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I know that the definition of molecularity of a reaction is number of species reacting in an elementary step. But considering the theory of microscopic reversibility for elementary reactions, each reaction can be assumed to be reversible. In that case, if $$\ce{A <=> B + C}$$ then what is the molecularity of the reaction? Is it 1 or 2?

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  • $\begingroup$ For a reversible reaction, the appropriate arrows are double arrows $\ce{<-->}$ (feature not implemented in MathJax's mhchem) or equilibrium arrows $\ce{<=>}$. The use of resonance indicating arrows $\ce{<->}$ is wrong. However, an elementary reaction can be reversed, but can practically not be in equilibrium - the system can be in one state or the other only. $\endgroup$ May 5 '15 at 14:29
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For the reaction $$\ce {A <=> B + C}$$ its molecularity is unimolecular. If we were given $$\ce{B + C <=> A}$$ Its molecularity would be bimolecular. Thus we see molecularity is dependent on the number of reactant particles, even if it's in equilibrium.

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Just to add onto the other answer.
Since the equilibrium constant also changes when we reverse a reaction, so does the molecularity of the reaction.
So many properties of reversible reactions are defined in the way we write down the reaction.

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