UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 218

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QUESTIONS AND PROBLEMS

1. Describe the differences between absorption, emission, and fluorescence?
2. Calculate (a) the energy (in eV) of a 5.3- ˚ X-ray photon and (b) the energy (kJ/mol)
of a 530-nm photon of visible radiation. Given the following constants and unit conver-
sion factors: 1 ˚ ¼10 10 m, 1 nm ¼ 10 9 m, 1 nm¼10 ˚ ,1J¼ 6:2610 18 eV, Planck
constant ðhÞ¼6:6310 34 Js, speed of light ðcÞ¼310 8 m=s.
3. Use molecular orbital theories to explain: (a)Why hexane andwater can be used as a solvent
for UV absorption spectrometry? (b) Why CO 2 has two sorption bands in the mid-IR
range? (c) Why the frequency of bonds is in the increasing order of: CC<C

C<C C?

4. Explain: (a) How two s orbitals form a s bond? and (b) How two p orbitals form a
s bond?
5. Explain in general: (a) Why wavelength of less than approximately 180 nm is hardly
used in UV spectrophotometer? (b) Why IR is less sensitive than UV but more useful
than UV to deduce the structure of unknown chemicals? (c) Why CO 2 and water vapor
always present problem when measuring atmospheric contaminants using IR?
6. Define/describe the following terms: (a) the difference between absorption spectroscopy
and emission spectroscopy, (b) p electrons and n electrons, (c) sigma bonding and
sigma antibonding molecular orbital, (d) dispersive IR and nondispersive IR?
7. Which of the following electronic transition(s) is usually concerned with UV-VIS
spectroscopy of organic compounds: (a) s ! s*, (b) n ! p and p ! p*, (c) n ! s*?
8. Draw electron configuration (1s 2 ,2s 2 ,...) for (a) C and (b) O. The atomic number for C
and O are 6 and 8, respectively.
9. Draw the electron dot formula for: (a) O 2 , (b) H 2 O, and (c) CH 4 .
10. Explain: (a) Why do you think it takes more energy (shorter wavelengths, higher
frequencies) to excite the stretching vibration than the bending vibration? (Refer to
Fig. 8.9 for CO 2 ), (b) Why C

C triple bonds have higher stretching frequencies than

C

C double bonds, which in turn have higher stretching frequencies than CCsingle
bonds?
11. Refer to Figure 8.12 and explain why the IR absorption wave number for C

N is greater

than that for CN?
12. (a) Explain what the linear molecules are and what nonlinear molecules are?
(b) Calculate how many vibrational modes are there for CO and H 2 S?
13. Describe the difference between a typical UV spectrum and an IR spectrum?
14. Explain why the nonlinear deviation in IR is more severe than UV-VIS?
15. Which one of the following is true regarding Beer's law: (a) Absorbance is proportional to
both path length and concentration of absorbing species? (b) Absorbance is proportional
to the log of the concentration of absorbing species? (c) Absorbance is equal to P 0 /P?