Very often chemists need a longer version of reaction arrows (\rightleftharpoons) with the possibility to put text above and below. Analogous to amsmath's \xrightarrow and \xleftarrow this package provides the macro \xrightleftharpoons. It requires amsmath. To use it, \usepackage{chemarr}, then \xrightleftharpoons[below]{above}. The author is Heiko Oberdiek.