escaped text>' }
\end{beschreibung}
\begin{beispiel}
 \ch{"\ox{2,Ca}" O} \\
 \ch{"\ldots\," Na + "\ldots\," Cl2 -> "\ldots\," NaCl} \\
 \ch{'A~->~B'}
\end{beispiel}
In many cases you won't need to escape the input.  But when you get into
trouble when using a command inside \cmd{ch} try hiding it.

\subsection{Math}\secidx[math]{Escaped Input}
If you especially want to input math you just enclose it with \lstinline+$ $+.
This output is different from the escaped text as it is followed by a space.
The reasoning behind this is that I assume math will mostly be used to replace
stoichiometric factors.
\begin{beschreibung}
 \Befehl{ch}{ \string$<escaped math>\string$ }
 \Befehl{ch}{ \string\(<escaped math>\string\) }
\end{beschreibung}
\begin{beispiel}
 escaped text: \ch{"$x$" H2O} \\
 escaped math: \ch{$x$ H2O} \\
 also escaped math: \ch{\(x\) H2O} \\
 \ch{$2n$ Na + $n$ Cl2 -> $2n$ NaCl}
\end{beispiel}

The space that is inserted after a math group can be edited:
\begin{beschreibung}
 \Option{math-space}{<skip>}\Default{.1667em plus .0333em minus .0117em}
   A rubber length.
\end{beschreibung}
\begin{beispiel}
 \ch{$2n$ Na + $n$ Cl2 -> $2n$ NaCl} \\
 \chemsetup[chemformula]{math-space=.25em}
 \ch{$2n$ Na + $n$ Cl2 -> $2n$ NaCl} \\
 \ch{$A->B$}
\end{beispiel}
\secidx*{Escaped Input}

\section{Arrows}\label{sec:arrows}\secidx{Arrows}
\subsection{Arrow types}\secidx[types]{Arrows}
Arrows are input in the same intuitive way they are with \paket{mhchem}.
There are various different types:
\begin{beschreibung}
 \Befehl{ch}{ -> } \ch{->}\newline
   standard right arrow
 \Befehl{ch}{ <- } \ch{<-}\newline
   standard left arrow
 \Befehl{ch}{ -/> } \ch{-/>}\newline
   does not react (right)
 \Befehl{ch}{ </- } \ch{</-}\newline
   does not react (left)
 \Befehl{ch}{ <-> } \ch{<->}\newline
   resonance arrow
 \Befehl{ch}{ <> } \ch{<>}\newline
   reaction in both directions
 \Befehl{ch}{ == } \ch{==}\newline
   stoichiometric equation
 \Befehl{ch}{ <=> } \ch{<=>}\newline
   equilibrium arrow
 \Befehl{ch}{ \lstinline+<=>>+ } \ch{<=>>}\newline
   unbalanced equilibrium arrow to the right
 \Befehl{ch}{ \lstinline+<<=>+ } \ch{<<=>}\newline
   unbalanced equilibrium arrow to the left
 \Befehl{ch}{ <o> } \ch{<o>}\newline
   isolobal arrow
\end{beschreibung}
All these arrows are drawn with \TikZ.
\begin{beispiel}
 \ch{H2 + Cl2 -> 2 HCl} \\
 \ch{H2O + CO3^2- <=> OH- + HCO3-} \\
 \ch{A <- B} \\
 \ch{\{[CH2=CH-CH2]- <-> [CH2-CH=CH2]- \}} \\
 \ch{A <> B} \\
 \ch{H+ + OH- <=>> H2O} \\
 \ch{2 NO2 <<=> N2O4}
\end{beispiel}

% TODO: add description for \charrow

\subsection{Labels}\secidx[labels]{Arrows}
The arrows take two optional arguments to label them.
\begin{beschreibung}
 \Befehl{ch}{ ->[<above>][<below>] }
\end{beschreibung}
\begin{beispiel}
 \ch{A ->[a] B} \\
 \ch{A ->[a][b] B} \\
 \ch{A ->[\SI{100}{\celsius}] B}
\end{beispiel}
The label text can be parsed seperately from the arrow. The recipe is easy:
leave blanks.
\begin{beispiel}
 \ch{A ->[H2O] B} \\
 \ch{A ->[ H2O ] B} \\
 \ch{A ->[ "\ox{2,Ca}" F2 ] B} \\
 \ch{A ->[$\Delta$,~ \[H+ \]] B}
\end{beispiel}
% TODO: correct info about []

If you leave the blanks \chemformula\ treats the groups inside the square
brackets as seperated input types.  The arrow reads its arguments
\emph{afterwards}.  As you can see the arrows \enquote{grow} with the length
of the labels.  What stays constant is the part that protrudes the labels.  As
you also can see in the last example square brackets inside the arrow
arguments should be produced using \cmd{[} and \cmd{]}.  They keep their usual
meaning outside \cmd{ch}.  These commands are necessary since the usual
grouping (\ie hiding the brackets inside curly brackets) doesn't work due to
the way \cmd{ch} reads its argument.
\begin{beispiel}
 \ch{A ->[a] B} \\
 \ch{A ->[ab] B} \\
 \ch{A ->[abc] B} \\
 \ch{A ->[abc~abc] B} \\
 % needs the `chemfig' package:
 \setatomsep{15pt}
 \ch{A ->[ "\chemfig{-[:30]-[:-30]OH}" ] B} \\
 \ch{A ->[[]] B} vs. \ch{A ->[\[\]] B}
\end{beispiel}

\subsection{Customization}\secidx[customization]{Arrows}
These are the options which enable you to customize the arrows:
\begin{beschreibung}
 \Option{arrow-offset}{<dim>}\Default{.75em}
   This is the length that an arrow protrudes a label on both sides.  This
   means an empty arrow's length is two times \code{arrow-offset}.
 \Option{arrow-min-length}{<dim>}\Default{0pt}
   \sinceversion{3.6b}The minimal length an error must have unless two times
   \key{arrow-offset}
   plus the width of the label is larger.
 \Option{arrow-yshift}{<dim>}\Default{0pt}
   Shifts an arrow up (positive value) or down (negative value).
 \Option{arrow-ratio}{<factor>}\Default{.6}
   The ratio of the arrow lengths of the unbalanced equilibrium.  \code{.4}
   would mean that the length of the shorter arrow is $0.4\times$ the length
   of the longer arrow.
 \Option{compound-sep}{<dim>}\Default{.5em}
   The space between compounds and the arrows.
 \Option{label-offset}{<dim>}\Default{2pt}
   The space between the labels and the arrows.
 \Option{label-style}{<font command>}\Default{\cmd*{footnotesize}}
   The relative font size of the labels.
 \Option{arrow-penalty}{<num>}\Default{0}
   \sinceversion{4.0a}The penalty that is inserted after an arrow for
   (dis-)allowing line breaks.
\end{beschreibung}

The following code shows the effect of the different options on the \lstinline+<=>>+
arrow:
\begin{beispiel}
 standard: \ch{A <=>>[x][y] B} \\
 longer: \ch[arrow-offset=12pt]{A <=>>[x][y] B} \\
 higher: \ch[arrow-yshift=2pt]{A <=>>[x][y] B} \\
 more balanced: \ch[arrow-ratio=.8]{A <=>>[x][y] B} \\
 labels further away: \ch[label-offset=4pt]{A <=>>[x][y] B} \\
 larger distance to compounds: \ch[compound-sep=2ex]{A <=>>[x][y] B} \\
 smaller labels: \ch[label-style=\tiny]{A <=>[x][y] B}
\end{beispiel}

\subsection{Modify Arrow Types}\label{sec:arrows_modify}\secidx[types!modify]{Arrows}
The arrows are defined with the command
\begin{beschreibung}
 \Befehl{DeclareChemArrow}{<tokens>}\ma{<tikz>}
\end{beschreibung}
\ma{<tokens>} is the sequence of tokens that is replaced with the actual arrow
code.  For example the basic arrow is defined via
\begin{beispiel}[code only]
 \DeclareChemArrow{->}{\draw[-cf] (cf_arrow_start) -- (cf_arrow_end) ;}
\end{beispiel}
In order to define arrows yourself you need to know the basics of
\TikZ\footnote{Please see the \textsf{pgfmanual} for details.}.  There are
some predefined coordinates you can and should use:
\begin{description}
 \item[\code{(cf\_arrow\_start)}] The beginning of the arrow.
 \item[\code{(cf\_arrow\_end)}] The end of the arrow.
 \item[\code{(cf\_arrow\_mid)}] The mid of the arrow.
 \item[\code{(cf\_arrow\_mid\_start)}] The beginning of the shorter arrow in
   types like \lstinline+<=>>+.
 \item[\code{(cf\_arrow\_mid\_end)}] The end of the shorter arrow in types like
   \lstinline+<=>>+.
 \item[\code{cf}, \code{left cf}, \code{right cf}] \chemformula's own arrow
   heads.
\end{description}
\begin{beispiel}
 \DeclareChemArrow{.>}{\draw[-cf,dotted,red] (cf_arrow_start) -- (cf_arrow_end);}
 \DeclareChemArrow{n>}{\draw[-cf] (cf_arrow_start) .. controls ([yshift=3ex]cf_arrow_mid) .. (cf_arrow_end);}
 \ch{A .> B} \ch{A .>[a][b] B} \ch{A n> B}
\end{beispiel}
If you want to redefine an existing arrow there are two commands you can use:
\begin{beschreibung}
 \Befehl{RenewChemArrow}{<tokens>}\ma{<tikz>}
 \Befehl{ShowChemArrow}{<tokens>}
\end{beschreibung}
The second one gives the current definition, the first one redefines an existing
arrow.
\begin{beispiel}
 \texttt{\ShowChemArrow{->}} \\
 \RenewChemArrow{->}{\draw[->,red] (cf_arrow_start) -- (cf_arrow_end) ;}
 \texttt{\ShowChemArrow{->}} \\
 \ch{A -> B}
\end{beispiel}

\subsection{Standalone Arrows}\secidx[standalone]{Arrows}
\noindent\sinceversion{4.0}\chemformula\ offers a command that \emph{only
  accepts} the \enquote{arrow} input type:
\begin{beschreibung}
  \Befehl{charrow}{<arrow>}\oa{<above>}\oa{<below>}
\end{beschreibung}
\secidx*{Arrows}

\section{Names}\secidx{Names}
\subsection{Syntax}\secidx[syntax]{Names}
\chemformula\ has a built-in syntax to write text under a compound.  In a way
it works very similar to the arrows.
\begin{beschreibung}
 \Befehl{ch}{ !(<name>)( <formula> ) }
\end{beschreibung}
If an exclamation mark is followed by a pair of parentheses \chemformula\ will
parse it this way:
\begin{beispiel}
 \ch{!(ethanol)( CH2CH2OH )}
\end{beispiel}
The same what's true for the arrows arguments holds for these arguments: if
you leave blanks the different parts will be treated according to their input
type before the text is set below the formula.
\begin{beispiel}
 \ch{!(water)(H2O)} \quad
 \ch{!( "\textcolor{blue}{water}" )( H2O )} \quad
 \ch{!( $2n-1$ )( H2O )} \quad
 \ch{!( H2O )( H2O )} \quad
 \ch{!(oxonium)( H3O+ )}
\end{beispiel}
If for some reason you want to insert an exclamation mark \emph{without} it
creating a name you only have to make sure it isn't followed by parentheses.
\begin{beispiel}
 \ch{H2O~(!)} \\
 \ch{A!{}()}
\end{beispiel}

% TODO add description of \chname command

\subsection{Customization}\secidx[customization]{Names}
\chemformula\ provides two options to customize the output of the names:
\begin{beschreibung}
 \Option{name-format}{<commands>}\Default{\cmd*{scriptsize}\cmd*{centering}}
   The format of the name.  This can be arbitrary input.
 \Option{name-width}{<dim>|auto}\Default{auto}
   The width of the box where the label is put into.  \code{auto} will detect
   the width of the name and set the box to this width.
\end{beschreibung}
\begin{beispiel}
 \ch{!(acid)( H2SO4 ) -> B} \\
 \ch[name-format=\sffamily\small]{!(acid)( H2SO4 ) -> B} \\
 \ch[name-format=\scriptsize N:~]{!(acid)( H2SO4 ) -> B} \\
 \ch[name-width=3em,name-format=\scriptsize\raggedright]{!(acid)( H2SO4 ) -> B}
\end{beispiel}

\subsection{Standalone Names}\secidx[standalone]{Names}
\noindent\sinceversion{4.0}\chemformula\ offers a command that allows the
usage of the \enquote{name} syntax in normal text.  This is the command that
a bang is replaced with in \chemformula's formulas, actually.  Both arguments
are mandatory.
\begin{beschreibung}
  \Befehl{chname}\da{<under>}\da{<text>}
\end{beschreibung}
\secidx*{Names}

\section{Format and Font}\secidx{Format and Font}\label{sec:format}
In the standard setting \chemformula\ doesn't make any default changes to the
font of the formula output.  Let's take a look at a nonsense input which shows
all features:
\begin{beispiel}
 \newcommand*\sample{%
   \ch{H2C-C+C-CH=CH+ + CrO4^2-
       <=>[x][y]
     2.5 Cl^{-.} + 3_1/2 Na*OH_{(aq)} + !(name)( A^n ) "\LaTeXe"}
 }
 \sample
\end{beispiel}
\newcommand*\sample{%
  \ch{H2C-C+C-CH=CH+ + CrO4^2-
      <=>[x][y]
    2.5 Cl^{-.} + 3_1/2 Na*OH_{(aq)} + !(name)( A^n ) "\LaTeXe"}
}

Now we're going to change different aspects of the font a look what happens:
\begin{beispiel}[below]
 \sffamily Hallo \sample \\
 \ttfamily Hallo \sample \normalfont \\
 \bfseries Hallo \sample \normalfont \\
 \itshape Hallo \sample
\end{beispiel}
As you can see most features adapt to the surrounding font.

If you want to change the default format you need to use this option:
\begin{beschreibung}
 \Option{format}{<anything>}\Default{}
   Adds arbitrary code before the output of \cmd{ch}.
\end{beschreibung}
\begin{beispiel}
 \definecolor{newblue}{rgb}{.1,.1,.5}
 \chemsetup[chemformula]{format=\color{newblue}\sffamily}
 \sffamily Hallo \sample \\
 \ttfamily Hallo \sample \normalfont \\
 \bfseries Hallo \sample \normalfont \\
 \itshape Hallo \sample
\end{beispiel}

You can also specifically change the fontfamily, fontseries and fontshape of
the output.
\begin{beschreibung}
 \Option{font-family}{<family>}\Default{}
   Changes the fontfamily of the output with
   \lstinline+\fontfamily{<family>}\selectfont+.
 \Option{font-series}{<series>}\Default{}
   Changes the fontseries of the output with
   \lstinline+\fontseries{<series>}\selectfont+.
 \Option{font-shape}{<shape>}\Default{}
   Changes the fontshape of the output with
   \lstinline+\fontshape{<shape>}\selectfont+.
\end{beschreibung}
\begin{beispiel}
 \chemsetup[chemformula]{font-series=bx}
 Hallo \sample \\
 \sffamily Hallo \sample \normalfont \\
 \chemsetup[chemformula]{font-family=lmr,font-series=m} Hallo \sample \normalfont \\
 \itshape Hallo \sample
\end{beispiel}

If you're using \XeLaTeX\ or \LuaLaTeX\ and have loaded \paket{fontspec} you
have the possibilty to set the font with it:
\begin{beschreibung}
 \Option{font-spec}{\{<font>\}}\Default{}
   or with options
 \Option{font-spec}{\{[<options>]<font>\}}
\end{beschreibung}
\begin{beispiel}
 \chemsetup[chemformula]{font-spec={Linux Biolinum O}} \sample \\
 \chemsetup[chemformula]{font-spec={Augie}} \sample \\
 \chemsetup[chemformula]{font-spec={Latin Modern Sans}} \sample \\
 \bfseries \sample \normalfont
\end{beispiel}
\secidx*{Format and Font}

\section{Usage In Math Equations}\secidx{Math Environments}
The \cmd{ch} command can be used inside math equations.  It recognizes
\lstinline+\\+ and \lstinline+&+ and passes them on.  However, you can't use
the optional arguments of \lstinline+\\+ inside \cmd{ch}.
\begin{beispiel}
 \begin{align}
  \ch{
    H2O & ->[a] H2SO4 \\
    Cl2 & ->[x][y] CH4
  }
 \end{align}
 \begin{align*}
 \ch{
   RNO2      &<=>[ + e- ] RNO2^{-.} \\
   RNO2^{-.} &<=>[ + e- ] RNO2^2-
 }
 \end{align*}
\end{beispiel}
\secidx*{Math Environments}

\section{Usage with \TikZ{} or \paket*{pgfplots} and externalization}
\sinceversion{4.1}
\noindent
Since \chemformula{} uses \TikZ{} to draw reaction arrows and bonds they would
be externalized, too, if you use that facility with \TikZ{} or
\paket{pgfplots}\footnote{\CTANurl{pgfplots}}.  This may not be desirable
since they are very small pictures maybe containing of a single line.  This is
why \chemformula's default behaviour is to disable externalization for it's
bonds and arrows.  This can be turned on and off through the following option:
\begin{beschreibung}
  \Option{tikz-external-disable}{\default{true}|false}\Default{true}
    dis- or enable \TikZ' externalization mechanism for \chemformula's arrows
    and bonds.
\end{beschreibung}

If you should be using a formula that contains bonds or arrows inside of a
\code{tikzpicture} that is externalized you should locally enable it for
\chemformula, too:

\begin{beispiel}[code only]
  \begin{tikzpicture}
    \setchemformula{tikz-external-disable=false}
    \begin{axis}[xlabel={\ch{2 H+ + 2 e- -> H2}}]
      \addplot ... ;
    \end{axis}
  \end{tikzpicture}
\end{beispiel}

\section{Further Examples}
This section presents some examples of a possible usage.

\begin{beispiel}
 \begin{reaction}[Synthese von Alkanen]
  !(Synthesegas)( $n$ CO + $(2n+1)$ H2 )
  ->[\SI{200}{\celsius}][\[CoNi\]]
  C_{$n$}H_{$2n+2$} + $n$ H2O
 \end{reaction}
\end{beispiel}

\begin{beispiel}
 \begin{reactions*}
  "a)" && CH4    + Cl2 &-> CH3Cl  + HCl && "{\small Chlormethan/Methylchlorid}" \\
  "b)" && CH3Cl  + Cl2 &-> CH2Cl2 + HCl && "{\small Dichlormethan/Methylenchlorid}" \\
  "c)" && CH2Cl2 + Cl2 &-> CHCl3  + HCl && "{\small Trichlormethan/Chloroform}" \\
  "d)" && CHCl3  + Cl2 &-> CCl4   + HCl && "{\small Tetrachlormethan/Tetrachlorkohlenstoff}"
 \end{reactions*}
\end{beispiel}

\begin{beispiel}
 \chemsetup[ox]{parse=false}
 \ch{"\ox{\delm,C}" -{} "\ox{\delp,M}" \qquad ( <-> "\ox{\delp,C}" -{} "\ox{\delm,Br}" )} \\
 \ch[adduct-space=0pt]{X. + .Y <=> X-Y + Bindungsenergie} \\
 \ch[name-format=\normalsize]{
   !(\State{H}{f}\qquad)() !(\textcolor{red}{??})( CH4\gas{} ) +
   !(\num{0})( 2 O2\gas{} )
   ->
   !(\num{-94.3})( CO2\gas{} ) +
   !(\num{-57.9})( H2O\lqd{} ) +
   !(\num{-192.1})( "\State{H}" )
 }
\end{beispiel}

\begin{beispiel}
 \newcommand\cstack[1]{%
   \clap{%
     \begin{tabular}{c}
       #1
     \end{tabular}
   }%
 }
 \begin{reactions*}
  CH3MgBr + "\ox*{1,Cu}" X &-> "\glqq" CH3 "\ox*{1,Cu}\grqq" + MgBrX "\qquad X~$=$~Br,I,CN" \\
  2 MeLi + CuI             &-> !(\cstack{Dimethylcuprat\\(Gilmann-Cuprat)})( Me2CuLi ) + Li
 \end{reactions*}
\end{beispiel}

\begin{beispiel}
 % needs `chemfig'
 \begin{reactions*}
  H3C-CH3 + Cl2                           &->[$\Delta$][$h\nu$] H3CCH2Cl + HCl       & &"\Enthalpy{-27.1}" \\
  H3C-CH3 + "\Lewis{0.,Cl}"               &-> H3CCH2 "\Lewis{0.,\vphantom{H}}" + HCl & &"\Enthalpy{-5.0}" \\
  H3C-CH2 "\Lewis{0.,\vphantom{H}}" + Cl2 &-> H3CCH2Cl + "\Lewis{0.,Cl}"             & &"\Enthalpy{-23.0}"
 \end{reactions*}
\end{beispiel}

The following example shows how the cancelling of compounds could be done%
\footnote{Inspired by a question on TeX.SE: \url{http://tex.stackexchange.com/q/30118/5049}}.
\begin{beispiel}
 % needs `cancel'
 \begin{align*}
  \ch{\cancel{HCOOH\aq} + H2O\lqd{}  &<=> H3O^+\aq{} + \cancel{HCOO^-\aq}} \\
  \ch{\cancel{HCOO^-\aq} + H2O\lqd{} &<=> \cancel{HCOOH\aq} + OH^-\aq}\\[-1ex]
  \cline{1-2}
  \ch{H2O\lqd{} + H2O\lqd{} &<=> H3O^+\aq{} + OH^-\aq}    
 \end{align*}
\end{beispiel}

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\end{document}
