# How to get text above and below double harpoon (reversible reaction) symbols in an equation

I'm working on the Chemwiki Dynamic Textbook Project at UC Davis, California and I've run into a problem. I need to formulate a symbol containing double harpoons (as in a reversible reaction, e.g. \rightleftharpoons); however, I need to have text both above and below the harpoons. I have an image I can email, but I can't post with an image because I'm new to this.

I can find code for \overleftrightarrow and \underleftrightarrow, but that's not quite what I need. I need both, on the same expression. If anyone has a minute to help me, I would greatly appreciate it!

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If you don't mind using TikZ, a quite similar (if not the same) problem was solved in this question. –  sgbmyr Apr 4 '11 at 23:26
possible duplicate of Extending arrows with overset text –  Caramdir Apr 5 '11 at 2:11

You could use the features provided by the mhchem package. A little example, taken from the package documentation:

\documentclass{article}
\usepackage{mhchem}

\begin{document}

\ce{Zn^2+
<=>[\ce{+ 2OH-}][\ce{+ 2H+}]
$\underset{\text{amphoteres Hydroxid}}{\ce{Zn(OH)2 v}}$
<=>C[+2OH-][{+ 2H+}]
$\underset{\text{Hydroxozikat}}{\cf{[Zn(OH)4]^2-}}$
}

\end{document}


The result:

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\documentclass{article}
\usepackage{mathtools}
\begin{document}

$\mathrm{A} \xrightleftharpoons[k_2]{k_1} \mathrm{B}$

\end{document}

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Welcome, can you please extend your answer to cover the duplicate suggested under the question? –  percusse Jul 2 '14 at 23:49
done. I replaced \rightleftharpoons with \xrightleftharpoons. –  Fred Senese Jul 5 '14 at 17:55
\rm was deprecated around twenty years ago. It's not part of the LaTeX commands. –  Manuel Jul 5 '14 at 19:12
Yes, I know. It still works, though, and it's shorter than \mathrm but point taken. I edited my response. –  Fred Senese Jul 7 '14 at 19:50

An alternative to mhchem would be the chemmacros bundle (including chemformula) by our user @cgnieder.

The syntax is simple and (pretty) intuitive, in many ways similar to the syntax of mhchem.

Here is a minimal example and the output.

\documentclass[border=10pt]{standalone}
\begin{document}
\ch{Zn^2+
<>[ + 2 OH- ][ + 2 H+ ]%Reaction in both directions
!(amphoteres~Hydroxid)( Zn(OH)2 v )
<=>[ + 2 OH- ][ + 2 H+ ]%Equilibrium arrow
!(Hydroxozikat)( [Zn(OH)4]^2- )
}
\end{document}![chemmacros output][1]


There is much more handy stuff in the bundle. Check out the documentation for more or the blog of the author.

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A simple one-line solution which worked for me was:

X \underset{k_2}{\stackrel{k_1}{\rightleftharpoons}} Y


But the harpoon arrows don't extend when the text above and below is longer.

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