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I want to define a command for chemical equations, using the mhchem package. The objective is to obtain this:

A + B = C (R1) C + D = E (R2)

Following the mhchem manual this should be something like:

\newcommand\reaction[1]{\begin{equation}\ce{#1}\end{equation}} \newcounter{reaction} \renewcommand{\thereaction}{R\,\arabic{reaction}}

which is similar to the solution given here Create New Type of Align Environment with Unique Counter

The command works(\reaction{A + B -> C}), but the counter does not. It looks like it keeps using the counter for the equation environment. What am I getting wrong?

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  • 1
    Maybe you should see in the mathtools documentation the way to customise equation labels, with the \newtagform and \usetagform commands. Commented Sep 23, 2022 at 14:06
  • Do you only have reactions in your document, or is it mixed in between other equations? Commented Sep 24, 2022 at 0:26
  • @Werner there are both chemical reactions and regular equations. Commented Sep 26, 2022 at 8:50

3 Answers 3

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I'd state your problem counts of two parts. First, the counter, which is solved by adding to the preamble:

\newcounter{reactionCount}

Next, the command delimiting the reaction.

\newcommand{\reaction}[1]{ \refstepcounter{reactionCount} % Adds 1 to the counter \[ \ce{#1} \tag{R\thereactionCount} \] % Using \[\] for simplicity. }

Here's a MWE:

\documentclass[preview, border=.2cm]{standalone} \usepackage[utf8]{inputenc} \usepackage[version=4]{mhchem} \newcounter{reactionCount} %Setting up the counter \newcommand{\reaction}[1]{ \refstepcounter{reactionCount} % Adds 1 to the counter \[ \ce{#1} \tag{R\thereactionCount} \] % Using \[\] for simplicity. } \begin{document} \reaction{A + B = C} \reaction{C + D = E} \end{document}

Numbered Reactions

This is a quick solution, built upon what code you showed. There are smarter people in this forum who can code a better one, even with more features. If this is not what you were looking for, be sure to add more details to your question.

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Extract from the mhchem Manual, from the passage following what you quoted in the question.

So far, so good. All reactions will be labelled exactly as all the equations. A few people asked for a different set of numbers for equations and reactions. One could use this code:

\makeatletter \newcounter{reaction} %%% >> for article << %\renewcommand\thereaction{C\,\arabic{reaction}} %%% << for article << %%% >> for report and book >> \renewcommand\thereaction{C\,\thechapter.\arabic{reaction}} \@addtoreset{reaction}{chapter} %%% << for report and book << \newcommand\reactiontag% {\refstepcounter{reaction}\tag{\thereaction}} \newcommand\reaction@[2][]% {\begin{equation}\ce{#2}% \ifx\@empty#1\@empty\else\label{#1}\fi% \reactiontag\end{equation}} \newcommand\reaction@nonumber[1]% {\begin{equation*}\ce{#1}\end{equation*}} \newcommand\reaction% {\@ifstar{\reaction@nonumber}{\reaction@}} \makeatother

With that, all reactions will be labelled independently of the equations.

\begin{equation}a+b\end{equation} \reaction{CO2 + C} \reaction*{CO2 + C} \reaction[react:co]{CO2 + C} \begin{equation}a+b\end{equation}

enter image description here

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You can replace equation with a different counter for reactions.

\documentclass{article} \usepackage{amsmath} \usepackage[version=4]{mhchem} \makeatletter \newcounter{reaction} %\counterwithin*{reaction}{section}% adjust to suit \renewcommand{\thereaction}{R\arabic{reaction}} %\renewcommand{\thereaction}{R\thesection.\arabic{reaction}} \newenvironment{reaction} {% \let\c@equation\c@reaction \let\theequation\thereaction \begin{equation}% } {\end{equation}} \begin{document} \begin{equation} a+b=c \end{equation} \begin{reaction} \ce{A + B -> C} \end{reaction} \begin{equation} c+d=e \end{equation} \begin{reaction} \ce{C + D -> E} \end{reaction} \end{document}

enter image description here

The same where numbering is by section.

\documentclass{article} \usepackage{amsmath} \usepackage[version=4]{mhchem} \makeatletter \newcounter{reaction} \counterwithin*{reaction}{section}% adjust to suit %\renewcommand{\thereaction}{R\arabic{reaction}} \renewcommand{\thereaction}{R\thesection.\arabic{reaction}} \newenvironment{reaction} {% \let\c@equation\c@reaction \let\theequation\thereaction \begin{equation}% } {\end{equation}} \begin{document} \section{First} \begin{equation} a+b=c \end{equation} \begin{reaction} \ce{A + B -> C} \end{reaction} \section{Second} \begin{equation} c+d=e \end{equation} \begin{reaction} \ce{C + D -> E} \end{reaction} \end{document}

enter image description here

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