Mixing multlined and align

Question

I’m trying to have a first equation split on two lines, first one being left aligned, second one right aligned, equation number centered for those two lines, and then a second (numbered) equation that would align at one particular point with a symbol from the second line.

I’ve looked a bit around but couldn’t find a non-manual spacing solution for this case.

Here is what the code would look like

\documentclass{article}

\usepackage{mathtools}
\usepackage{mhchem}

\begin{document}

\begin{align}
    \begin{multlined}[.88\linewidth]
        \ce{2CaCO3 + 5C + 2CaC2 + 4H2O + 2Ca(OH)2 + 2CO2} \\
        \ce{&-> 2CaC2 + 3CO2 + 2Ca(OH)2 + 2C2H2 + 2CaCO3 + 2H2O}
    \end{multlined}\\
    \ce{5C + 2H2O &-> CO2 + 2C2H2}.
\end{align}

\end{document}

excepted of course this does not work. If it did, it should look a bit like this:

2 CaCO₃ + 5 C + 2 CaC₂ + 4 H₂O + 2 Ca(OH)₂ + 2 CO₂
                         ——⟶ 2 CaC₂ + 3 CO₂ + 2 Ca(OH)₂ + 2 C₂H₂ + 2 CaCO₃ + 2 H₂O
             5 C + 2 H₂O ——⟶ CO₂ + 2 C₂H₂

with one equation number for the first two lines, vertically centered, and one equation number for the last line.

This code also provide a manual solution giving roughly the result I expect

\documentclass{article}

\usepackage[margin=2cm]{geometry}
\usepackage{mathtools}
\usepackage{mhchem}

\begin{document}

\begin{align}
    \begin{split}
        \ce{2CaCO3 + 5C + 2CaC2 + 4H2O& + 2Ca(OH)2 + 2CO2} \\                                                                                                                                                                                                                  
        \ce{&-> 2CaC2 + 3CO2 + 2Ca(OH)2 + 2C2H2 + 2CaCO3 + 2H2O}
    \end{split}\\
    \ce{5C + 2H2O &-> CO2 + 2C2H2}.
\end{align}

\end{document}

But it requires finding manually where to put the first & so I’m looking for a more general solution.

I would accept a solution using other environments as long as they match the requirements stated in my first paragraph (e.g. something that left/right align the first equation while using a split for instance).

Answer 1

I'd do it with split. The first line is artificially made the same size as the part before the arrow in the last line.

\documentclass{article}
\usepackage{geometry}

\usepackage{mathtools}
\usepackage[version=4]{mhchem}

\begin{document}

\begin{align}
\begin{split}
\sbox0{\ce{5C + 2H2O}}
\makebox[\wd0][l]{\ce{2CaCO3 + 5C + 2CaC2 + 4H2O + 2Ca(OH)2 + 2CO2}} \\
\ce{&-> 2CaC2 + 3CO2 + 2Ca(OH)2 + 2C2H2 + 2CaCO3 + 2H2O}
\end{split}\\
\ce{5C + 2H2O &-> CO2 + 2C2H2}.
\end{align}

\end{document}

I loaded geometry because the reactions would not fit in the standard text width of article.

Here's how to move the first line to the left margin.

\documentclass{article}
\usepackage[margin=2cm,showframe]{geometry}

\usepackage{mathtools}
\usepackage[version=4]{mhchem}

\makeatletter
\def\column@width#1{\ifcase\@xp#1\maxcolumn@widths\fi}
\def\shift@{%
  \ifmeasuring@
    0pt
  \else
    \dimexpr(\displaywidth+\column@width{1}-\column@width{2})/2\relax
  \fi
}
\def\movetoleft#1{%
  &\makebox[0pt][l]{\hspace{-\shift@}#1}%
}
\makeatother

\begin{document}

\begin{align}
\begin{split}
\movetoleft{\ce{2CaCO3 + 5C + 2CaC2 + 4H2O + 2Ca(OH)2 + 2CO2}} \\
\ce{&-> 2CaC2 + 3CO2 + 2Ca(OH)2 + 2C2H2 + 2CaCO3 + 2H2O}
\end{split}\\
\ce{5C + 2H2O &-> CO2 + 2C2H2}.
\end{align}

\end{document}