Arrows chemical equations

There are two alternatives I'd consider. The first is the well known mhchem package:

\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}

\ce{Na2SO4 ->[H2O] Na+ + SO4^2-}

\ce{(2Na+,SO4^2- ) + (Ba^2+, 2Cl- ) -> BaSO4 v + 2NaCl}

\end{document}

The second is the chemformula package from the chemmacros bundle:

\documentclass{article}
\usepackage{chemformula}
\begin{document}

\ch{Na2SO4 ->[ H2O ] Na+ + SO4^2-}

\ch{( 2 Na+ ,SO4^2- ) + (Ba^2+ , 2 Cl- ) -> BaSO4 v + 2 NaCl}

\end{document}

Their syntax is slightly different and both are bit picky about spaces that need to be there in order to parse the input, however in different places... you'll also notice small differences in the output for example with regard to spacing.

But since you've mentioned the chemist package (which is part of the XyMTeX bundle): the reactions are also possible with it:

\documentclass{article}
\usepackage{chemist}
\begin{document}

\begin{chemmath}
  Na_{2}SO_{4}
  \reactrarrow{0pt}{1.5cm}{\ChemForm{H_2O}}{}
  Na^{+} + SO_{4}^{2-}
\end{chemmath}

\begin{chemmath}
  (2 Na^{+},SO_{4}^{2-}) + (Ba^{2+},2 Cl^{-})
  \reactrarrow{0pt}{1cm}{}{}
  BaSO_{4} + 2 NaCl
\end{chemmath}

\end{document}

Here is a chemfig solution:

\documentclass{article}
\usepackage{chemfig}
\begin{document}
\schemestart
\chemfig{Na_2SO_4}
\arrow{->[\footnotesize\chemfig{H_2O}]}
\chemfig{Na^+}\+\chemfig{SO_2^{-}}
\schemestop

\bigskip

\schemestart
(2\chemfig{Na^+}, \chemfig{SO_4^{2-}})
\+
(\chemfig{Ba^{2+}}, 2\chemfig{Cl^{-}})
\arrow(.mid east--.mid west)
\chemname[1pt]{\chemfig{BaSO_4}}{\chemfig{-[,0.75]-[5,.3,,,-stealth]}}\+2\chemfig{NaCl}
\schemestop
\end{document}