3-31 No. 6

Iodine vapor dissociates according to the equation:

I₂ à 2I                        K = (I)² / (I₂)

The equilibrium constant for this reaction is 3992 and 0.0002 at 1000°C and 600°C respectively. What is the heat of reaction and % dissociation at 900°C?

The equilibrium constant is always defined as   pi (species)^(stoich. coeff).  But the stoichiometry itself is chosen.  You must connect the assumed stoichiometry of the reaction to the equilbrium coefficient value.

Example:  I2 ßà 2I,    K = (I)^2 / (I2)

But, if the reaction is written as 2I ßà I2, then K’ = (I2) / (I)^2

and K = 1/K’

You’ll get the right answer as long as the expression for K matches the stoichiometry assumed in calculating what K is.