[curiouscat]

Wikipedia tells me that the following reaction is one of the ways of generating HCl gas:


NaCl + H2SO4    NaHSO4 + HCl

So, is this a equilibrium reaction or does it go to completion? Does anyone know? i.e. Say I added excess of 98% H2SO4 to NaCl (s) would the NaHSO4 just precipitate out & the NaCl get consumed entirely (ignoring the tiny bit of water coming in with the 98% H2SO4)?

Also, the subsequent stage i.e.

NaCl + NaHSO4  HCl + Na2SO4

does it go to completion as well? Is that typically then a solid-solid reaction? Wikipedia says temps. of 200 C & above are needed so I wasn't sure.

If you tried it in solution you'd have to put a pressurized reactor to get this up to 200 C & besides you'd probably end up with a mixed solution of HCl & Na2SO4?

[Zyklonb]

So, is this a equilibrium reaction or does it go to completion? Does anyone know? i.e. Say I added excess of 98% H2SO4 to NaCl (s) would the NaHSO4 just precipitate out & the NaCl get consumed entirely (ignoring the tiny bit of water coming in with the 98% H2SO4)?

Every reaction is an equilibrium reaction, but for all practical uses it goes nearly to completion. That's how I make HCl (g) and yields are at least 95%. NaHSO₄ doesn't completely percipitate as it's slightly soluble in the solution.

Also, the subsequent stage i.e.

NaCl + NaHSO4  HCl + Na2SO4

does it go to completion as well? Is that typically then a solid-solid reaction? Wikipedia says temps. of 200 C & above are needed so I wasn't sure.

This equilibrium depends a lot on the temps, but at over 200 deg. C it also goes nearly all the way to the right.

[curiouscat]

Every reaction is an equilibrium reaction,

Indeed!

but for all practical uses it goes nearly to completion.

OK. That's what I wanted to know. Thanks.

That's how I make HCl (g) and yields are at least 95%. NaHSO₄ doesn't completely percipitate as it's slightly soluble in the solution.

Perfect. Thanks. That helps a lot. To hear from someone actually using this.

A follow up question: Have you tried absorbing the HCl gas in water? Theoretically I feel I ought to have no trouble making, say, 30% w/w HCl like this. But just wanted to reconfirm my thinking.

This equilibrium depends a lot on the temps, but at over 200 deg. C it also goes nearly all the way to the right.

How do you execute this stage? Do you add water or do you do it as solids? Wondering what apparatus you used.

[unsu]

A follow up question: Have you tried absorbing the HCl gas in water? Theoretically I feel I ought to have no trouble making, say, 30% w/w HCl like this. But just wanted to reconfirm my thinking.

Of course, HCl(g) dissolves in water very quickly! It dissolves efficiently even if the gas-discharge tube is not submerged into the water in the receiver. It is recommended to cool the receiver with ice.

In the first year in my inorganic labs we did an experiment to demonstrate quick dissolution of HCl gas in water: all you need is to fill a bottle with HCl gas and close the bottle with a stopper provided with a capillary tube facing inward. Invert the bottle with the gas and immerse its neck in a bath with water. You will observe a fountain! This is because HCl dissolves so quickly in water so the pressure inside the bottle quickly drops and you observe the fountain.

The reaction of NaCl(s) with conc. sulfuric acid goes nearly to completion because HCl gas escapes the reaction mixture, so the formation of the products is favoured. It won't reach equilibrium.
Heating is not required for this reaction. Once you add the sulfuric acid to solid NaCl, the reaction starts immediately. You can use 70% sulfuric acid.

[curiouscat]

In the first year in my inorganic labs we did an experiment to demonstrate quick dissolution of HCl gas in water: all you need is to fill a bottle with HCl gas and close the bottle with a stopper provided with a capillary tube facing inward. Invert the bottle with the gas and immerse its neck in a bath with water. You will observe a fountain! This is because HCl dissolves so quickly in water so the pressure inside the bottle quickly drops and you observe the fountain.

Yes. I can recall that too now that you mentioned it. Been a long time!

You can use 70% sulfuric acid.

One reason I prefer 98% is that the product NaHSO4 would precipitate out substantially. If I use 70% H2SO4 a lot of the NaHSO4 will stay in dissolved form in the associated water.

[Zyklonb]

Heating is not required for this reaction. Once you add the sulfuric acid to solid NaCl, the reaction starts immediately. You can use 70% sulfuric acid.

Heating is required for the second reaction. The first dissociation of sulfuric acid produces H⁺ and HSO₄^-, which of course turns into sodium bisulfate. The bisulfate wont react with NaCl at room temps, for this heating is required.
No, you can't use 70% sulfuric acid for either reaction, 90% is practically the minimum.

[curiouscat]

Heating is required for the second reaction. The first dissociation of sulfuric acid produces H⁺ and HSO₄^-, which of course turns into sodium bisulfate. The bisulfate wont react with NaCl at room temps, for this heating is required.

Heating is no problem. But if it happened to a significant extent at Temp. of approx 150 C that'd be most convenient for my purposes. 200 C is a bit tricky.

Context: This is for on scale. Steam is available at 10 bar. But that won't heat to 200 C. For that I'd have to design a new heating system. 

[unsu]

External heating is not required but you can do it if you feel it is necessary. The reaction itself can be exothermic.
70-85% sulfuric acid works, this procedure is taken from the lab manual, I did it myself and I know it works. However, if you want NaHSO4 to precipitate, use 96%.

HBr and HI are prepared by the similar procedure, where H3PO4 86% is used instead of H2SO4

[curiouscat]

External heating is not required but you can do it if you feel it is necessary.

For the second step I meant.

NaCl + NaHSO4  HCl + Na2SO4


 There it is needed, right?

[unsu]

NaCl + NaHSO4  HCl + Na2SO4

Yes, for this step heating is necessary, I totally agree

[curiouscat]

Follow up question: Say, I only did the room temp. reaction i.e. start with NaCl & end up with NaHSO4 (hopefully!)

What's a good way to assay the NaHSO4 content of the solids left at the end. i.e. To see how much un-reacted NaCl (if any) remains.

The idea is to eventually be able to sell bulk, industrial grade NaHSO4, if at all feasible.

[Zyklonb]

You could use silver nitrate (or any soluble silver salt) to qualitatively test for chloride. I'd do two or three recrystallizations before hand though.
As for measuring the % of sodium bisulfate, testing the pH would give you an idea, but residual sulfuric acid could mess that up, again two or three recrystallizations should solve both problems.

[curiouscat]

As an aside, what  happens if you add H2SO4 to an aq. solution of NaCl? Do you form Na2SO4?

e.g. If I were to add a stoichiometric amount of H2SO4 to an aq. soln of NaCl & then evaporate out all the water / volatiles would the residual crystals be Na2SO4 or NaCl or NaHSO4 or a mixture?

My guess is I'd get Na2SO4 preferentially & the excess Cl- ions get driven off as volatile HCl.