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I have been doing a bit of literature review, and have come across a few equations which use an equal sign (=) instead of an arrow, for example:

$$\ce{Fe^2+ + 1/2O2 = 2Fe^3+ + H2O}$$

What does the equal sign mean? Is the same as a single arrow $\ce{->}$ or the double arrow $\ce{<=>}$?

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    $\begingroup$ the gist is that it means a net or overall reaction. $\endgroup$
    – MaxW
    Dec 9, 2017 at 4:54

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The IUPAC Green Book (Quantities, Units and Symbols in Physical Chemistry, Third Edition, 2007) reads:

2.10.1 Other symbols and conventions in chemistry
[...]
(iv) Equations for chemical reactions

(a) On a microscopic level the reaction equation represents an elementary reaction [...] A single arrow is used to connect reactants and products in an elementary reaction. An equal sign is used for the "net" reaction, the result of a set of elementary reactions. [...]

$\ce{H + Br2 -> HBr + Br} \quad$ one elementary step in HBr formation

$\ce{H2 + Br2 = 2 HBr} \quad$ the sum of several such elementary steps

(b) On a macroscopic level, different symbols are used connecting the reactants and products in the reaction equation, with the following meanings:

$\ce{H2 + Br2 = 2 HBr}\quad$ stoichiometric equation

$\ce{H2 + Br2 -> 2 HBr}\quad$ net forward reaction

$\ce{H2 + Br2 <--> 2 HBr}\quad$ reaction, both directions

$\ce{H2 + Br2 <=> 2 HBr}\quad$ equilibrium

The two-sided arrow $\ce{<->}$ should not be used for reactions to avoid confusion with resonance structures [...]

2.12.1 Other symbols, terms, and conventions used in chemical kinetics
[...]
(ii) Composite mechanisms
A reaction that involves more than one elementary reaction is said to occur by a composite mechanism. The terms complex mechanism, indirect mechanism, and stepwise mechanism are also commonly used. Special types of mechanisms include chain-reaction mechanisms, catalytic reaction mechanisms, etc.

Examples
A simple mechanism is composed of forward and reverse reactions $\quad\quad\quad\quad\quad\quad\ce{A -> B + C}$
$\quad\quad\quad\quad\ce{B + C -> A}$
It is in this particular case conventional to write these in one line
$\quad\quad\quad\quad\quad\quad\ce{A <--> B + C}$

However, it is useful in kinetics to distinguish this from a net reaction, which is written either with two one-sided arrows or an "equal" sign
$\quad\quad\ce{A <=> B + C}$
$\quad\quad\ce{A = B + C}$

When one combines a composite mechanism to obtain a net reaction, one should not use the simple arrow in the resulting equation.
$\quad$Example $$\begin{align} \ce{A &\to B + C}\quad\text{ unimolecular elementary reaction} \\ \ce{B + C &\to D +E}\quad\text{ bimolecular elementary reaction} \\ \hline \ce{A &= D + E}\quad\text{net reaction (no elementary reaction,no molecularity)} \end{align}$$

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  • $\begingroup$ Déjà Vu $\endgroup$
    – andselisk ♦
    Dec 9, 2017 at 9:10
  • $\begingroup$ I'd say, the core fundamentals warrant a repetition if that makes them easier to find for future questioners. $\endgroup$
    – mhchem
    Dec 9, 2017 at 9:32
  • $\begingroup$ I just like when it's dry, that's all:) $\endgroup$
    – andselisk ♦
    Dec 9, 2017 at 9:38
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    $\begingroup$ Rest assured. We covered arrows and we covered equals. I think there cannot be any more questions that could be answered by this quote. :-) $\endgroup$
    – mhchem
    Dec 9, 2017 at 9:42
  • $\begingroup$ Right, this sounds fair, +1:) $\endgroup$
    – andselisk ♦
    Dec 9, 2017 at 9:45

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