Mixing chemistry reaction and mathematics in equation

I would like to type a list of chemical reactions with their associated reaction constants, pH limits and redox potentials. The reaction would be on the left, with the reaction starting at the left edge of the page, and the reaction constant would one line below, right-aligned to the rightmost part of the page. I tried a bunch of things with align, flalign, multline, ... but there always seem to be an issue. Here are my two best tries:

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage[separate-uncertainty=true, locale=US, range-phrase={\,-\,}]{siunitx}
\usepackage[version=4, arrows=pgf]{mhchem}
\usepackage{chemmacros}

\begin{document}

\verb+flalign+ with \verb+aligned+ in it:
\begin{flalign}
&\ce{Cu2(OH)3Cl(s) <=> 2Cu^{2+} + Cl- + 3HO-} && \\
&&&
\begin{aligned}
K_a &= \frac{h^3}{[\ce{Cu^{2+}}]^2[\ce{Cl-}]} \\
\pH &= \frac{1}{3} (pK_a - 2 \log[\ce{Cu^{2+}}] - \log[\ce{Cl-}]) \\
&= \frac{1}{3} (\num{8.35} - 3 \log c_{\text{tra}} )
\end{aligned} \nonumber
\\
& \ce{Cu2(OH)3Cl(s) + 2e- <=> Cu2O(s) + HO- + H2O + Cl-} && \\
&&&
\begin{aligned}
E &= E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06}\log\left(\frac{[\ce{Cu^{2+}}]}{[\ce{Cu+}]}\right) \\
&= E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06}\log\left(\sqrt{\frac{h^3}{K_a [\ce{Cl-}]}} \frac{[\ce{HO-}]}{\sqrt{K_{s1}}}\right) \\
&= E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06} \frac{1}{2} \log h
+ \num{0.06} \left(\frac{1}{2} \log \frac{K_e^2}{K_a K_{s1}} - \frac{1}{2}\log[\ce{Cl-}] \right) \\
&= \SI{0.47}{V} - \num{0.03}\log c_{\text{tra}} - \num{0.03} \pH
\end{aligned} \nonumber
\end{flalign}

\verb+subequations+ with \verb+flalign+ in it:
\begin{subequations}
\begin{flalign}
& \ce{Cu2(OH)3Cl(s) + 2e- <=> Cu2O(s) + HO- + H2O + Cl-} &
\end{flalign}
\begin{flalign*}
&& E = E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06}\log\left(\frac{[\ce{Cu^{2+}}]}{[\ce{Cu+}]}\right) \\
&& = E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06}\log\left(\sqrt{\frac{h^3}{K_a [\ce{Cl-}]}} \frac{[\ce{HO-}]}{\sqrt{K_{s1}}}\right) \\
&& = E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06} \frac{1}{2} \log h
+ \num{0.06} \left(\frac{1}{2} \log \frac{K_e^2}{K_a K_{s1}} - \frac{1}{2}\log[\ce{Cl-}] \right) \\
&& = \SI{0.47}{V} - \num{0.03}\log c_{\text{tra}} - \num{0.03} \pH
\end{flalign*}
\end{subequations}

\end{document}

The first example (aligned within flalign) has correct numbering and spacing, but the chemical reactions (left side) and equations (right side) can not overlap, and that is not good when the reaction or equations are too long. The second example has correct width, but numbering is not that good (due to subequations being used), and the spacing between the reaction and the equations is too big.

Does anybody knows a way to have this kind of structure implemented in LaTeX?

Using a mix with alignat and aligned enviroments.

\documentclass{article}
\usepackage{amsmath}
\usepackage[version=4, arrows=pgf]{mhchem}
\usepackage{chemmacros}
\usepackage{siunitx}

\begin{document}

\begin{alignat}{2}
&\ce{Cu2(OH)3Cl(s) <=> 2Cu^{2+} + Cl- + 3HO-} &&  \\
&\begin{aligned}
K_a &= \frac{h^3}{[\ce{Cu^{2+}}]^2[\ce{Cl-}]} \\
\pH &= \frac{1}{3} (pK_a - 2 \log[\ce{Cu^{2+}}] - \log[\ce{Cl-}]) \\
&= \frac{1}{3} (\num{8.35} - 3 \log c_{\text{tra}} )
\end{aligned} && \nonumber \\
&\ce{Cu2(OH)3Cl(s) + 2e- <=> Cu2O(s) + HO- + H2O + Cl-} && \\
&\begin{aligned}
E &= E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06}\log\left(\frac{[\ce{Cu^{2+}}]}{[\ce{Cu+}]}\right) \\
&= E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06}\log\left(\sqrt{\frac{h^3}{K_a [\ce{Cl-}]}} \frac{[\ce{HO-}]}{\sqrt{K_{s1}}}\right) \nonumber\\
&= E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06} \frac{1}{2} \log h
+ \num{0.06} \left(\frac{1}{2} \log \frac{K_e^2}{K_a K_{s1}} - \frac{1}{2}\log[\ce{Cl-}] \right) \\
&= \SI{0.47}{V} - \num{0.03}\log c_{\text{tra}} - \num{0.03} \pH
\end{aligned} &&
\end{alignat}

\end{document}

You need to make the aligned to have zero width.

In the following I use \DeclareUnicodeCharacter{00B0}{} in order to remove weird warnings: if you really want you should give an appropriate definition, because by default ° translates to \textdegree that's invalid in math mode.

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage[separate-uncertainty=true, locale=US, range-phrase={\,-\,}]{siunitx}
\usepackage[version=4, arrows=pgf]{mhchem}
\usepackage{chemmacros}

\DeclareUnicodeCharacter{00B0}{}% what's ° in your code?

%flush right aligned
\NewDocumentEnvironment{myaligned}{b}
{\makebox[0pt][r]{\begin{aligned}#1\end{aligned}}}
{}

\begin{document}

This flushes left the reaction and right the equations, as you asked;
now some filler text in order to have two lines.
\begin{flalign}
&\ce{Cu2(OH)3Cl(s) <=> 2Cu^{2+} + Cl- + 3HO-} && \\
&&&\begin{myaligned}
K_a &= \frac{h^3}{[\ce{Cu^{2+}}]^2[\ce{Cl-}]} \\
\pH &= \frac{1}{3} (pK_a - 2 \log[\ce{Cu^{2+}}] - \log[\ce{Cl-}]) \\
&= \frac{1}{3} (\num{8.35} - 3 \log c_{\text{tra}} )
\end{myaligned} \nonumber
\\
&\ce{Cu2(OH)3Cl(s) + 2e- <=> Cu2O(s) + HO- + H2O + Cl-} && \\
&&&\begin{myaligned}
E &= E°(\ce{Cu^{2+}}/\ce{Cu+})
+ \num{0.06}\log\left(\frac{[\ce{Cu^{2+}}]}{[\ce{Cu+}]}\right) \\
&= E°(\ce{Cu^{2+}}/\ce{Cu+})
+ \num{0.06}\log\left(\sqrt{\frac{h^3}{K_a [\ce{Cl-}]}}
\frac{[\ce{HO-}]}{\sqrt{K_{s1}}}\right) \\
&= E°(\ce{Cu^{2+}}/\ce{Cu+}) + \num{0.06} \frac{1}{2} \log h
+ \num{0.06} \left(\frac{1}{2} \log \frac{K_e^2}{K_a K_{s1}}
- \frac{1}{2}\log[\ce{Cl-}] \right) \\
&= \SI{0.47}{V} - \num{0.03}\log c_{\text{tra}} - \num{0.03} \pH
\end{myaligned} \nonumber
\end{flalign}

\end{document}

The equations don't go up to the right margin, but sit to the left of the equation numbers. I don't think you want to move them further right.