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2

In the align below the last element of the first row is slightly off compared to the next rows. It looks like the root cause is mostly the distance between minus and zero, but also between the equal sign and minus. Why are they so different?

\begin{align}
\cee{Fe^2+_{(aq)} + 2e- -> Fe_{(s)}&(reduction) &E^0&= -0.41V}\\
\cee{Al_{(s)} -> Al^3+_{(aq)} + 3e- &(oxidation) &E^0&= +1.66V}\\
\cee{3Fe^2+_{(aq)} + 2Al_{(s)} -> 2Al^3+_{(aq)} + 3Fe_{(s)}&(overall) &E_{cell}&= +1.25V}
\end{align}

enter image description here

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1  
If you know Italian, see this: youtu.be/VPsS5_b5U30 – CarLaTeX 5 hours ago
up vote 9 down vote

The - defaults to a unary minus and is set that way. However, typically one doesn't have a unary plus. Force it using {+}:

enter image description here

\documentclass{article}

\usepackage[version=3]{mhchem}

\begin{document}

\begin{align}
  \cee{Fe^2+_{(aq)} + 2e- -> Fe_{(s)} &\ (reduction)  & E^0 &= -0.41V} \\
  \cee{Al_{(s)} -> Al^3+_{(aq)} + 3e- &\ (oxidation) & E^0 &= {+}1.66V} \\
  \cee{3Fe^2+_{(aq)} + 2Al_{(s)} -> 2Al^3+_{(aq)} + 3Fe_{(s)} & \ (overall) & 
    E_{cell} &= {+}1.25V}
\end{align}

\end{document}
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3  
No, the reason is very different. If you try a simple \begin{align}A&=-2\\B&=+2\end{align}, you can see what I mean. – egreg 1 hour ago
up vote 5 down vote

The reason is that \cee does a different scanning than usual and gets tricked by +, which does have a special meaning inside \ce or \cee, because it's used for reactions.

Indeed, if you try putting that part outside \cee (which it should be), the problem doesn't show.

Here's a better version.

\documentclass{article}

\usepackage[version=3]{mhchem}
\usepackage{siunitx}
\sisetup{retain-explicit-plus}

\begin{document}
\begin{align}
E&=-0.1\\
F&=+0.1
\end{align}

\begin{align}
  \cee{Fe^2+_{(aq)} + 2e- -> Fe_{(s)} &\ (reduction)}  & E^0 &= \SI{-0.41}{\V} \\
  \cee{Al_{(s)} -> Al^3+_{(aq)} + 3e- &\ (oxidation)} & E^0 &= \SI{+1.66}{\V} \\
  \cee{3Fe^2+_{(aq)} + 2Al_{(s)} -> 2Al^3+_{(aq)} + 3Fe_{(s)} & \ (overall)} &
    E_{\mathrm{cell}} &= \SI{+1.25}{\V}
\end{align}

\end{document}

enter image description here

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