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I want to centre the third equation and then align it with the arrows. Is this possible?

\begin{align}

\intertext{Volmer-reaction:}
\begin{split}
    \text{Acidic electrolyte:\hspace{1cm}}    H_{ad}+H^++e^-&\rightarrow(H_2)_{ad}\\
    \text{Basic electrolyte:\hspace{1cm}}     H_{ad}+H_2O+e^-&\rightarrow (H_2)_{ad}+OH^-.
\end{split}\\
\intertext{Absorption:}
H_{ad} &\rightarrow H_{ab}.
\intertext{Heyrovsky-reaction:}
\begin{split}
    \text{Acidic electrolyte:\hspace{1cm}} H_{ad}+H^++e^-&\rightarrow(H_2)_{ad}.\\
    \text{Basic electrolyte:\hspace{1cm}}  H_{ad}+H_2O+e^-&\rightarrow (H_2)_{ad}+OH^-.
\end{split}\\
\intertext{Tafel-reaction:}
2 H_{ad}&\rightarrow (H_2)_{ad}.
\end{align}

It would be great if someone can help me :)

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1 Answer 1

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You can move all arrows more to the center, provided you:

  1. equalize the widths of the two relevant left-hand sides and
  2. typeset the texts in a box with reduced width, so the texts will stick to its left and fool TeX into thinking that they occupy only the reduced width.

I warmly suggest using chemformula.

\documentclass{article}
\usepackage{mathtools}
\usepackage{chemformula}

\newlength{\mylen}

\begin{document}

\sbox0{Acidic electrolyte:}
\sbox2{Basic electrolyte:}
\sbox4{\ch{H_{ad} + H+ + e-}}
\sbox6{\ch{H_{ad} + H2O + e-}}
\setlength{\mylen}{\dimexpr2em+\wd2-\wd0+\wd6-\wd4}
\noindent{Volmer-reaction:}
\begin{align}
\begin{split}
  \makebox[2em][r]{Acidic electrolyte:\hspace{\mylen}}    \ch{H_{ad} + H+ + e- &-> (H2)_{ad}} \\
  \makebox[2em][r]{Basic electrolyte:\qquad}     \ch{H_{ad} + H2O + e- &-> (H_2)_{ad} + OH-}
\end{split}\\
\intertext{Absorption:}
\ch{H_{ad} &-> H_{ab}}.
\intertext{Heyrovsky-reaction:}
\begin{split}
  \makebox[2em][r]{Acidic electrolyte:\hspace{\mylen}} \ch{H_{ad} + H+ + e- &-> (H2)_{ad}} \\
  \makebox[2em][r]{Basic electrolyte:\qquad}  \ch{H_{ad} + H2O + e- &-> (H2)_{ad} + OH-}
\end{split}\\
\intertext{Tafel-reaction:}
\ch{2 H_{ad}&-> (H2)_{ad}}
\end{align}

\end{document}

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