Yes, Daniel's MathSlate may be a good option. it is easily customisable, has a nice drag-and-drop interface, and emphasises the logic/structure of an expression.
See https://moodle.org/mod/forum/discuss.php?d=283205
A while back, I did experiment with a ChemSlate version of MathSlate that supported the mhchem extension to TeX/MathJax. However, I was focusing on basic (reaction) chemical equations and got 'bogged-down' in the technical details of the conversion of MathML, MathJax snippets, TeX-mhchem syntax etc. So, my attention switched to a simpler (to my mind!) system based on the Atto Equation editor, instead.
In the context of equilibrium expressions, MathSlate may well be a better fit. Perhaps an EquilibriumSlate?