An interesting piece of trivia, about the industrial production of Hydrogen Chloride.

Hydrogen chloride, or, HCl, is generally agreed to be the simplest acid. But this does not automatically imply that HCl is also the cheapest acid. Why? One way in which HCl can be produced, is by allowing equal volumes of hydrogen gas and chlorine gas to combine:


Cl2 + H2 -> 2HCl


The problem with this reaction is the known fact, that it will generally take place explosively. Unlike the equivalent reaction for oxygen:


O2 + 2H2 -> 2H2O


Which can either take place as an explosion, or in a controlled way, the reaction that produces HCl directly, will take place in an uncontrolled way. What this does is to complicate how HCl is produced industrially. One process that can be used is, that concentrated sulphuric acid, which is, BTW, the cheapest available acid, can be made to react with either NaCl or CaCl2 in their solid form. And usually, CaCl2 is chosen as a starting material, from which to produce HCl, because, choosing this salt produces not one but two useful products:


H2SO4 (l) + CaCl2 (s) -> 2HCl (g) + CaSO4 (s)


That additional reaction product, CaSO4, is also referred to as “Gypsum” when hydrated, or as “Plaster Of Paris” when dry, and is a popular construction material. The reaction written above is actually the main way in which drywall is manufactured, with HCl being given off as a byproduct. Yet, instead of using CaCl2, it will work equally well to use NaCl:


H2SO4 (l) + 2NaCl (s) -> 2HCl (g) + Na2SO4


The reason why this second reaction is less-favoured, is the mere fact that Na2S04 is less of a useful product commercially. It’s also referred to as “Glauber Salt”.

But, regardless of which reaction has been chosen as a method to mass-produce HCl, doing so consumes sulphuric acid in equivalent quantities, and also produces either sulphate as a byproduct.