How to align chemical reactions in multiple environments?

Question

How do I align (by arrow) chemical reactions in different environments (between text)?

Example: Two arrays.

\begin{eqnarray}
    A & \longrightarrow & A' \\
    B & \longrightarrow & B'
\end{eqnarray}

some text

\begin{eqnarray}
    C & \longrightarrow & C' \\
    D & \longrightarrow & D'
\end{eqnarray}

For A/B and C/D the centering works. But they are not all (A,B,C,D) centered.

How do I manage this problem?

Thank you in advance!

Answer 1

I would avoid eqnarray and use amsmath's align instead, see the reasons here.

Then I'd use amsmath's \intertext if the text between the reaction is short and the reactions form some sort of set. If they don't or if the text is too long then alignment might not be so important after all or even look strange.

And last but not least I'd use chemformula for the chemistry:

\documentclass{article}

\usepackage{amsmath}
\usepackage{chemformula}

\begin{document}

\begin{align}
  \ch{A & -> NO_{$x$}} \\
  \ch{B & -> H2O} \\
  \intertext{some text}
  \ch{C & -> SO4^{2-}} \\
  \ch{D & -> NH4+}
\end{align}

\end{document}

Should you want your reactions numbered independently from equations then you can do the same thing with chemmacros' reactions environment – this also uses chemformula for the chemicals:

\documentclass{article}

\usepackage{chemmacros}
\chemsetup{modules=reactions}

\begin{document}

\begin{reactions}
  A & -> NO_{$x$} \\
  B & -> H2O
  \intertext{some text}
  C & -> SO4^{2-} \\
  D & -> NH4+
\end{reactions}

\end{document}