Why do Alkaline Metals React Violently with Water?

[susperia_knvb]

just a general interest thing for chemists.
i was shown this youtube video the other day:
http://youtube.com/watch?v=gRuoWkOlaHM

it is the show Brainiac and they are demonstrating Alkaline Metals reacting with water. They show brief Lithium, Sodium and Potassium reactions;

then drop 2g of Rubdium and Caesium into different bathtubs.
and wow. :)

must see!

 

[jonegil]

be careful susperia...alkaline metals are very dangerous reacting with water...a single piece of potassium can make a violent explosion

 

[susperia_knvb]

id never do it myself... i just like the power of these metals and seeing them actually do it. people in our chem class are always joking about what would happen and now we have seen :) so the desire is gone :) but just wow.

 

[jonegil]

lol...yeah..but recently there were a bunch of kids in my city who started to make bombs with aluminium and sulphuric acid and throw them to the sidewalks...they were arrested some weeks ago and they actually hurt a two years old boy

 

[susperia_knvb]

wow. that's so stupid.seriously some kids are so thick its unbelievable. i heard something about hydrogen balloon bombs in a rural town in australia once. not sure how they worked thou.

 

[jonegil]

yep...hydrogen is xplosiv 2 in contact wit oxygen

 

[mrjeffy321]

I have heard that this show (Brainiac) has been known to fake some of their experiments, specifically some of the explosive ones…and even more specifically the video clip you mention with the Alkali metals.
Not that what they show is an inaccurate portrayal of what would actually happen with Cs or Rb are dropped in water, but the magnitude/destructive force might have been exaggerated/changed for the sake of making it more entertaining for the viewing audience at home.

 

[chemisttree]

Brainiac may have exaggerated the effect but these guys didn't!

http://video.google.com/videoplay?docid=3825610222960975525

 

[chemisttree]

Actually, the rubidium stunt may not have been exaggerated. I once was cleaning out a schlenk that contained several grams of rubidium residue. I added butanol directly to it without a septum or an argon blanket. REALLY STUPID! The butanol was 2-butanol instead of tert-butanol... also stupid. I squirted in a small amount of the butanol and nothing serious happened. Apparently a thin crust of oxide had managed to form on the stuff and was moderating (temporarily) what was soon to be an extremely vigorous reaction. Suddenly, the schlenk shattered with a very loud bang sending very small glass shards everywhere in the hood. I was left holding a small glass sidearm. I at least had the good sense to conduct this behind a blast shield but I'm still amazed that my hands weren't trashed.

Lesson: Rubidium should be left to slowly oxidize by itself in the hood or covered with oil. A tertiary alcohol should be added in small increments only to oil covered metal under an argon (not nitrogen) blanket. Blastproofing is a must and the schlenk should be wrapped with duct tape and placed in a plastic bucket behind a blast shield during the operation. Signage should indicate that only the responsible party is to approach the hood until the operation is complete. Work with a buddy...

 

[susperia_knvb]

haha. Rubidium is awesome stuff. its so amazing that such small quanities of an element can create such a big bang! especially Ceasium... if that clip is indeed true... well it split the bathtub in half! 2grams!

there once was a teacher at our school who threw a block (maybe 3-4 possibly 5grams) og Na straight into a metal sink filled with water... with no safety screen (he had a hangover) and bang! there was molten sodium on the roof and kids yelling... the sink deflected the whole blast upwards thou so lucky no one was hurt!