Non-NMR Spectroscopies Solutions: #9

9. * (1993 F B1E) IR absorption. The photochemical decomposition of HI in the gas phase at 266 nm results in the production of H2 and I2 in the gas phase. It is found experimentally that two molecules of HI are decomposed per photon absorbed. This information suggests two possible mechanisms in which the heat released is indicated for each step (recall that negative DH corresponds to positive heat release):

Mechanism I

DH (kcal/mol)

hn + HI ----> H + I

(fill in from part D)

I + HI ----> H + I2

+35

H + H + M ----> H2 + M

-104

Mechanism II

DH (kcal/mol)

hn + HI ----> H + I

(fill in from part D)

H + HI ----> H2 + I

-33

I + I + M ----> I2 + M

-36

In each mechanism M represents a third body that stabilizes the collision process.

Would it be possible to follow concentrations of reactants and products by infrared absorptions? Discuss.

It's possible to follow the concentration of the reactant (HI) since it has a dipole moment and is therefore IR active. It is not possible to follow the concentrations of the products (H2, I2, or H and I, depending on which reaction you thought the question referred to.) In either case, there is no induced dipole from the vibration and therefore the products are not IR active.