Aluminum, iodine and water?

Question

What exactly happens in the reaction involving powdered aluminum and crushed iodine? I know that adding a few drops of water produces a lot of purple smoke and that it will flare up but what is the chemistry involved? And also, I know that the white residual left in the cup is Al2I6 but why is it acidic when water is added to it? It has a pH of between 1 and 2. What is the smoke that is produced and what causes the exothermic reaction?

Answer 1

iodine is an oxidizer, a good taker of electrons

Aluminum is a great loser of electrons

you have a redox reaction happening where Al  loses electrons to become Al+3 , and Iodine takes electrons to become I-1 iodide

since the reaction is forming the most stable material Al2I6, the reaction must be exothermic, when products are more stable than reactants,... the excess energy must be released.

it is the release of this heat that evaporates some of the iodine crystals into the purple iodine vapor

Al2I6 does a hydrolysis reaction with water:

Al2I6  in water -->  Al(OH)3  & 3 HI

the byproduct Al(OH)3 is stable & not soluble (a very weak base)

but the byproduct HI is a very strong acid

what is the smoke? upon adding water?  could be steam,

the purple smoke was iodine vapor