“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 … )

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Will the Water-Gas Shift Reaction become a suitable means to fuel a fleet of Cars?

More than a century ago, there was a famous chemical reaction that could supply (combustible) fuel gas to cities, which was based on first turning coal into coke, and then passing steam over white-hot coke:

 


C(s) + H2O -> CO + H2

 

The gas mixture that resulted, of carbon monoxide and hydrogen gas, was both a source of fascination before the year 1900, and a reason why some city blocks ended up blowing themselves up, over the first Historic gas leaks.

What some people may not know, is that this chemical reaction was already accompanied by another reaction:

 


CO + H2O -> CO2 + H2

 

That coincidentally resulted in the gas-mixture containing a higher amount of H2 than it would contain CO, just because there was already steam present, when the water-gas was made. This second reaction is called the Water-Gas Shift Reaction.

There is one Historical context in which this reaction was always useful (as the cited article already points out). There have always been industrial consumers of hydrogen, who could not afford hydrogen produced by hydrolysis, but who could afford hydrogen that is partially contaminated with carbon monoxide. BTW, the CO2 can be removed from the gas mixture, by passing it over quicklime, producing some form of calcium carbonate. The only problem with using quicklime in the modern era is the fact that quicklime itself is produced primarily, by heating calcium carbonate, thereby releasing the equivalent amount of CO2 in advance… (:1)

But there is a context in which this reaction is almost useless. It uses a large supply of coal as its energy source, and also produces CO2 as a byproduct, the latter of which needs to be controlled, if a population of cars is to be fuelled with hydrogen.

I suppose, though, that some countries could be so desperate for an alternative source of energy, or so determined, that they’d consider trapping all the CO2 thus produced, before selling hydrogen to fuel-cell-powered -car owners. Those would probably be countries that also have a lot of coal as a resource. Those countries might also need to refine their fuel-cell technology, into fuel-cells which won’t become poisoned by the carbon monoxide still present in the hydrogen, which quicklime would not trap.

I tend to think of this reaction as a trip down memory lane.


 

Afterthought:

Carbon monoxide reacts with methanol to form acetic acid, which can in turn be trapped by quicklime, just as carbon dioxide once was. In this case, the precipitate will be calcium acetate. Therefore, special fuel cells might not be needed.

 

(Update 2/18/2020, 18h05 : )

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