# Tag chemical equations with letters and numbers

Just started in the writing of chemistry stuff using LaTeX.

Question

Is it possible to tag chemical equations with a letter and numbers as R1, R2 and so on?

Output and MWE

\documentclass[12pt,twoside]{report}

\usepackage[spanish,es-noquoting]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{nccmath}
\usepackage{chemformula}

\begin{document}

\begin{align}
&\ch{B + H+ <=> BH+}\\
&\ch{AH+ + B <=> A + BH^{+}} \label{reaccion_neta}
\end{align}

The sum \ref{reaccion_neta} is:

\begin{align}
\Delta G^0_{1} + \Delta G^0_{ref}&=-RT(ln(Ka_1)+ln(Ka^{-1}_{ref}) \\
G^0_{BH^+} - G^0_{B} + G^0_{A}- G^0_{AH+} &=-RT(ln(Ka_1)+ln(Ka^{-1}_{ref})\\
&= RT2.303pKa_{1}- RT2.303pKa_{ref}
\end{align}

\end{document}

• We appreciate it if there is only one question per post. That allows us to keep focussed. – Masroor Aug 31 '17 at 17:17
• There are various packages for typesetting chemistry related stuff. You could have a look at mhchem or chemmacros. Both offer the possibility to align and number reaction equations. This answer might be interesting as well. – leandriis Aug 31 '17 at 17:23
• Unrelated: use \ln not ln for the natural logaritm, and you'd probably want _{\mathrm{ref}} instead of _{ref}. – Torbjørn T. Aug 31 '17 at 17:34
• How would you like the "letter numbering" to be relative to the "equation numbering"? (A) (B) (1) (2) (3)...? – Werner Aug 31 '17 at 17:41
• @Hernan Miraola As already commented before, there already is an answer to this part of your question here: tex.stackexchange.com/a/147854/134144 Ludovic C. defined a new environment for numbering reaction equations with R1... while keeping the counter of math equations untouched. – leandriis Aug 31 '17 at 19:05

Here you are. I define an environment, somewhat like subequations, which re-defines the equation counter as a new chemeqn counter. Also, I propose another alignment (second group) based on alignat, which I find nicer:

\documentclass[12pt,twoside]{report}

\usepackage[spanish,es-noquoting]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{mathtools, nccmath}
\usepackage{chemformula}

\makeatletter
\newcounter{chemeqn}
\newenvironment{chemequations}{\let\c@equation\c@chemeqn\def\theequation{R\thechemeqn}}{}
\makeatother

\begin{document}

\begin{chemequations}
\begin{align}
&\ch{B + H+<=> BH+}\\
&\ch{AH+ + B<=> A + BH^{+}} \label{reaccion_neta}
\end{align}

\begin{alignat}{2}
\ch{B &+ H+ & & <=> BH+}\\
\ch{AH+ &+ B & & <=> A + BH^{+}} \label{reaccion_neta}
\end{alignat}
\end{chemequations}

The sum \eqref{reaccion_neta} is:
\begin{align}
\Delta G^0_{1} + \Delta G^0_\textrm{ref}&=-RT(\ln(Ka_1)+\ln(Ka^{-1}_\textrm{ref}) \\
G^0_{BH^+} - G^0_{B} + G^0_{A}- G^0_{AH+} &=-RT(\ln(Ka_1)+\ln(Ka^{-1}_\textrm{ref})\\
&= RT2.303pKa_{1}- RT2.303pKa_\textrm{ref}
\end{align}

\end{document}