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Topic: HCl fumes  (Read 9025 times)

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Offline bakerbg

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HCl fumes
« on: May 07, 2011, 07:58:53 PM »
I got this question in my exam:

when SiCl4 is exposed to humidity in the atmosphere ( H2O) white fumes were observed evolving. What is the name and formula of the white fumes?

I say it is Hydrogen chloride gas, HCl(g). Am I right?

Offline Teddy3333

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Re: HCl fumes
« Reply #1 on: May 07, 2011, 10:57:27 PM »
I have found Wikipedia to be very helpful. This is what it said about "Silicon tetrachloride:"
Quote
Like other chlorosilanes, silicon tetrachloride reacts readily with water:

    SiCl4 + 2 H2O → SiO2 + 4 HCl

In contrast, carbon tetrachloride does not hydrolyze readily. The differing rates of hydrolysis are attributed to the greater atomic radius of the silicon atom, which allows attack at silicon. The reaction can be noticed on exposure of the liquid to air, the vapour produces fumes as it reacts with moisture to give a cloud-like aerosol of hydrochloric acid.[2] With methanol and ethanol it reacts to give tetramethyl orthosilicate and tetraethyl orthosilicate:

    SiCl4 + 4 ROH → Si(OR)4 + 4 HCl

You were right, I believe.

Offline bakerbg

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Re: HCl fumes
« Reply #2 on: May 07, 2011, 11:45:35 PM »
The article from wikipedia said it is Hydrochloric acid which is observed as white fumes, NOT Hydrogen chloride gas ( HCl(g) ) which is colourless.

How would you best describe the white fumes teddy3333? HCl(g) or HCl(aq)

Offline Arkcon

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Re: HCl fumes
« Reply #3 on: May 08, 2011, 07:26:55 AM »
The white fumes you see are HCl (g), which is very hygroscopic, pulling moisture out of the air.  Technically, it is HCl (aq), but calling it that is cheating people out of information:  (l) and (aq) imply an accumulation of fluid, in a vessel of some sort, maybe out of the way, and seal-able.  Those white fumes are going everywhere, behaving like a gas.  That's just from a safety perspective mind you, if you have some reaction that only proceeds anydrous, these fumes aren't good for it, but still, you specify you want HCl (g) anhydrous, you don't call the fumes HCl (aq)
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline bakerbg

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Re: HCl fumes
« Reply #4 on: May 08, 2011, 10:36:50 PM »
Thanks for your reply. Given the fact that the question specify that the "white fumes" were only seen when SiCl4 was exposed to Humid air - I think they are expecting you to write the Aqueous form Of HCl. You know these examiners: always matching wits ! lol

Offline vmelkon

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Re: HCl fumes
« Reply #5 on: May 21, 2011, 01:16:51 PM »
What about the SiO2?

Offline bakerbg

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Re: HCl fumes
« Reply #6 on: May 21, 2011, 01:33:56 PM »
...Guess the SiO2 just settle at the bottom of the reacting vessel- The question didn't ask.

SiCl4(l) + 2H2O(l)  :rarrow: SiO2(s) + 4 HCl(g)    colourless gas evolve


HCl(g) + aq :rarrow:HCl(aq)                             White fumes


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