Box around reaction chemmacros

Question

I've been trying to use tcolorbox to frame a reaction using the reaction environment from chemmacros. The use of \begin{tcolorbox}and \end{tcolorbox} does work:

\documentclass{exam}
\usepackage{chemmacros}
\usepackage{tcolorbox}
\chemsetup{modules = all}

\begin{document}

Document starts

\begin{tcolorbox}
\begin{reaction}
    3 O2 -> 2 O3
\end{reaction}
\end{tcolorbox}

document continues here

\end{document}

This produces the document with no errors; however, as expected, the box spans the text width. I want the box to fit the reaction, so I tried to use the tcbox command. However, if I put it before the reaction environment, I get a bad math delimiter error and the reaction number appears inside the box, and if I use it inside it won't compile (it says LaTeX error: "kernel/empty-search-pattern" Empty search pattern. For immediate help type H . \end).

Any suggestions?

Answer 1

When chemmacros starts a reaction it basically does three things:

• it temporarily sets the equation counter to be equal to the reaction counter
• it sets the tag form to reaction; for this it loads the mathtools package because the latter allows easy control of the tag form of an equation.
• it then puts the reaction content inside the formula controlling macro – per default chemformula's \ch.

The last point is the problem when you try to use a box macro inside a reaction.

But knowing the above we can simulate a reaction using equation which then allows us to put frames around parts of the content:

\begin{equation}
  \refstepcounter{reaction}\usetagform{reaction}
  \ch{...} \tag{\thereaction}
\end{equation}

For example using tcolorbox and its library theorems we can now put a framed box around the content using \tcboxmath or \tcbhighmath (see the tcolorbox manual for details):

\documentclass{article}
\usepackage{chemmacros}
\usepackage{tcolorbox}
\tcbuselibrary{theorems}

\chemsetup{
  formula = chemformula ,
  modules = reactions
}

\begin{document}

Document starts
\begin{equation}
  \refstepcounter{reaction}\usetagform{reaction}
  \tcboxmath{\ch{3 O2 -> 2 O3}} \tag{\thereaction}
\end{equation}
document continues here
\begin{equation}
  a + b = c
\end{equation}

\end{document}

BTW: please don't start new paragraphs with a math or chemistry display unless it is absolutely necessary…

Answer 2

I can propose this, based on empheq (needless to load amsmath since it loads mathtools):

\documentclass[svgnames]{exam}
\usepackage{chemmacros}
\usepackage{empheq}
\usepackage{tcolorbox}
\chemsetup{modules = all}
\newcommand*\widecolorbox[1]{\fboxsep=2ex\fboxrule=1pt\fcolorbox{black}{Gainsboro!40}{\hspace{1em}#1\hspace{1em}}}

\begin{document}

Document starts

%
\begin{empheq}[box =\widecolorbox]{equation}
\ch{
 3 O2 -> 2 O3}
\end{empheq}
%
document continues here

\end{document}