Reaction between hydrogen halide and sulfuric acid

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In summary, this reaction occurs when concentrated sulfuric acid and sodium bromide are mixed. The halides are protonated and the reaction proceeds until the halides are displaced by gaseous hydrogen iodides. This can be easily confirmed by using less concentrated acid.
  • #1
yucheng
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I am studying the reaction

$$\ce{NaX(s) + H2SO4(conc.) -> NaHSO4 + HX \uparrow} \tag{1}$$

with the conditions similar to this video below

Solid halides with concentrated sulfuric acid


What reaction is this? Precipitation reaction? In other words, will sodium bisulfate be precipitated? I believe it would in the case of concentrated sulfuric acid

Does it make a difference if sulfuric acid is in its molecular form as opposed to dissociated form?
This is related to the question: what if concentrated acid was replaced with dilute acid? Apparently this works for bromides (see Handbook of Preparative Inorganic Chemistry see page 285) But isn't sodium bisulfate soluble? I'm thinking of putting table salt in dilute sulfuric acid etc. Will we get hydrochloric acid solution?

Why would this reaction even occur in the first place for both dilute and concentrated sulfuric acid? (See my post below). I hope there is a better reason than "thermodynamics".....

Thanks in advance!
 
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  • #2
My first thought was that (1) appears to be a Bronsted-Lowry acid-base reaction, right? Why would the halides be protonated? Is this related to displacement of weak acid by strong acids? But aren't HCl, HBr, HI stronger than H2SO4?
 
  • #3
I would classify it as a double displacement.

The driving factor here is the fact that produced hydrides of halogens are gaseous and - if you use a strong acid - they have no solution to dissolve in. In effect they quickly disperse and run away from the mixture. Thermodynamically it is the entropic factor -TΔS.

This can be easily confirmed by using less concentrated acid: there will be no reaction, just a simple dissolution.
 
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  • #4
Borek said:
This can be easily confirmed by using less concentrated acid: there will be no reaction, just a simple dissolution.

I was reading this post:
https://chemistry.stackexchange.com...oncentrated-sulfuric-acid-and-sodium-bromide/
"This reaction occurs at all conditions of temperature and concentrations, even in dilute solutions, where the reagents and products are transformed into ions."

Is the above statement true, regarding reaction of NaBr and H2SO4? If it does, that this is a simple double displacement reaction is rather doubtful......

Also, it appears that reaction with dilute sulfuric acid has indeed been done:
PREPARATION of CONSTANTBOILING HYDROBROMIC ACID
https://pubs.acs.org/doi/pdf/10.1021/ed014p187
(see the second page of this paper for details, or the Handbook I cited above)
 
  • #5
This stackexchange statement is wrong for me - writing the reaction taking place in a solution using compounds and not as a net ionic one is a nonsense. In the solution there is no reaction - you deal with ions only:

H+ + HSO4- + Na+ + Br- → Na+ + HSO4- + H+ + Br-

You can group these ions any way you like, but it doesn't mean anything has changed/reacted.

But: heating the solution changes the situation. Even in relatively diluted solutions of halide acids you can expect halide hydrogen to be volatile enough to evaporate. And any solution containing both H+ and X- ions can be considered to be an "acid solution".

With heating we are basically back to thermodynamics and -TΔS, there is no other explanation.

That's all ignoring the oxidation of halide ions to elemental halides.
 
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  • #6
Borek said:
This stackexchange statement is wrong for me - writing the reaction taking place in a solution using compounds and not as a net ionic one is a nonsense. In the solution there is no reaction - you deal with ions only:

H+ + HSO4- + Na+ + Br- → Na+ + HSO4- + H+ + Br-

You can group these ions any way you like, but it doesn't mean anything has changed/reacted.
I believe that at high enough concentration, sodium hydrogen sulfate might be precipitated out of the solution... perhaps comparing lattice enthalpies with NaBr?

What about this:
https://chemistry.stackexchange.com...ized-by-h2so4-whereas-nabr-isnt-oxidised?rq=1

The author of the answer argues that oxidation of hydrogen iodide is spontaneous, whereas that of hydrogen bromide is not spontaneous. However, in the video I've shown above, the reaction proceeds without heating. Perhaps the displacement of the halide from its salt is highly exothermic such that the temperature is just right for oxidation of hydrogen bromide to happen?

In relation to the video, as well, isn't the textbook mentioned in the question (the OP) wrong?
 
  • #7
yucheng said:
What about this:
https://chemistry.stackexchange.com...ized-by-h2so4-whereas-nabr-isnt-oxidised?rq=1

The author of the answer argues that oxidation of hydrogen iodide is spontaneous, whereas that of hydrogen bromide is not spontaneous. However, in the video I've shown above, the reaction proceeds without heating. Perhaps the displacement of the halide from its salt is highly exothermic such that the temperature is just right for oxidation of hydrogen bromide to happen?

In relation to the video, as well, isn't the textbook mentioned in the question (the OP) wrong?
Well, the oxidation behaviour of sulphuric acid depends a lot on concentration, and somewhat on temperature. And while the reaction potential gives the thermodynamic limits on possible reactions, not all possible reactions will be kinetically spontaneous.
Note that the reaction of HI or HBr with H2SO4 will compete against the reaction with alcohols. When there is no alcohol to react with, and no alcohol or product water to dilute the acid, HBr does get oxidized (as in the experiments). But HI is so much more reactive with sulphuric acid that it gets oxidized even in presence of alcohol.
 

Related to Reaction between hydrogen halide and sulfuric acid

What happens when hydrogen chloride reacts with sulfuric acid?

When hydrogen chloride (HCl) reacts with sulfuric acid (H2SO4), there is no significant reaction because HCl is a stronger acid than H2SO4. The reaction typically only results in the mixing of the two acids without any notable chemical change.

Why does hydrogen bromide react differently with sulfuric acid compared to hydrogen chloride?

Hydrogen bromide (HBr) reacts differently with sulfuric acid because HBr is a weaker acid than HCl. Sulfuric acid, being a strong oxidizing agent, can oxidize HBr to bromine (Br2), resulting in the formation of sulfur dioxide (SO2), water (H2O), and bromine gas (Br2).

What are the products of the reaction between hydrogen iodide and sulfuric acid?

When hydrogen iodide (HI) reacts with sulfuric acid, the products include iodine (I2), sulfur dioxide (SO2), and water (H2O). HI is a strong reducing agent and reduces the sulfuric acid to sulfur dioxide while being oxidized to iodine.

Can sulfuric acid be used to prepare hydrogen fluoride from sodium fluoride?

Yes, sulfuric acid can be used to prepare hydrogen fluoride (HF) from sodium fluoride (NaF). The reaction is as follows: NaF + H2SO4 → NaHSO4 + HF. This reaction is feasible because HF is a weaker acid than H2SO4, allowing the displacement to occur.

Why is sulfuric acid not suitable for preparing hydrogen bromide and hydrogen iodide from their respective salts?

Sulfuric acid is not suitable for preparing hydrogen bromide (HBr) and hydrogen iodide (HI) from their respective salts (NaBr and NaI) because it acts as an oxidizing agent and oxidizes the produced HBr and HI to bromine (Br2) and iodine (I2), respectively. This oxidation prevents the isolation of pure HBr and HI gases.

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