Lecture 31

The Atmosphere: Composition and Stratification and Energy Transfers

Reading Assignment: Review the chapter on gas laws in your general chemistry book (Chapter 5 in Chang) or the information on gas laws linked below. Read the chapter on atmospheric composition, linked below, by Dr. Michael Pidwirny at Okanagan University College. Read chapater 9 in Manahan.

Homework: HW-10, due Friday April 4.

Summary and Important Terms for Chapters 9 & 10

Links and Additional Resources:

Ohio State University Review of gas laws

Chem Tutor Gas Law Page

Atmospheric Composition by Dr. Michael Pidwirny at Okanagan University College

Glossary of Terms by Dr. Michael Pidwirny at Okanagan University College

TechExpo physical constants

(also called insolation) = 1.34 x 10³ w/m²
Average temperature = 15 °C -- Must be in balance (equilibrium)

Energy transfer through interaction of adjacent molecules --but no bulk movement of matter.

Movement of entire masses of air.

Energy in the form of Kinetic energy of molecules

Heat of vaporization--stored in water vapor

Electromagnetic radiation--the only way energy is transfered through a vacuum.

The science of the atmosphere

Short term variation in atmosphere

Long term averages of weather

Water content of air

The percent saturation of water in the atmosphere

The temperature at which water vapor condenses to liquid water

Provide a surface area for water vapor to condense to liquid water.

Translational Energy

Chemical bond is two electrons shared between atoms

When a photon of energy is absorbed by a molecule, one of these shared electrons moves from a "ground-state" molecular orbital to an empty "excited state" molecular orbital. When this process occurs, the paired electrons (in the unexcited molecule) become unpaired (in the excited state of the molecule). There are two possible configurations for unpaired electrons in the excited state that are called "singlet" and "triplet." Singlet refers to a situation where both electrons have different spin quantum numbers (+1/2 and –1/2) and triplet refers to the situation where both electrons have the same spin quantum number (either +1/2 or –1/2). Chemists represent these electronic energy transformations with energy level diagrams, and Figure 31.1 illustrates the energy changes that would occur if a molecule of hydrogen were to absorb one photon of energy. A requirement of the quantum theory is that the energy absorbed be equal to the difference in energy between the excited and ground states of the molecule. Electrons in the exicted state are unpaired and very reactive. Much of the chemistry that occurs in the atmosphere is explained by the presence of these highly reactive species. A stable, unpaired electron on a molecule is called a "free-radical."

Figure 31.1  Energy changes in a molecule of hydrogen when a photon of energy is absorbed.

Figure 31.2  Molecules absorbing sufficient energy to break chemical bonds and form molecular fragments containing unpaired electrons (free racicals).

Nitrogen oxides are very important in atmospheric chemistry because nitrogen contains 5 (an odd number) of valence electrons and therefore is often found in the atmosphere as a free radical.

Table 31.1 Atmospheric gases and composition of the clean atmosphere.

 

Table 31.2 Concentrations of trace substances in the troposphere and in polluted urban air (concentrations expressed in ppb).

Figure 31.2  Major regions of the earth’s atmosphere (not to scale).

Figure 31.3 The attenuation of incoming solar radiation in the earth's atmosphere.

Figure 31.4 The atmospheric oxygen cycle.

ENV 440 - Course Topics

Environmental Chemistry -- ENV 440
Last Updated:  03/31/2008