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I mean, how can we reverse the neutralization reaction of HCl and NaOH to get back HCl and NaOH.

I want to extract HCl and NaOH from a mixture of common salt and distilled water in equal proportions at home.

Basically, how to make this reaction happen:

$$\ce{H2O + NaCl -> HCl + NaOH}$$

What temperature or pressure conditions are required?
Also, would the extracted components be pure? (I know common salt contains iodine and even distilled water is not pure, that's why I am asking this.)

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2 Answers 2

up vote 10 down vote accepted

Unless time travel is an option, you could

  1. Electrolyze the $\ce{NaCl}$ solution to obtain a solution of $\ce{NaOH}$, and $\ce{H2}$ and $\ce{Cl2}$ as gases.

  2. Collect the gases and photolyze them. The dissociation energy of $\ce{Cl2}$ is $243\, \mathrm{kJ \cdot mol^{-1}}$, irradiation at $\lambda$ < 490 nm will cleave $\ce{Cl2}$ to chlorine radicals and initiate the chain reaction to yield $\ce{HCl}$ gas.

$$\ce{Cl2 ->[h\nu] 2 Cl*}$$

$$\ce{Cl* +\ H2 -> HCl + H*}$$ $$\ce{H* +\ Cl2 -> HCl + Cl*}$$


Unless you know exactly about the risk of both processes, don't do it! This video gives a nice impression of how violently hydrogen and chlorine react upon irradiation! Note that it has nothing to do with the laser involved. A flashlight would give the same dramatic effect!


The answer above, giving practical advices on how to get $\ce{HCl}$ and $\ce{NaOH}$ back apparently suggests that you can not simply revert the neutralisation reaction.

The neutralisation is irreversible.

In order to find out why this is the case, you might want to have a look at thermodynamics of this exothermic reaction, particularly at the entropy changes.

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That vid seems really cool. It would be fun trying that out! Just kidding. Just wanna know, after the burst in the video, was HCl finally formed or was the reaction stopped? –  Awal Garg Mar 6 '14 at 13:01
Yes, HCl is definitely formed here. I updated the answer and added the mechanism. The video is really impressive and it is fun to carry out - in a well-vented lecture hall or in a fumehood, but not in the living room ;) –  Klaus Warzecha Mar 6 '14 at 15:00
thats it. surely gonna try this once I get through with my exams. thnx for your help. –  Awal Garg Mar 6 '14 at 15:20
Is it really entropy that dominates the huge free energy release on neutralization? I figured most of it would come from the enthalpy change on formation of the additional, strong O-H bond. –  Nicolau Saker Neto Mar 6 '14 at 22:35
Strictly speaking, it is theoretically possible to reverse neutralization using super-critical steam on crystalline $NaCl$. However, it requires extreme conditions. –  permeakra Mar 7 '14 at 10:38

Direct hydrolysis of NaCl is not the problem. The problem is isolating the products before they recombine to reverse the reaction. That's a good trick given the net free energy of formation. Dry HCl $gas$ is polar but $not$ $ionic$. Reduction to practice is left as an exercise of the alert reader.

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Thnx for your answer. I didn't understand your last line - 'Reduction to practice is left as an exercise of the alert reader'. –  Awal Garg Mar 7 '14 at 10:03
That is the punchline. Science describes but engineering builds. A diffusion-selective membrane will do the separation. There is no such survivable membrane. –  Uncle Al Mar 7 '14 at 20:36

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