Sciencemadness Discussion Board

Mysterious Brown Gas. Not Bromine.

SimplyChem16 - 20-7-2014 at 18:16

I had recently performed the "smoke bomb" reaction between Potassium Nitrate, and sugar. After the reaction had completed, a thick paste of yellowish/white material covered the reaction vessel. I had read this was Potassium Carbonate, so I decided to try to neutralize it simply using Hydrochloric Acid. Well, much to my surprise, lots of brown looking gas was evolved, and frankly, moved my butt away from that thing as quickly as I could. Could anybody out there provide an explanation to what happened? :)
(Nitrogen Dioxide Perhaps?)

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gdflp - 20-7-2014 at 18:24

It is unlikely that much K2CO3 was formed. Mostly it would be potassium nitrite (KNO2) depending on the ratio of sugar to nitrate. When this reacts with an acid nitrous acid (HNO2) is formed which is unstable and decomposes to water and nitrogen oxides. The brown gas is most likely nitrogen dioxide.

[Edited on 21-7-2014 by gdflp]

Zyklon-A - 20-7-2014 at 18:26

Quote: Originally posted by SimplyChem16  

(Nitrogen Dioxide Perhaps?)

Yep, your right.
Nitrogen dioxide (or more correctly speaking, dinitrogen tetroxide) is evolved when a nitrite (in this case KNO2) reacts with an acid:
2 KNO2 + 2 HCl (aq) → 2 KCl + 2 HNO2 ↔ 2 HNO2 → NO2 + NO + H2O

[EDIT] Looks like gdflp beat me to it, anyway.
Quote:

I had read this was Potassium Carbonate, so I decided to try to neutralize it simply using Hydrochloric Acid.

There probably is some K2CO3, but if the KNO3 was in excess, then it would have been partially reduced to KNO2:
2 KNO3 + H2CO (relative empirical formula for sucrose) → CO2 + H2O + 2 KNO2

Vs. the correct stoichiometry of the oxidation reaction:
2 KNO3 + 2 H2CO (relative empirical formula for sucrose) → 2 CO2 + 2 H2O + K2O

And finally, the CO2 reacts partially with the K2O to produce K2CO3:
CO2 + K2O ↔ K2CO3
. At STP, the reaction goes to completion, but at the high temperatures of the reaction, it's at equilibrium.

[Edited on 21-7-2014 by Zyklon-A]

SimplyChem16 - 20-7-2014 at 18:54

Here's a video, since I have the opportunity to provide you with one :)

http://tinypic.com/r/55p9js/8

Thanks for the replys!

Brain&Force - 20-7-2014 at 18:59

Most certainly NO2. However, it is a mixture of colorless dinitrogen tetroxide and nitrogen dioxide, which imparts the brownish color.

Zyklon-A - 20-7-2014 at 19:24

Quote: Originally posted by Brain&Force  
However, it is a mixture of colorless dinitrogen tetroxide and nitrogen dioxide, which imparts the brownish color.

Are you sure N2O4 is colorless?
Per Absorption of Nitrous Gases:
Capture.PNG - 41kB

Which states that at 30°, only ~ 12% N2O4 dissociates into NO2. So, perhaps N2O4 exhibits color as well?

I guess you're probably right, ~12% is likely enough to produce the color, we see at STP.

[Edited on 21-7-2014 by Zyklon-A]

Amos - 20-7-2014 at 21:00

In any case, SimplyChem, if the smell of nitrogen dioxide isn't enough to keep you away, remember that it is acutely toxic, in case you didn't already know. Nasty stuff.

SimplyChem16 - 20-7-2014 at 21:18

To be honest, I hadn't smelled the "characteristic sharp, biting odor" that comes with the gas. There was a fair amount of wind though :)

papaya - 21-7-2014 at 09:25

If you could improve the reaction in a way that the end products contain mostly KNO2, then you have a simple way to get nitrites (unless you can buy them).

Fantasma4500 - 21-7-2014 at 12:31

interesting.. nitrites from KNO3 is said to be relatively hard
NaNO3 is preferred at least for charcoal and molten NaNO3
surprisingly pure product if you let all the impurities settle

also NO2 doesnt have a sharp biting odor if you ask me, i'd say its more roundish and thick / choking, ofcourse more choking the more there is in the air of it
SO2 has a sharp biting odor however -- if you ask me, supposing not 100% of all chemists would agree