Is there a convention as to what order to write reactants and products in a reaction?
There is no convention.

There might be some cases where one choice simply makes more sense to people. For example I have yet to see anybody write

$$\ce{NaCl(s) &lt;=&gt;[H2O] Cl-(aq) + Na+(aq)},$$

but there is no convention dictating that you have to write $\ce{Na+}$ before $\ce{Cl-}$.