dawsonsuen
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Questions about synthesising hydrochloric acid.
I've recently got lots of KHSO4 around and I want to convert them into hydrochloric acid.
But I'm not sure which equation takes place..
NaCl+KHSO4-->HCl+NaKSO4
or...
2NaCl+2KHSO4-->2HCl+Na2SO4+K2SO4
Or...
Will it just be a double replacement reaction?
KHSO4+NaCl-->NaHSO4+KCl
Also, I've heard that using NaHSO4 and NaCl to generate HCl requires a temperature of 200°C.
So would using KHSO4 instead of NaHSO4 suggest that I would need an even higher temperature for the reaction to take place?
Thanks.
[Edited on 28-8-2015 by dawsonsuen]
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gdflp
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Since both NaHSO4 and KHSO4 are ionic solids, I would guess that the temperature needed for the reaction to occur is quite
similar. Displacement reactions don't occur easily without a solvent, so you won't get displacement reactions. Although both equations have the same
stoichiometry, it is correctly written as the second equation since you are not producing the double salt, rather just a mixture.
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dawsonsuen
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Thank you for your response gdflp!
So I guess upon heating the mixture of KHSO4 and NaCl at about 200°C, I should expect to see HCl gas forming and I will be able to tell when the
reaction is complete (when the mixture stops producing gas)?
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Bert
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Thread Moved
28-8-2015 at 10:00 |
Praxichys
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Assuming those two chemicals have some water in them, you will get an equilibrium of:
NaCl
KHSO4
KCl
NaHSO4
K2SO4
Na2SO4
HCl
H2SO4
All floating in a soup of:
HSO4-
SO4-
Cl-
K+
Na+
H+
The best way to separate out the HCl is to distill it off. It will come with some water. This will drive the equilibrium and the reaction will go to
completion. You will get much better mixing and thus a higher yield if you run this in a fairly watery solution. If you choose to run it dry, powder
the ingredients and grind them together as finely as possible, but expect low yield. Collect the HCl by bubbling the distillate into ice cold water,
then re-distill, collecting the fraction corresponding to the 20% azeotrope. Or, you could titrate it to find the concentration and use it without
re-distilling.
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Bot0nist
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Watch out for suck back if you bubble your HCl gas into cold water. The gas will dissolve quickly in the cold water, and as the generation of the gas
slows, the rapid pressure change will cause cold HCl solution to get sucked up your tubing, and if it runs into your hot reaction vessel it could
cause a dangerous situation. Read up about the proper use of a suck back trap to prevent this.
[Edited on 29-8-2015 by Bot0nist]
U.T.F.S.E. and learn the joys of autodidacticism!
Don't judge each day only by the harvest you reap, but also by the seeds you sow.
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annaandherdad
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Praxichys--I made HCl about a year ago with NaHSO4 and NaCl, and although I didn't make quantitative measurements, I wouldn't say that the yield was
small. I should go back and do it again, paying attention to what's being said on this thread. But I just ground the dry ingredients together and
heated, and it seemed to work fine.
I also did it with H2SO4, but I like NaHSO4 better since I worry about a flask of hot H2SO4 breaking.
Botonist---An inverted funnel works well to prevent suck-back with HCl, and I didn't have trouble with HCl getting out into the room around the
funnel. I did have trouble with it leaking around stoppers, however, and attacking all my plastic tubing.
The water should be in an ice bath or in a larger bath of cold water, since the reaction of HCl gas with H2O is exothermic, and when it gets hot it
limits the amount of HCl that will dissolve. I mean, the water gobbles the HCl at first, of course, but there is a limit. Also, you don't want to
make the acid too concentrated, for the same reason---if it is, and you store it in a bottle, there is a danger of HCl coming out of solution if the
weather gets warm and creating an overpressure in the bottle.
Any other SF Bay chemists?
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dawsonsuen
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Thank you all for the reply.
I have a similar setup with NurdRage's video on making hydrochloric acid to prevent such back so don't worry  .
I have actually tried the method of making hydrochloric acid by sulfuric acid and sodium chloride. But I just didn't want to waste the valuable
sulfuric acid to make hydrochloric acid plus I have got a lot of potassium bisulfate left over from making nitric acid.
So I guess the best option here is to mix the potassium bisulfate and sodium chloride with little water, just enough to cover the salts and heat it up
to about 110°C and collect the distillate?
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byko3y
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You need a high temperature to shift the equilibrium NaHSO4 + NaCl <=> Na2SO4 + HCl to the right, because NaHSO4 is far less acidic then HCl,
and HCl generated is quickly absorbed by Na2SO4-H2O, while for the H2SO4 (conc) + NaCl case there's no way for HCl to be absorbed. The same problem
applies to H3PO4 + NaCl.
Also, melting point of anhydrous NaHSO4 is 315°C, and at this temperature it starts to give away water, becoming Na2S2O7 whose melting point is
400°C, so this is pretty much the minimal temperature you need to obtain close to quantitative yield - 400°C.
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dawsonsuen
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But if NurdRage and annaandherdad has tried this approach, shouldn't this work fine?
annaandherdad: Do you still remember the temperature you used for this approach?
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blogfast25
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One commercial production of HCl is NaCl + conc. H2SO4.
In the first step:
NaCl + H2SO4 === > HCl(g) + NaHSO4
The second step does require some heating:
NaCl + NaHSO4 === > HCl(g) + Na2SO4
So overall:
2 NaCl + H2SO4 === > 2 HCl(g) + Na2SO4
The equilibria are to the right because HCl distils off.
I've done this myself and yield is near-quantitative.
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annaandherdad
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dawsonsuen: No, I didn't measure the temperature, but I doubt if it was as high as 315 C. There might have been a small amount of water in my
ingredients. Blogfast seems to say that the second stage (which uses NaHSO4+heat) is nearly quantitative.
Any other SF Bay chemists?
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blogfast25
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Yes. The whole process, the NaHSO4 step included, is near-quantitative. Small amounts of moisture will affect yield a little, in accordance with the
amount of water.
This is to be expected as one of the reaction products is continuously being removed from the reagent mixture.
[Edited on 29-8-2015 by blogfast25]
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