Explanation of the Zinc and Iodine Demonstration

Solid elemental zinc and an excess of solid elemental iodine are ground together in a mortar and pestle to ensure a uniform and finely ground mixture and placed into a large test tube.  A large (6-L) flask is inverted and placed above the test tube to capture any vapor that is created.  A small volume of water is added to the mixture to initiate the reaction, and the flask is very quickly replaced.

As some of the iodine dissolves and becomes more mobile, it attacks the zinc to form zinc iodide.  This is a highly exothermic reaction; the heat of the initiation speeds up the reaction, which generates more heat; the great heat not only causes the test tube to become too hot to handle (as demonstrated when I put my hand on the test tube and quickly withdrew it in pain), but the heat immediately causes the sublimation of the excess iodine.  This sublimation creates a great cloud of purple vapor, which rises and fills the inverted flask.

Why didn't the reaction begin without the water?  Apparently the activation energy is too large.  Adding the water provides a new path unavailable before.

(1)       Zn(s)   +  I₂(s)  -->  ZnI₂(s)             DH  =  -208 kJ/mol

(2)    ZnI₂(s)   +   H₂O(l)   -->   ZnI₂(aq)     DH  =  -56.23 kJ/mol

When the reaction begins in the presence of water, sufficient thermal energy is produced to melt and vaporize the iodine.  The vaporization of iodine accounts for the violet smoke.  Since water can be recovered after the reaction, it acts in this reaction as a catalyst.  Reactions between solids at room temperature are frequently slow since the reaction can only occur at the solid surfaces.  The rate increases rapidly with time due to the temperature increase that follows the exothermic reactions, at which time the water may n longer be necessary to maintain a high reaction rate.  The initial thin coating of ZnI₂(s) on the zinc is solubilized by the drop of water.

Concepts:  Catalyst, Sublimation, Exothermic

Time Required:  15 minutes

Chemicals Needed:  powdered zinc, iodine crystals

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