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Topic: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?  (Read 23684 times)

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Offline Bob Sacamano

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3M hydrochloric acid was added to Sn(s) and KI(s) and heated under reflux in an inert atmosphere. Why must the reaction be carried out under an inert atmosphere?

Initially I had thought it was to prevent the moisture in the air from entering the system. But there is already plenty of water in the solution because aq HCl was added. Then I thought that the the following reaction would take place:

Sn (s) + 2 HCl (aq) → SnCl2 (aq) + H2 (g)

Then I have the Sn(II) required to form the stannous iodide but its bonded to Cl... And besides that still doesn't explain why the reaction must be carried out under an inert atmosphere.

Offline Borek

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #1 on: September 25, 2010, 05:27:57 PM »
What is atmosphere composition?
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Offline Bob Sacamano

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #2 on: September 25, 2010, 05:54:47 PM »
N2 - inert
H2O - already present in the system
Ar - inert

Leaving oxygen as the most likely suspect.

So assuming the inert atmosphere is to prevent the oxidation of the SnCl2, the SnCl2 must be somehow converted to SnI2 making SnCl2 an intermediate.

But I don't get why a chlorinated intermediate is formed first? why isn't the SnI2 formed immediately?


Offline Borek

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #3 on: September 25, 2010, 07:04:25 PM »
Perhaps because HI is much more expensive and difficult to obtain?
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Offline Bob Sacamano

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #4 on: September 25, 2010, 07:21:05 PM »
Yes, but the potassium iodide is also in solution. Why do we need the HCl to form stannous chloride before it is converted to stannous iodide?

Previously we synthesized tin(iv) iodide but did not use HCl as a solvent.

Is the stannous chloride complex somehow required to form as an intermediate in the process of synthesizing stannous iodide?

Offline Borek

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #5 on: September 26, 2010, 03:36:40 AM »
You need to oxidize tin - what will be oxidizing it in just a KI solution?

How did you made SnCl4?
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Offline Bob Sacamano

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #6 on: September 26, 2010, 02:44:24 PM »
We didn't make SnCl4 to get SnI4. We just heated Sn and KI in a glacial acetic acid / acetic anhydride solution at very high temperature and then crystallized the SnI4 out of the solution.

I don't see why the Cl was needed for the one reaction but not the other.

Offline Borek

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #7 on: September 26, 2010, 03:06:57 PM »
Was the solution in contact with the air (oxygen)?
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Offline Bob Sacamano

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #8 on: September 26, 2010, 03:23:31 PM »
Ahhhhh. I see. In that reaction the Sn is being oxidized by atmospheric oxygen to yield tin dioxide. And in the other reaction the tin is being oxidized by the Cl.

But why does the Cl induce the 2+ oxidation state and oxygen induce the +4 oxidation state?

Offline Borek

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #9 on: September 26, 2010, 04:55:45 PM »
It is not Cl- that does the oxidation.

And atmospheric oxygen was not to yield tin dioxide. I think it is a clever combination of reagents - including acetic anhydride - that removes any traces of water that can be produced in the oxidation process, thus leaving just Sn(IV) that precipitates with iodide.
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Offline Bob Sacamano

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #10 on: September 27, 2010, 11:35:47 AM »
Hmmm.

SnI2 case:

Sn(s) + 2H+ :rarrow: Sn2+ + H2

Now, we still have the Cl- and I- in solution and have oxidized the Sn to a 2+ state. Why does stannous iodide now crystalize instead of stannous chloride? Since chloride is a better nucleophile won't it bond preferentially?

SnI4 case:

The acetic anhydride will react with any water in the system to produce more acetic acid. Is H+ again acting as an oxidizing agent in this reaction? If so, why is this Sn complex oxidized to a 4+ state? The higher heat? and why is this system allowed to interact with atmospheric oxygen?

Offline Borek

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #11 on: September 27, 2010, 12:32:01 PM »
I guess the solubility of the iodides is much smaller.

You are right about H+ being an oxidizing agent.

Oxygen is much stronger oxidizing agent than H+.
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Offline Bob Sacamano

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #12 on: September 27, 2010, 03:12:18 PM »
These are my proposed mechanisms:

SnI4 case:

Sn(s) + O2 :rarrow: SnO2

SnO2 + 4I- + 4H+ :rarrow: SnI4 + 2H2O

SnI2 case:

Sn(s) + 2H+ :rarrow: Sn2+ + H2

Sn2+ + 2I- :rarrow: SnI2

Am I on the right track without these?





Offline Borek

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #13 on: September 27, 2010, 03:37:46 PM »
I told you - I doubt SnO2 was present in the meantime.

Seems to me like you learned organic chemistry before inorganic, people that go that way try to find some strange mechanisms for inorganic reactions. That's not the way inorganic chemistry is done ;)

You have to write probable half reaction for oxidizing agent - how it gets reduced. How the tin is oxidized is quite obvious, it just loses 4 electrons.
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Offline Bob Sacamano

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Re: Why must stannous iodide (SnI2) be synthesized under an inert atmosphere?
« Reply #14 on: September 27, 2010, 09:40:36 PM »
Ok I'll take another stab at it. Although I must admit that my redox chemistry is lacking.

Sn(s) :rarrow: Sn4+ + 4e-

O2 + 4e- :rarrow: 2O22-

2O22- + 4H+ :rarrow: 4H2O

Sn4+ + 4I- :rarrow: SnI4

That's my best guess. Although I still don't see why stable O2 would take up the electrons to form O2-.


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