People in my lab sometimes want to be able to typeset the equation for a reversible chemical reaction in LaTeX:

I've created a macro that allows you to typset that as follows:

{\rm H}_2{\rm O} \rates{k_{\rm 1}}{k_{\rm 2}} {\rm H}^+ + {\rm OH}^-

Here is the macro definition:

\def\longrightharpoonup{\relbar\joinrel\rightharpoonup}
\def\longleftharpoondown{\leftharpoondown\joinrel\relbar}

\def\longrightleftharpoons{
  \mathop{
    \vcenter{
      \hbox{
      \ooalign{
        \raise1pt\hbox{$\longrightharpoonup\joinrel$}\crcr
	  \lower1pt\hbox{$\longleftharpoondown\joinrel$}
	  }
      }
    }
  }
}

\newcommand{\rates}[2]{\displaystyle
  \mathrel{\longrightleftharpoons^{#1\mathstrut}_{#2}}}

Note that I have to use the \mathstrut command in the defintion since otherwise the subscript on k₁ seems to be a bit too close to the arrow. I also go into displaystyle automatically; this is not useful if you are in a different mode.

Download an example file.