How do I get better double arrows for balanced chemical reactions?
There are many double arrows in the Arrows subset of the DejaVu family of fonts, for example: ⇄, ⇆, ⇋, and ⇌. (You can highlight these directly from this FAQ then copy and paste them into the Math markup area.)
See How do I use a symbol not provided in Math's Catalog of characters? for information on accessing these symbols directly in Math.