e , rate of disappearance of any of the reactants is same as   the rate
of appearance of the products But in the following reaction, two moles of
HI decompose to produce one mole each of H2 and I2,
2HI(g)   ®   H2(g)   +   I2(g)
For expressing the rate of such a reaction where stoichiometric
coefficients of reactants or products are not equal to one, rate of
disappearance of any of the reactants or  the rate of appearance of
products is divided by their respective stoichiometric coefficients Since
rate of consumption of HI is twice the rate of formation of H2 or I2, to
make them equal, the term D[HI]  is divided by 2