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I was recently subject to some atrocious teaching. One of the gems that found me was the equilibrium arrow usage. I am accustomed to write $\ce{<=>}$. Yet, the entire class had to use ⇆ throughout the semester. Teacher's argument was that each harpoon from $\ce{<=>}$ has a meaning already, the movement of a single electron in a mechanism.

I concur, but I can't think of an situation in which the meaning of a harpoon is unclear because of the context. This is unlike Y being the symbol for both EDTA and yttrium, allowing $\ce{[YY]^{-1}}$ to have a meaning.

I tried to find the correct character, but not even the IUPAC Gold Book (1) (2) has a clear stance on it.

Kurzd
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1 Answers1

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According to IUPAC: Quantities, Units and Symbols in Physical Chemistry (Green Book), the arrows are used differently for composite mechanisms in chemical kinetics.

In case of a simple mechanism that is composed of a forward and a reverse elementary reaction (i.e. reactions that occur at the molecular level in one step), it is conventional to write these in one line using two double-sided arrows $(\rightleftarrows)$.
For example:

$$\begin{align} \mathrm{A} &\to \mathrm{B + C}\\ \mathrm{B + C} &\to \mathrm{A}\\ \hline \mathrm{A} &\rightleftarrows \mathrm{B + C} \end{align}$$

This is distinguished from a net reaction, which is written with two one-sided arrows $(\rightleftharpoons)$.
For example:

$$\begin{align} \mathrm{A} &\rightleftarrows \mathrm{B + C}\\ \mathrm{B + C} &\rightleftarrows \mathrm{D + E}\\ \hline \mathrm{A} &\rightleftharpoons \mathrm{D + E} \end{align}$$

The two-sided arrow $(\leftrightarrow)$, which indicates resonance structures, shall not be used for reactions.

I guess that every elementary reaction that is proceeding in both directions $(\mathrm{A} \rightleftarrows \mathrm{B + C})$ may also be considered a net equilibrium reaction $(\mathrm{A} \rightleftharpoons \mathrm{B + C})$, but not every net equilibrium reaction $(\mathrm{A} \rightleftharpoons \mathrm{D + E})$ is an elementary reaction that occurs at the molecular level in one step $(\mathrm{A} \rightleftarrows \mathrm{D + E})$. Therefore, I assume that one-sided arrows $(\rightleftharpoons)$ may be used for all equilibrium reactions, whereas double-sided arrows $(\rightleftarrows)$ could be wrong when one combines composite mechanisms to obtain net reactions.

  • Is there a reason why you don't use \ce{<=>} for the two one-sided arrows ? In my case, such a notation gives me problems when I convert to pdf via wkhtmltopdf. – mannaia Jul 13 '19 at 06:33