# Align and label in chemical equation

1. How to only label the second, third and sixth line with 1, 2, 3?

2. the arrow with catalyst in first line is a little bit longer than others. How can I extend others or let it shorter?

3. How to label the second and third line with A, and fourth, fifth and sixth with B?

\begin{align*}
\ce{3H+ + 3BrO3^- + 5CH2(COOH)2 & ->[Ce3+] 3BrCH(COOH)2 + 4CO2 + 5H2O + 2HCOOH}\\
\mathrm{A:} \ce{Br- + BrO3^- + 2H+ & -> HBrO2 + HBrO}\\
\ce{Br- + HBrO2 + H+ & -> 2HBrO}\\
\mathrm{B:}\ce{Br- + BrO3^- + 2H+ & -> HBrO2 + HBrO}\\
\ce{BrO2. + Ce3+ H+ & -> HBrO2 + Ce4+}\\
\ce{2HBrO2 &-> BrO3- + H+ +HBrO}
\end{align*}
• How should lines with dual labels (both numbers and letters) be typeset? Should it be, say, (3B), (3,B), (3)(B), or something else? Please advise. – Mico Apr 17 '17 at 10:29
• @Mico Is it possible for one equations have 2 lables? A, B on the left and 1, 2, 3 on the right? – Chao Song Apr 17 '17 at 10:33
• And what should happen with the A: and B: items currently on lines 2 and 4? – Mico Apr 17 '17 at 10:35
• I add it manually, is it possible to use \something{} to get the same result with the A and B? – Chao Song Apr 17 '17 at 10:40

For the first objective, change from an align* to an align environment and use \notag directives on lines 1, 4, and 5.

For the second, load the mathtools package and replace ->[Ce3+] with & ->[$\mathclap{\ce{Ce3+}}$].

For the third, simply use two extra implicit columns and insert \textnormal{(A)} and \textnormal{(B)} directives as needed.

And do replace \mathrm{A:} with \textnormal{A}\colon.

\documentclass{article}
\usepackage{mathtools,mhchem}
\usepackage[a4paper,margin=2.5cm]{geometry} % set page size parameters appropriately
\begin{document}
\begin{align}
&&\ce{3H+ + 3BrO3^- + 5CH2(COOH)2
& ->[$\mathclap{\ce{Ce3+}}$]
3BrCH(COOH)2 + 4CO2 + 5H2O + 2HCOOH} \notag\\
\textnormal{(A)}&&
\textnormal{A}\colon \ce{Br- + BrO3^- + 2H+
& -> HBrO2 + HBrO}\\
\textnormal{(A)}&&
\ce{Br- + HBrO2 + H+
& -> 2HBrO}\\
\textnormal{(B)}&&
\textnormal{B}\colon \ce{Br- + BrO3^- + 2H+
& -> HBrO2 + HBrO} \notag\\
\textnormal{(B)}&&
\ce{BrO2. + Ce3+ H+
& -> HBrO2 + Ce4+} \notag\\
\textnormal{(B)}&&
\ce{2HBrO2
&-> BrO3- + H+ +HBrO}
\end{align}
\end{document}

With flalign*, \phantom and \notag:

\documentclass{article}
\usepackage{geometry}
\usepackage{mathtools}
\usepackage[version=4]{mhchem}

\begin{document}

\begin{flalign*}
& & \ce{3H+ + 3BrO3^- + 5CH2(COOH)2 & ->[Ce3+] 3BrCH(COOH)2 + 4CO2 + 5H2O + 2HCOOH}\notag\\
& (\mathrm A) & \mathrm{A}\colon \ce{Br- + BrO3^- + 2H+ & ->[\phantom{Ce3+}] HBrO2 + HBrO}}\label{eq-1}\\
& (\mathrm A) & \ce{Br- + HBrO2 + H+ & ->[\phantom{Ce3+}] 2HBrO}\label{eq-2}\\
& (\mathrm B) & \mathrm{B}\colon \ce{Br- + BrO3^- + 2H+ & ->[\phantom{Ce3+}] HBrO2 + HBrO}\notag\\
& (\mathrm B) & \ce{BrO2. + Ce3+ H+ & ->[\phantom{Ce3+}] HBrO2 + Ce4+}\notag\\
& (\mathrm B) & \ce{2HBrO2 &->[\phantom{Ce3+}] BrO3- + H+ +HBrO}*\label{eq-3}
\end{flalign*}

\end{document}

• How to refer the \tag{1}? \ref{1 =}doesn't work. – Chao Song Apr 17 '17 at 11:57
• You didn't say you want a referable number. In this case, use flalign (the unstarred version), and \notag with the lines you don't want to refer to. Of course the letters (A) and (B) at the beginning of lines are not referable. – Bernard Apr 17 '17 at 12:02