Question

4. How would you use IR to distinguish between liquid bromine (Bra) and gaseous hydrogen bromide (HBr)? Explain. Normal : BIU x*— 53 folge E T IIT (3pts) 5. What, if any, precautions must you take when using salt plates for IR sample preparations? : BIU , * . 25 = @ T

Answer 1

4. If the molecule in its equilibrium position has a dipole moment, as is always the case for the heteroatomic molecules, this dipole moment will in general change if the internuclear distance changed. Thus, on the basic of classical electrodynamics the molecular vibration would lead to the emission of light at the oscillation frequency. Conversely, the oscillator could be set in vibration by absorption of light at this frequency. Therefore, all heteroatomic molecules in principle are said to be infrared active, that is they can absorb or emit infrared radiation. Contrary, all homoatomic diatomic molecules do not have any dipole moment and cannot set in vibration by absorption the infrared light. These molecules are said to be infrared inactive.

As Br2 is homo diatomic molecule which does not has any dipole moment and hence IR inactive while HCl gas is hetroatomic molecule which generates dipolemoment when vibrations are induced in the molecule. Therefore HCl is IR active.

5. To prepare a liquid sample to IR analysis, firstly place a drop of the liquid on the face of a highly polished salt plate (such as NaCl, AgCl or KBr), then place a second plate on top of the first plate so as to spread the liquid in a thin layer between the plates, and clamps the plates together. Finally wipe off the liquid out of the edge of plate. You can mount the sandwich plate onto the sample holder. After finishing your experiment, clean the plates with isopropanol (No water!) and return them to the desiccators.

Precaution: Volatile liquid can’t be prepared with this method, because it will evaporate while its spectrum is being obtained. If the liquid sample is toxic or smelly, please don’t use this method. In addition, NaCl and KBr are dissolved into water, and thus they can’t be used for aqueous samples.