\documentclass[fleqn,12pt,a4paper]{article} % normal
% \documentclass[fleqn,a0,portrait]{a0poster} % poster
\usepackage{amsmath,chemarrow}
\begin{document}

% \tiny
% \footnotesize
% \small
\normalsize
% \large
% \Large
% \LARGE
% \huge
% \Huge

\newcommand{\aqua}{\ensuremath{\text{H}_{\text{2}}\text{O}}}
\newcommand{\acid}{\ensuremath{\text{H}_{\text{3}}\text{O}^{\text{+}}}}
\newcommand{\base}{\ensuremath{\text{OH}^{\text{$-$}}}}



% \section{size check}
%     
% \fbox{{\symbol{65}}}
% \fbox{{\symbol{66}}}
% \fbox{{\symbol{67}}}
% \fbox{{\symbol{68}}}
% \fbox{{\symbol{69}}}
% \fbox{{\symbol{70}}}
% \fbox{{\symbol{71}}}
% 
% {\Huge
% \fbox{{\arro\symbol{65}}}
% \fbox{{\arro\symbol{66}}}
% \fbox{{\arro\symbol{67}}}
% \fbox{{\arro\symbol{68}}}
% \fbox{{\arro\symbol{69}}}
% \fbox{{\arro\symbol{70}}}
% \fbox{{\arro\symbol{71}}}
% }


\section{chemarrow}


\begin{displaymath}
\aqua + \aqua\ \chemarrow\ \acid + \base
\end{displaymath}


\section{rarrowfill 2.5em}


\begin{displaymath}
\aqua + \aqua \rarrowfill{2.5em} \acid + \base
\end{displaymath}

\section{larrowfill 2.5em}


\begin{displaymath}
\aqua + \aqua \larrowfill{2.5em} \acid + \base
\end{displaymath}


\section{rightleftharpoonsfill 2.5em}


\begin{displaymath}
\aqua + \aqua \rightleftharpoonsfill{2.5em} \acid + \base
\end{displaymath}


\section{leftrightharpoonsfill 2.5em}


\begin{displaymath}
\aqua + \aqua \leftrightharpoonsfill{2.5em} \acid + \base
\end{displaymath}



\section{autorightleftharpoons}


\begin{displaymath}
\aqua + \aqua \autorightleftharpoons{$k_a$}{$k_b$} \acid + \base
\end{displaymath}


\section{autoleftrightharpoons}


\begin{displaymath}
\aqua + \aqua \autoleftrightharpoons{$k_a$}{$k_b$} \acid + \base
\end{displaymath}


\section{autorightarrow}


\begin{displaymath}
\aqua + \aqua \autorightarrow{$k_a$}{$k_b$} \acid + \base
\end{displaymath}


\section{autoleftarrow}


\begin{displaymath}
\aqua + \aqua \autoleftarrow{$k_a$}{$k_b$} \acid + \base
\end{displaymath}




\end{document}
