“Does Chlorine react with Acid?”

A question which I was once asked was, “Does chlorine react with acid?” And when I gave a quick answer to this question, I assumed that when the person asking it said “Chlorine”, he meant Cl2 gas. What I had forgotten was, that some people, including the person who was asking this question, take the word “Chlorine” to be a synonym for ‘Chlorine Bleach’. And of course, chlorine bleach reacts with acid! But to understand how such communication errors take place, one must actually understand, ‘What is chlorine bleach?’

Most preparations of chlorine bleach, are based on either sodium hypochlorite (NaOCl), or potassium hypochlorite (KOCl), with a generous amount of the corresponding hydroxide added to it, to stabilize it. This modern mixture is prepared industrially, by allowing chlorine gas to react with sodium hydroxide solution, for the sake of argument, which generates a mixture of sodium chloride – i.e., salt – and sodium hypochlorite:

Cl2(g) + 2NaOH(aq) -> NaCl + NaOCl + H2O

What happens next is that Engineering separates the (useful) sodium hypochlorite from the (useless) sodium chloride, and adds more sodium hydroxide to it, to arrive at the final mixture.

The treacherous fact about this sort of mixture is, that the reaction that produced it is readily reversible. Sodium hypochlorite solutions will react under any of the following conditions, to cause dangerous gasses, such as Cl2, but not only Cl2 to be generated, where Cl2 is also a gas that was used in Chemical Warfare, in WW1:

  • When mixed with anything that would neutralize the sodium hydroxide, including any acid,
  • When mixed with so-called reducing agents, such as ammonia,
  • When mixed with salt,
  • In fact, sodium hypochlorite solutions are not 100% stable, when neutral…
  • Etc., etc., etc..

But just to be clear, if one starts with the deadly gas that is Cl2, No, that does not react readily with common acids.

(Update 5/06/2020, 15h15 … )

One of the types of reagents which modern Chemistry offers a Chemist, and which was not available in the earlier days of Chemistry, is so-called “Hypochlorous Acid”:

[H]+ + [OCl]-

And, given what I just wrote above, the next question which the reader might ask himself is, ‘Why can Chemists work with hypochlorous acid at all? Shouldn’t it just revert to chlorine gas, being an acidic solution that contains hypochlorite ion?’

And the answer to this question is, that hypochlorous acid preparations generally decay, and that they must be prepared within hours if not minutes of being used. And, how fast they decay depends, on how concentrated this acid is. They tend to be prepared electrochemically, in that a solution containing hydrogen chloride has an electric current passed through it, in a setup that uses a lead-oxide anode. The chloride gets electro-oxidized – transformed – into hypochlorite, and the corresponding acid forms.

Obviously, real Chemists must also work with “vent hoods”, that are expensive devices which capture gasses evolved in chemical experiments, since solutions such as hypochlorous acid solution, are slowly giving off gasses as they decay.


 

I suppose that an interesting follow-up question might be, ‘How is hypochlorous acid useful, in the lab?’ The real answer is, that it’s only useful in a limited number of situations. It can be used in a synthesis, where a chlorine atom is supposed to replace an atom or group, that would formerly have given rise to a positive ion, while it’s more common that the chlorine atom will replace a group, that would give rise to a negative ion. In other words, it would be more conventional to start with ethanol, for the sake of argument, and to perform a substitution on it, where the hydroxyl group is replaced with a chlorine atom. More conventional, because the hydroxyl group by itself, tends to form the negative hydroxide ion, to correspond to the negative chloride ion. This substitution can be performed, by allowing ethanol to react with phosphorus pentachloride, which should be strong enough a chlorinating agent:

CH3CH2OH + PCl5 -> CH3CH2Cl + H3PO4

I did not bother to balance the above equation for brevity, but what we’d obtain is chloroethane. Hydroxide ion and chloride ion are usually both negative:

[OH]-, [Cl]-

It can happen that a Chemist wants to synthesize ‘chloramine’, which is a derivative of ammonia, in which one of its hydrogen atoms has been replaced with a chlorine atom:

NH2Cl

Related substances exist, in which 2 or 3 hydrogen atoms have been replaced with chlorine atoms:

NHCl2, NCl3

If the intention is to synthesize these through a substitution reaction, the fact that a hydrogen ion is usually positive, while a chloride ion is usually negative, gets in the way. In this situation specifically, ammonia can be made to react with hypochlorite, in the lab, in the form of hypochlorous acid, and an elimination reaction will produce the desired chloramine, as well as water:

NH3 + HOCl -> NH2Cl + H2O

NH2Cl + HOCl -> NHCl2 + H2O

NHCl2 + HOCl -> NCl3 + H2O

 


 

If someone undertook the folly of mixing chlorine bleach with ammonia, then another gas which would form, is ‘hydrazine’. The reason this would also form, is the fact that monochloramine is forming, in the presence of more, supplied ammonia. And these two molecules will undergo the following reaction:

NH3 + NH2Cl -> N2H4(g) + HCl(g)

Hydrazine is a gas related to ammonia, which some people already know as painful to inhale, but even more painful to inhale, and AFAIK, more toxic than either chloramine or ammonia gas are already toxic.

Dirk

 

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