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Where does the required energy for Na + Cl -> NaCl come from?

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Enthalpy:
Energy barriers to reactions:

The reactants are almost always molecules, not atoms. Lone atoms are energetically so unfavorable, except for rare gases, that they form molecules under normal conditions.

Take H2+Cl2 :rarrow: 2HCl: the weaker bond, Cl-Cl, is already 243kJ/mol strong, so at 2478J/mol=298K the probability of one being broken is around e-98 = 10-43, so again even Avogadro's N gives no significant chance. H-H is stronger: 436kJ/mol, very close to H-Cl: 432kJ/mol.

But then, reactions must go against the reactant molecules, which is more difficult, and heat doesn't suffice for that. They happen thanks to reaction mechanisms that often involve more exotic species that are very far from thermal equilibrium, must be created somehow, and have a limited lifespan. For HCl synthesis, classical steps are:
Cl+H2 :rarrow: HCl+H  and  H+Cl2 :rarrow: HCl+Cl
which together make the global reaction as a chain reaction and need more reasonable chance to proceed.

Something must start each chain. For instance light can dissociate Cl2. Later in the reaction, strong heat can suffice, or some impurity, catalyst... As long as Cl or H meet only Cl2 or H2, either the chain propagates or nothing happens, but if they meet Cl or H or something else, the chain stops by creating only a complete molecule. Consider 100 or 10000 events in a chain - it varies even more than that. So some mechanism must create new lone atoms often enough. If it's heat, you need an initial zone big and rich enough that the newly produced heat isn't lost immediately: you need a minimum energy, say from a spark.

Such process involving extremely rare species explain why reactions take ms or h or eons to proceed despite a molecule bumps an other every ns (gas) or ps (liquid). More so if one step demands energy rarely available from the local temperature: in the example, the "limiting step" is the one that breaks H2.

Much of the science of chemical synthesis seeks intermediate reactions able to proceed, and even better, that produce mostly the desired compound and no other. This involves solvents, catalysts, varied intermediate reactants...

Enthalpy:

--- Quote from: Corribus on November 17, 2022, 02:51:32 PM ---@Enthalpy

The webbook @ NIST provides gas phase thermochemistry data as well as spectroscopic constant for diatomic NaCl molecule, which I assume are determined in the gas phase.
https://webbook.nist.gov/cgi/cbook.cgi?ID=C7647145&Mask=1E9F

A brief search shows that dimers and linear polymer chains also form under certain conditions.
https://aip.scitation.org/doi/10.1063/1.471389
--- End quote ---

Thanks Corribus!

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