L7 - Gas phase chemistry

Question 1

What are the primary components of dry air and their approximate percentages?

Answer 1

Nitrogen (78%), Oxygen (21%), Argon (0.934%), and Carbon Dioxide (0.04%)

Question 2

How much water vapor is present in the atmosphere?

Answer 2

Between 0–3%, depending on temperature and location.

Question 3

What units are used to express gas concentrations in the atmosphere?

Answer 3

Molecules per cubic centimeter (molecules cm⁻³)

Question 4

What is Avogadro’s number, and how is it used in gas calculations?

Answer 4

Avogadro’s number is
6.02×10 ^23, used to convert moles to molecules

Question 5

What is the ideal gas law and its significance in atmospheric chemistry?

Answer 5

P=nRT, which helps determine the number of molecules in a given volume under specific conditions

Question 6

What is the molecular density of air at standard temperature and pressure (STP)?

Answer 6

2.7×10 ^19 molecules cm⁻³.

Question 7

What does 1 ppm represent in gas concentration?

Answer 7

One molecule of gas per million molecules of air

Question 8

How is 1 ppb different from 1 ppm?

Answer 8

1 ppb represents one molecule per billion (10⁹) molecules of air

Question 9

What are unimolecular reactions?

Answer 9

Reactions involving the decomposition or rearrangement of a single molecule (e.g.,
A→B+C)

Question 10

What are bimolecular reactions?

Answer 10

Reactions involving two reactants (e.g.,
A+B→C+D)

Question 11

What are trimolecular reactions?

Answer 11

Reactions involving three reactants, often association reactions (e.g.,
A+B+M→AB+M)

Question 12

What is the rate of reaction for a simple bimolecular reaction?

Answer 12

Rate=k[A][B], where
k is the rate coefficient

Question 13

What are the units of the rate coefficient for a first-order reaction?

Answer 13

s ^-1

Question 14

What are the units of the rate coefficient for a second-order reaction?

Answer 14

molecules
−1cm ^3 s ^−1

Question 15

What are the units of the rate coefficient for a third-order reaction?

Answer 15

molecules ^−2cm ^6 ^−1

Question 16

How is the chemical lifetime of a reactant defined?

Answer 16

The time it takes for a reactant to fall to
1/e of its original concentration

Question 17

What is the equation for the chemical lifetime of a first-order reaction?

Answer 17

T=1/k A

Question 18

How is the chemical lifetime of a second-order reaction calculated?

Answer 18

T=1/(k AB[B])

Question 19

How is the chemical lifetime of a third-order reaction calculated?

Answer 19

T=1/(k ABM [B][M])

Question 20

What is photochemistry?

Answer 20

The study of chemical reactions initiated by sunlight

Question 21

What is photolysis?

Answer 21

The decomposition of molecules by the action of light.

Question 22

Write the general equation for a photolysis reaction

Answer 22

A+hν→products

Question 23

What is the rate equation for photolysis?

Answer 23

d[A]/dt=−J[A], where
J is the photolysis rate coefficient

Question 24

What happens when NO₂ absorbs light with a wavelength less than 424 nm?

Answer 24

It photolyzes to form NO and O (NO2 + hν→NO+O)

Question 25

How is ozone (O₃) formed in the atmosphere?

Answer 25

O+O2 +M→O 3+M

Question 26

How does ozone react with NO?

Answer 26

NO+O 3→NO 2 +O2

Question 27

What is the most common order of reactions in atmospheric chemistry?

Answer 27

Bimolecular reactions, which are overall second order

Question 28

What is required to initiate atmospheric photochemistry?

Answer 28

Access to atomic oxygen (O) and energy from sunlight.

Question 29

Why is the photochemical cycle of NO, NO₂, and O₃ important?

Answer 29

It plays a crucial role in ozone formation and regulation