CHEMISTRY 253
Spring, 2015 - Dixon
Homework Set 3.1 Solutions
Text Problems not to be turned in
Ch. 5: Problems: 2, 5, 9
2. Deduce whether the following molecules will absorb infrared light due to internal vibrational motions:
a) H2 – No. This molecule is a homonuclear diatomic molecule with no dipole moment. It can’t absorb infrared light.
b) CO – Yes. The CO bond is somewhat polar (due to differences in element electronegativities despite the formal positive charge on O). It can absorb in the stretching mode.
c) Cl2 – No. This molecule is a homonuclear diatomic molecule.
d) O3 – Yes. This molecule is bent, and not all O atoms are equivalent (the middle O has a partial positive charge vs. end O atoms).
e) CCl4 – Yes. Even though the molecule is symmetrical, it still can undergo asymmetrical bond vibration transitions and bond bending transitions.
f) NO – Yes. The NO bond is somewhat polar. It can absorb in the stretching mode.
5. Carbon dioxide is also released into the atmosphere when calcium carbonate rock (limestone) is heated to produce quicklime, i.e. calcium oxide, used in the manufacture of cement:
Calculate the mass, in metric tons, of CO2 released per metric ton of limestone used in this process. What is the mass of carbon that the air gains for each gram of carbon dioxide that enters the atmosphere?
mass(CO2)/mass(CaCO3)
= (1 mol CO2/1 mol CaCO3)(44.01 g CO2/mol CO2)( 1 mol CaCO3/100.91 g mol CaCO3)
mass(CO2)/mass(CaCO3) = 0.440
mass C gained/g of CO2 = mass fraction C in CO2 = 0.273 g
9. Fully fluorinated compounds such as tetrafluoromethane and hexafluoroethane are released as by-product wastes into the air in the production of aluminum. They were also briefly considered as CFC replacements. Will such molecules have a sink in the troposphere?
No. The C-F bonds are very strong so that reaction with OH is negligible and photolysis by short wavelength UV is needed.
Will they act as greenhouse gases?
Yes. All polyatomics (3 or more atoms) absorb IR light.
Would your answers be the same for monofluoromethane and monoflouroethane?
No. Those compounds have H atoms present which can be extracted by OH reactions. Their lifetimes would still be long enough to be greenhouse gases as they should have longer lifetimes than methane and ethane.
Review Questions: 2, 4-7, 9-12, 17, 19, 20
2. What is the wavelength range, in mm, for infrared light?
In the range 0.75 mm to 1000 mm (although the range of ~50 to 1000 mm is not of great practical importance for emissions from earth).
In what portion of this range does the Earth receive IR from the Sun?
The Earth receives IR light from the sun over the whole range (considering that the sun is a blackbody emitter), but the solar IR light becomes insignificant at longer wavelengths (~4 mm) compared to IR light emitted from the Earth or the Earth’s atmosphere.
What are the wavelength limits for the thermal IR range?
The thermal IR (region of spectrum where the Earth emits significant quantities of light) is 5 to 100 mm.
4. Explain what is meant by the terms symmetric and antisymmetric bond-stretching vibrations and by angle-bending vibrations.
The terms symmetric and antisymmetric bond-stretching vibrations refer to whether atoms are moving away from a central atom in concert (both away or toward) or in opposition (one moving away while the other moving toward). Angle-bending vibrations refer to external atoms keeping the same distance from a central atom but moving toward (or away) from each other.
5. Explain the relationship between the frequency of vibrations in a molecule and the frequencies of light it will absorb.
The change in frequency of vibration should match the frequency of the light. In general, this will be close to the actual frequency of vibration.
6. Why don’t N2 and O2 absorb thermal IR?
These molecules don’t absorb thermal IR because they have no dipole moment.
Why don’t we consider CO and NO to be trace gases which could contribute to enhancing the greenhouse effect?
While both CO and NO absorb thermal IR, the absorption bands occur at wavelengths too short to be of much value for absorbing thermal IR.
7. What are the two main anthropogenic sources of carbon dioxide in the atmosphere? What is its main sink? What is fixed carbon?
The main two anthropogenic sources of carbon dioxide are combustion of fossil fuels and cement production.
The main sink of carbon dioxide is into the ocean
Fixed carbon refers to carbon removed from the atmosphere through biota in a form other than CO2 or carbonate.
9. Explain what is meant by positive and negative feedback. Give an example of each as it affects global warming.
Positive feedbacks are changes from an increase in temperature that results in a further increase in temperature while a negative feedback is a change from an increase in temperature that results in a lesser increase in temperature.
An example of a positive feedback is that warming can decrease snow cover and decrease albedo resulting in greater warming.
An example of a negative feedback is that warming can increase water vapor and cloud cover. This can increase the albedo and decrease the amount of warming. (In this particular case, we are ignoring the positive feedback from water vapor acting as a greenhouse gas).
10. What is meant by the term atmospheric window as applied to the emission of IR from the Earth’s surface? What is the range of wavelengths of this window?
The term atmospheric window refers to the region of the IR spectrum where there is only weak absorption of light. This refers to the range from 8 to 13 mm.
11. What reaction is the dominant tropospheric sink for methane?
The dominant tropospheric methane sink is oxidation initiated by the hydroxyl radical.
12. What are four important trace gases that contribute to the greenhouse effect?
Carbon dioxide, methane, nitrous oxide, and ozone are four important trace gases that contribute to the greenhouse effect. One could also include chlorofluorocarbons and/or make the case that carbon dioxide is not a trace gas.
17. What are the main sources and sinks of N2O in the atmosphere?
The main sources are in production of nylon and in release from soils, which can be enhanced with the use of fertilizer. The main sink is reaction in the stratosphere with either sunlight or O atoms.
19. By which two mechanisms does light interact with atmospheric particles?
Light can either be scattered or absorbed by atmospheric particles.
20. Explain how sulfate aerosols in the troposphere affect the air temperature at the Earth’s surface, by both the direct and indirect mechanisms.
Sulfate aerosols act to reduce air temperatures at the Earth’s surface by scattering light back to space (direct effect) or by increasing cloud brightness by making clouds contain more, but smaller and more reflective cloud droplets.