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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.

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    $\begingroup$ Yeah, and at the same time the normal-headed arrow has the meaning of two-electron movement in a mechanism. The argument is just silly. $\endgroup$
    – Jan
    Commented Jan 3, 2016 at 23:23
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    $\begingroup$ Clayden, Greeves, Warren, and Wothers (Oxford University Press) uses the harpoon form, that's good enough for me. For diagrammatic purposes, or if the conditions for the reverse reaction were starkly different, normal arrows can make more sense but I agree, -1 to this teacher. $\endgroup$
    – Spontification
    Commented Jan 4, 2016 at 1:01
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    $\begingroup$ Related: What is the difference between “reaction in both directions” and “equilibrium”? $\endgroup$
    – user7951
    Commented Jan 4, 2016 at 13:50

1 Answer 1

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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.

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  • $\begingroup$ 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. $\endgroup$
    – mannaia
    Commented Jul 13, 2019 at 6:33

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