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Sciencemadness Discussion Board » Fundamentals » Chemistry in General » Mystery of SCl2 Hydrolysis - Original Theory

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AJKOER

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posted on 29-9-2012 at 17:53

Mystery of SCl2 Hydrolysis - Original Theory

If one searches the internet on exactly what happens when Sulfur dichloride reacts with water, one draws a somewhat surprising result - little to nothing. A typical response is from Wikipedia (http://en.wikipedia.org/wiki/SCl2 ) to quote:

"SCl2 hydrolyzes with release of HCl. Old samples contain Cl2."

Yes, no editing here, that is it. Here is another source from a study sponsored by the United Kingdom's Health & Safety Executive with some more, but seemingly doubtful, comments with references:

"The only reference on its hydrolysis reaction is that it decomposes completely rapidly and completely on treatment with water, forming HCl, thiosulphate, polythionic acids [22, 23]. Unfortunately there are no further data on its reaction with water." See topic 2.2.10 at http://www.hse.gov.uk/research/crr_pdf/2001/crr01331.pdf

A not too informative hydrolysis of SCl2 is quoted as (see page 10 item 12 at http://aero.spss.edu.hk/~alam/F7%20Chemistry/Ans%20for%20Tex... ):

SCl2 (l) + H2O (l) ---> HOCl (aq) + HSCl (aq)

so HOCl and HSCl may be intermediate products.

There may also some be some insight provided by the reported hydrolysis of S2Cl2 given by:

S2Cl2 + 2 H2O --> 2 HCl + H2S + SO2

See http://pubs.acs.org/doi/abs/10.1021/ja01412a501

There is also some reported web observations (see http://www.chemicalforums.com/index.php?topic=27007.0 ) to quote from the opening thread: "It hydrolyses fairly quickly in moist air to S and HCl."

So the dots to be connected by any good theory are S, HCl, HOCl (intermediate along with HSCl), Cl2 (latter), and perhaps H2S and SO2 consistent with S2Cl2 and the reported presence of "thiosulphate, polythionic acids".

So let me be so audacious as to possible reaction sequences that appears to fill in all the reported observations. My speculation:

2 SCl2 + 4 H2O --> 4 HOCl + 2 H2S --> 2 HOCl + 2 HCl (g) + 2 H2O + 2 S (c) --> 4 HCl (g) + 2 H2O + SO2 + S (c)

Possible side reactions:

1. HCl + HOCl <--> Cl2 + H2O

where as the solution becomes increasing acid, the equilibrium can move to the right. This agrees with the Chlorine observation upon aging. The gases release of the HCl may explain the limited initial Cl2 observation.

2. S + HOCl --> SO2 + HCl

as HOCl reputedly will oxidize S to SO2 and even to H2SO4 (See Watt's Dictionary of Chemistry, Vol 2, page 16). The formation of SO2 is needed to explain the presence of thiosulphates.

3. Per Wikipedia (http://en.wikipedia.org/wiki/Polythionic_acid
) to quote:

"Interaction hydrogen sulfide and a sulfur dioxide. This yields a complex mixture of various oxyacids of sulfur of different structures, called Wackenroder solution.

H2S + H2SO3 -> H2S2O2 + H2O
H2S2O2 + 2H2SO3 -> H2S4O6 + 2H2O "

The presence, even temporary, of Hydrogen sulfide appears to be consistent with the reported presence of polythionic acids, which would also require the formation of SO2.

4. With respect to the formation of thiosulfates, per this link ( http://www.transtutors.com/chemistry-homework-help/s-and-p-b... ), to quote:

"The resulting solution is boiled with powdered sulfur as, Na2SO3 + S —> Na2S2O3"

Wikipedia also notes (see http://en.wikipedia.org/wiki/Thiosulfate
) to quote:

"Thiosulfates are stable only in neutral or alkaline solutions, but not in acidic solutions, due to decomposition to sulfite and sulfur, the sulfite being dehydrated to sulfur dioxide:

S2O32- (aq) + 2 H+ (aq) -> SO2 (g) + S (s) + H2O"

so these products, per my cited reaction, is consistent with the reported presence of thiosulphate.

5. Lastly, some of the colloidal created Sulfur could react with Chlorine:

S (c) + Cl2 --> SCl2

The created Sulfur dichloride would then be recycled.
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Note, as my reaction starts as (rescaled):

SCl2 + 2 H2O --> 2 HOCl + H2S

and the quoted given reaction is (upon adding H2O to both sides):

SCl2 + H2O (+ H2O) ---> HOCl + (HSCl + H2O)

I am implying that the hydrolysis of HSCl is:

HSCl + H2O --> HOCl + H2S

which lends itself, in my opinion, to a more productive understanding of subsequence reactions.

Comments welcome.

[Edited on 30-9-2012 by AJKOER]

AJKOER

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posted on 30-9-2012 at 05:46

OK, for the record it appears that the acid HSCl is called Thiohypochlorous acid.

Now, per Wiki (http://en.wikipedia.org/wiki/Sulfenyl_chloride) to quote: "According to IUPAC nomenclature they are named as alkyl thiohypochlorites, i.e. esters of thiohypochlorous acid" where the 'they' refers to sulfenyl chlorides, a functional group with the connectivity R-S-Cl.

While other sources (see, for example page 9 at http://books.google.com/books?id=7cOE43QZ1u4C&pg=PA9&... ) also indicate that the SCl anion is associated with the sulfenyl chloride group, nevertheless, the acid is not called Hydrosulfenylchloric acid.

In fact, per this link (http://www.guidechem.com/reference/dic-409091.html ), HSCl is confirmed as Thiohypochlorous acid.

[Edited on 30-9-2012 by AJKOER]

AJKOER

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posted on 1-10-2012 at 12:31

OK, I have actually find a reference via an answer sheet available online as a pdf from one of the 12 Microsoft® Innovative Schools worldwide, and is the only one in Asia (see http://www.microsoft.com/casestudies/Microsoft-Windows-XP/Fu... ), that confirms my reaction products. To repeat my speculated reaction equation:

2 SCl2 + 4 H2O --> 4 HOCl + 2 H2S --> 2 HOCl + 2 HCl (g) + 2 H2O + 2 S (c) --> 4 HCl (g) + 2 H2O + SO2 + S (c)

or, upon eliminating 2 H2O from the first and last equations:

2 SCl2 + 2 H2O --> 4 HCl (g) + SO2 + S (c)

The attached answer sheet pdf file reference from the Fungkai School (HK) on page 4 states to quote:

" For SCl2 + H2O -> HOCl + HSCl (or accept S + SO2 + HCl )"
----------------------------------------------------------------------------

I have also found a second reference (see page 6 at http://wenku.baidu.com/view/0cc172d9ce2f0066f53322c1.html ) that gives the reaction as (unbalanced):

SCl2 + H2O <-----> H2SO3 + S + HCl

which also consistent with my answer as it could be expressed as:

2 SCl2 + 3 H2O --> 4 HCl (g) + H2SO3 + S (c)

except my reaction is not reversible as I am expecting that HCl as a gas is permitted to escape from the reaction chamber.

It also appears that a precise respond to the hydrolysis of SCl2 requires specification of how much H2O and is the system sealed. The reported observation of the creation Chlorine suggests that the system could be in equilibrium when sealed (a reversible reaction) and with a drop in pH, some Cl2 could be liberated upon opening.
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Attachment: AL Chemistry 1991 Paper II Marking Scheme (2).pdf (146kB)
This file has been downloaded 4815 times

[Edited on 2-10-2012 by AJKOER]

AndersHoveland

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posted on 1-10-2012 at 13:25

I remember reading that the reaction is:

2 SCl2 + 2 H2O --> SO2 + S + 4 HCl

This is the same thing that AJKOER said, just in a different form.

AJKOER

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posted on 2-10-2012 at 05:30

A bit off topic, but perhaps more generally relevant, a few questions.

So why is the only answers to the hydrolysis of SCl2 found in Chinese high school prep exams?

Why is it that this high school have doctorates in Chemistry teaching Chemistry, and I have a teacher in my American HS in upstate New York (Rockland County to be precise) who was also the gym instructor/coach?

If this was a Chinese website, would this new topic be in Beginnings?

Is this singular example representative of what's happening in the Science race in the world?

May be I should amend the name of this thread from "Mystery of SCl2 Hydrolysis - Original Theory" to "Mystery of SCl2 Hydrolysis - Original Theory Except where Chemistry is Really Taught"

[Edited on 2-10-2012 by AJKOER]