Use the equation feature designed for math but works okay for chemistry. You got some nice equilibrium arrows. This will insert an equation at the position of your cursor and open the editor.
Shortcut for typing Chemical reaction or Chemical.
Quickest way to type chemical reaction chemical equation reaction arrows in Word 2007 above. It is designed for math but works okay for chemistry. While you need specialized input methods to draw chemical structures in Word you can compose the simplest formulas by making use of the tools that are already available. To use a built-in formula select Design Equation.