How to change distance within a reversible edge and increase the font size in chemfig?

Question

I have the following LaTeX code:

\documentclass{article}
\usepackage{chemfig}
\setarrowdefault{15,3,thick}
\begin{document}
\schemestart
 $A_{1}$
\arrow($A_{2}$--){<=>[$k_{1}$][$k_{2}$]}[35] $A_{2}$
\arrow($A_{2}$--){<=>[$k_{3}$][$k_{4}$]}[-35] $A_{3}$
\schemestop
\end{document}

Could you please help me how to change distance within the reversible edges ("<=>") and set (a) the font size of the nodes and the text on the arrows and (b) distances of $k_{i}$s from the arrows?

Answer 1

Looking at the example on page 29 chapter III.6, this is indeed customizable. Here is possible solution:

\documentclass{article}
\usepackage{chemfig}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,arrows}

\setarrowdefault{15,3,thick}
\begin{document}
\makeatletter
\tikzset{ 
    ddbond/.style n args={4}{ 
        draw=none, 
        decoration={% 
            markings,
            mark=at position 0 with { 
                \coordinate (CF@startdeloc) at (0,\dimexpr#1\CF@double@sep) 
                coordinate (CF@startaxis) at (0,\dimexpr-#1\CF@double@sep); 
            },
            mark=at position 1 with { 
                \coordinate (CF@enddeloc) at (0,\dimexpr#1\CF@double@sep)
                coordinate (CF@endaxis) at (0,\dimexpr-#1\CF@double@sep); 
                \draw[-{Stealth[left]}, line width=0.4mm] (CF@startdeloc)--node[rotate=#2, above = 0.1cm]{#3}(CF@enddeloc); 
                \draw[{Stealth[left]-}, line width=0.4mm] (CF@startaxis)--node[rotate=#2, below = 0.1cm]{#4}(CF@endaxis); 
        }
     },
  postaction={decorate}
 }
}
\makeatother 
\chemfig{A_1-[-135,5,,,ddbond={+1.5}{45}{$k_1$}{$k_2$}]A_2-[135,5,,,ddbond={+2.5}{-45}{$k_3$}{$k_4$}]A_3}

\end{document}

You would need to tune the parameters, but that should be easy to do. The space between arrows are set by the first parameter (+1.5, +2.5), the other parameters influence the label on the arrow. You would also have to tweak the angles, but in principle you should be able to make what you want.