»not very elegant« is not a very precise description of what's wrong...
First of all I'd use a list for the, well, list, i.e., enumerate
. The labels can easily be adjusted with enumitem
.
Then I'd make the arrows longer using the last optional argument of the \arrow
command:
\arrow[<angle>,<length factor>]
I'd also shorten the bond length a bit, maybe \setatomsep{2em}
.
Last but not least I'd use chemfig
's \+
command in the schemes for a bit more spacing around the compounds.
Then I get this (the lines just show the page dimensions so we're sure nothing sticks into the margin):

\documentclass[12pt]{article}
% \usepackage[portuguese]{babel}% not necessary for the example
\usepackage[utf8]{inputenc}
\usepackage{chemformula}% already loaded by chemmacros
\usepackage{chemfig}
% \usepackage{chemmacros}% not necessary for the example
\usepackage{enumitem}
\usepackage{showframe}% to visualize page dimensions
\begin{document}
\begin{enumerate}[label=(\alph*)]
\item\setatomsep{2em}
\schemestart
\chemfig{RO-Si(-[6]OR)(-[2]OR)-OR} \+ \ch{H2O}
\arrow(.mid east--.mid west){->[ Hidrólise]}[,2]
\chemfig{RO-Si(-[6]OR)(-[2]OR)-OH}
\schemestop
\item
\schemestart
\chemfig{RO-Si(-[6]OR)(-[2]OR)-OR} \+ \ch{H2O}
\arrow(.mid east--.mid west){->[ Condensação]}[,2]
\chemfig{RO-Si(-[6]OR)(-[2]OR)-OH}
\schemestop
\end{enumerate}
\end{document}
Of course the chemistry is still wrong but that was not your question, right? After correcting it the equations got a little but too long but that can be fixed by adjusting the lengths of the arrows and the bonds:
\documentclass[12pt]{article}
% \usepackage[portuguese]{babel}% not necessary for the example
\usepackage[utf8]{inputenc}
\usepackage{chemformula}% already loaded by chemmacros
\usepackage{chemfig}
% \usepackage{chemmacros}% not necessary for the example
\usepackage{enumitem}
\usepackage{showframe}% to visualize page dimensions
\begin{document}
\begin{enumerate}[label=(\alph*)]
\item\setatomsep{1.8em}
\schemestart
\chemfig{RO-Si(-[6]OR)(-[2]OR)-OR} \+ \ch{H2O}
\arrow(.mid east--.mid west){->[ Hidrólise]}[,1.8]
\chemfig{RO-Si(-[6]OR)(-[2]OR)-OH} \+ \ch{HOR}
\schemestop
\item
\schemestart
2 \chemfig{RO-Si(-[6]OR)(-[2]OR)-OR}
\arrow(.mid east--.mid west){->[ Condensação]}[,1.8]
\chemfig{RO-Si(-[6]OR)(-[2]OR)-O-Si(-[6]OR)(-[2]OR)-OR} \+ \ch{H2O}
\schemestop
\end{enumerate}
\end{document}

mychemistry
package could help.chemformula
package twice...