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.
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
, 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:
HCl+H and H+Cl2
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
, 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...