\xleftrightharpoons{abc}The command \xleftrightharpoons{abc} is used to create an extensible left-right harpoon arrow with custom annotations above and below the arrow. This is typically used in chemical equations to denote reversible reactions, where the text above and below the arrow can specify conditions or catalysts involved in the reaction.
A reversible chemical reaction with a catalyst specified above the arrow.
A + B \xleftrightharpoons{\text{catalyst}} CA reversible reaction with rate constants k1 and k2 specified below and above the arrow, respectively.
X \xleftrightharpoons[k_1]{k_2} YA reversible reaction with enthalpy change (ΔH) and Gibbs free energy change (ΔG) specified below and above the arrow.
R \xleftrightharpoons[\Delta H]{\Delta G} P