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Topic: Potassium halides  (Read 4289 times)

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Offline Big-Daddy

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Potassium halides
« on: February 28, 2013, 04:09:27 PM »
My syllabus reads "describe and explain the reactions of potassium halides with concentrated sulfuric acid, halogens and silver nitrate solution, and what is observed". I don't really understand what this is asking. Does it mean KX with conc. H2SO4, KX with X2 and KX with AgNO3?

The alternative is that there is some special reaction or mechanism - which you could tell me of - which occurs when you mix together all of these ingredients.

If we are going by the first case, separate reactions, then please correct my understanding:

KX with conc. H2SO4 (or conc H3PO4) will form HX solution and H2SO4- ion as these oxidize the X-. (Not sure about "oxidation" - the X is losing an electron but it is also gaining H and the oxidation state isn't really changing. Please clarify) If X is Br or I then you will get some Br2 or I2 in solution, with conc H2SO4, not with H3PO4.

KX with halogens - X- ions can change, so that a smaller halogen oxidizes a larger halide to turn the smaller halogen into a small halide and the larger halide into a large halogen. This halogen will be dissolved in the solution (even for I2). If F2 is entered into solution it will react with the water too quickly to form HF and so won't oxidize whatever halide ions are currently in there. In the special case of I2 being entered into solution with I- from KI, the I3- ion will be formed, and though the ion itself is red, the solution will turn from yellow (dilute) to brown (concentrated) depending on concentration of I3-. (I don't understand how it can turn further than red?)

KX with AgNO3. X- ions in solution go with the Ag+ ion in solution to form the AgX precipitate. All AgX precipitates darken in sunlight due to some Ag-X bonds being broken to produce Ag (s); AgI darkens quickest, AgCl slowest (AgF is soluble so does not precipitate).

Please help me! Please tell me if my understanding is correct in each case and add any additions or corrections you would make. I will very much appreciate it!
« Last Edit: February 28, 2013, 04:24:59 PM by Big-Daddy »

Online Borek

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Re: Potassium halides
« Reply #1 on: March 01, 2013, 03:32:13 AM »
I don't see anything blatantly wrong.
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Offline AWK

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Re: Potassium halides
« Reply #2 on: March 01, 2013, 04:23:57 AM »
Quote
KX with AgNO3. X- ions in solution go with the Ag+ ion in solution to form the AgX precipitate. All AgX precipitates darken in sunlight due to some Ag-X bonds being broken to produce Ag (s); AgI darkens quickest, AgCl slowest (AgF is soluble so does not precipitate).
AgI does not decompose at all.
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Online Borek

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Re: Potassium halides
« Reply #3 on: March 01, 2013, 04:58:18 AM »
AgI does not decompose at all.

Strange. I do remember it being listed as one of the photosensitive silver halogens used in B&W photography. Wikipedia (all possible disclaimers here) claims AgI is always contaminated with metallic silver due to the decomposition. BielaƄski in his Inorganic Chemistry states "all silver halogens decompose in the light".
« Last Edit: March 01, 2013, 05:28:21 AM by Borek »
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Offline Big-Daddy

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Re: Potassium halides
« Reply #4 on: March 01, 2013, 04:52:35 PM »
Thank you very much.

I would always tend to trust Wikipedia myself.

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