What does an equal sign mean in chemical equations? [duplicate]








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[Arrows used in chemical reactions](/questions/86856/arrows-used-in-chemical-reactions)

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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 $\ce{<=>}$?




**The [IUPAC Green Book](https://www.iupac.org/fileadmin/user_upload/publications/e-resources/ONLINE-IUPAC-GB3-2ndPrinting-Online-Sep2012.pdf) (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](https://www.iupac.org/fileadmin/user_upload/publications/e-resources/ONLINE-IUPAC-GB3-2ndPrinting-Online-Sep2012.pdf#page=67)**
> 
> 
> (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](https://www.iupac.org/fileadmin/user_upload/publications/e-resources/ONLINE-IUPAC-GB3-2ndPrinting-Online-Sep2012.pdf#page=83)  
> 
>  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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