L7 - Gas phase chemistry Flashcards
What are the primary components of dry air and their approximate percentages?
Nitrogen (78%), Oxygen (21%), Argon (0.934%), and Carbon Dioxide (0.04%)
How much water vapor is present in the atmosphere?
Between 0–3%, depending on temperature and location.
What units are used to express gas concentrations in the atmosphere?
Molecules per cubic centimeter (molecules cm⁻³)
What is Avogadro’s number, and how is it used in gas calculations?
Avogadro’s number is
6.02×10 ^23, used to convert moles to molecules
What is the ideal gas law and its significance in atmospheric chemistry?
P=nRT, which helps determine the number of molecules in a given volume under specific conditions
What is the molecular density of air at standard temperature and pressure (STP)?
2.7×10 ^19 molecules cm⁻³.
What does 1 ppm represent in gas concentration?
One molecule of gas per million molecules of air
How is 1 ppb different from 1 ppm?
1 ppb represents one molecule per billion (10⁹) molecules of air
What are unimolecular reactions?
Reactions involving the decomposition or rearrangement of a single molecule (e.g.,
A→B+C)
What are bimolecular reactions?
Reactions involving two reactants (e.g.,
A+B→C+D)
What are trimolecular reactions?
Reactions involving three reactants, often association reactions (e.g.,
A+B+M→AB+M)
What is the rate of reaction for a simple bimolecular reaction?
Rate=k[A][B], where
k is the rate coefficient
What are the units of the rate coefficient for a first-order reaction?
s ^-1
What are the units of the rate coefficient for a second-order reaction?
molecules
−1cm ^3 s ^−1
What are the units of the rate coefficient for a third-order reaction?
molecules ^−2cm ^6 ^−1
How is the chemical lifetime of a reactant defined?
The time it takes for a reactant to fall to
1/e of its original concentration
What is the equation for the chemical lifetime of a first-order reaction?
T=1/k A
How is the chemical lifetime of a second-order reaction calculated?
T=1/(k AB[B])
How is the chemical lifetime of a third-order reaction calculated?
T=1/(k ABM [B][M])
What is photochemistry?
The study of chemical reactions initiated by sunlight
What is photolysis?
The decomposition of molecules by the action of light.
Write the general equation for a photolysis reaction
A+hν→products
What is the rate equation for photolysis?
d[A]/dt=−J[A], where
J is the photolysis rate coefficient
What happens when NO₂ absorbs light with a wavelength less than 424 nm?
It photolyzes to form NO and O (NO2 + hν→NO+O)
How is ozone (O₃) formed in the atmosphere?
O+O2 +M→O 3+M
How does ozone react with NO?
NO+O 3→NO 2 +O2
What is the most common order of reactions in atmospheric chemistry?
Bimolecular reactions, which are overall second order
What is required to initiate atmospheric photochemistry?
Access to atomic oxygen (O) and energy from sunlight.
Why is the photochemical cycle of NO, NO₂, and O₃ important?
It plays a crucial role in ozone formation and regulation
