Chapter 11 The Air Around Us

Nivaldo J. Tro

http://www.cengage.com/chemistry/tro

Mark Erickson • Hartwick College

Chapter 11 The Air Around Us

Air Bags• Impact trips a sensor that activates the reaction:

• The gaseous product of the reaction occupies 450 times more space than does the solid reactant.

A Swarm of Particles• The air around us, a swarm

of molecules not unlike a swarm of gnats, is tasteless, odorless, and invisible; yet we can feel its effects, and we depend on it every moment of our existence.

Pressure: A Swarm of Particles• Gas molecules are in constant

motion, colliding with each other and with the walls of their container.

• The sum of these collisions is called pressure (force per unit area).

• Pressure units: 1 atm– 14.7 pounds per square inch– 1 atmosphere– 760 torr– 760 millimeters of Hg

Pressure• Pressure is directly proportional to the number of gas

molecules in the air. Pressure can change.

• Ears have small cavities within them that trap gas molecules.

• When the external pressure and the internal pressure are not equal, the eardrum is stressed.

• Pressure equalization– Ears– Sinuses

Pressure• The pressure principle at work

in a barometer allows us to drink through a straw.

• The heights of liquids in a barometer tube depend on the identity of the liquid. For convenience, more dense substances (like mercury) are commonly used in barometers.

Pressure and Weather• High atmospheric pressure

redirects storms (sign of fair weather).

• Low atmospheric pressure tends to draw storms in (sign of rainy weather).

• Changes in pressure are responsible for wind.

Gas Property Relationships• Fundamental properties of gases:

– Pressure (represented by P)– Amount (measured in moles, represented by n)– Volume (usually expressed in liters and represented by V)– Temperature (expressed in Kelvin (K) and represented by

T)

• If one of these properties is changed, the others will also change.

Boyle’s Law• If the pressure of a gas is

increased at a constant temperature, its volume will decrease.

• So, pressure and volume are inversely related: P1V1 = P2V2.

• Molecules in a smaller space collide more frequently with each other and the walls of their container.

Concept Check 11.1

• A weather balloon is inflated to a volume of 55.0 L at sea level where the atmospheric pressure is 755 mm Hg. Find the volume of the balloon as it rises to an altitude where the atmospheric pressure reduces to 135 mm Hg. (Assume constant temperature.)

Concept Check 11.1 Solution

• V1 = 55.0 L P1 = 755 mm Hg constant T

• P2 = 135 mm Hg P2 = ?

• Boyles Law: P1V1 = P2V2

1 12

2

2

=

755 mm Hg =

PVV

P

V 55.0 L

135 mm Hg = 308 L

Charles’ Law• If the temperature of a gas is

increased at constant pressure, its volume will increase.

• So, temperature and volume are directly related:

• As molecules absorb heat energy, they strike each other and the walls of their container with more force, producing larger force per unit (pressure).

1 2

1 2

V V

T T

Concept Check 11.2

• A balloon was filled at an initial temperature of 25ºC and an initial volume of 3.0 L. What would be its volume at −15ºC? (Assume constant pressure.)

Concept Check 11.2 Solution• V1 = 3.0 L T1 = 25ºC constant P

• V2 = ? T2 = −15ºC

• Use Charles Law: T must be in Kelvin

• Temperature conversion to K:

• Solving for V2:

1 2

1 2

V V

T T

2 12

1

258 K =

T VV

T

3.0 L

298 K = 2.6 L

o o1 2 = 25 C +273 = 298 K = -15 C +273 = 258 KT T

Combined Gas Law

• Sometimes more than one property of a gas changes at one time.

• The Combined Gas Law combines Boyle’s Law and Charles’ Law.

1 1 2 2

1 2

PV PV

T T

Concept Check 11.3• A weather balloon is initially inflated to a volume of 53.0 L at a

pressure of 1.00 atm and a temperature of 25ºC. The balloon then levels off at an altitude where the atmospheric pressure drops to 0.40 atm and the temperature drops to −20.ºC. What will be the volume of the balloon at the final altitude?

Concept Check 11.3 Solution

• P1 = 1.00 atm V1 = 53 L T1 = 25ºC

• P2 = 0.40 atm V2 = ? T2 = −20.º C

• Combined Gas Law: T must be in Kelvin

• Temperature conversion to K:

• Solving for V2:

1 1 2 2

1 2

PV PV

T T

1 2 12

2 1

1.00 atm =

PT VV

P T

253 K 53 L

0.40 atm 298 K 2 = 1.1 10 L

o o1 2 = 25 C + 273 = 298 K = -20 C + 273 = 253 KT T

The Atmosphere: What’s in It?• Nitrogen

– Tasteless, colorless, nonflammable, relatively inert– Nitrogen compounds are a limiting factor in plant growth.

• Oxygen– Oxygen reacts with glucose during respiration.

• Carbon dioxide– Central to plant growth through photosynthesis

• Argon, neon, and helium– Chemically unreactive

The Atmosphere: What’s in It?

The Atmosphere: A Layered Structure

Four Sections of the Atmosphere• Troposphere

– All Earth-bound life exists and all weather phenomena occur here.

– Ozone exists here as a pollutant.

• Stratosphere– Contains UV-absorbing ozone– Ozone is a natural and

necessary component of this section.

• Mesosphere and Ionosphere– “Falling stars” and the aurora

borealis occur here.

Problems in the Troposphere: Sources

• SO2

– Emitted from coal-burning power plants and industrial smelters

• PM-10– Particulate matter (PM) with diameters less than or equal

to 10 μm originating from agricultural tilling, construction, and unpaved roads.

• CO– Emitted from motor vehicles

• O3

– Formed by the action of sunlight on vehicle exhaust• NO2

– Emitted from motor vehicles• Pb

– Emitted primarily from smelters and battery plants

Adverse Effects• SO2

– Respiratory irritant and precursor to acid rain• PM-10

– Aggravates existing respiratory and cardiovascular disease, damage to lung tissue, and causes cancer

• CO– Diminishes the blood’s ability to carry oxygen

• O3

– Reduces lung function and prolonged exposure can permanently damage lung tissue

• NO2

– Brown color of smog and contributes to acid rain– Lung and eye irritant

• Pb– Damages kidneys, liver, and nervous system.

The Clean Air Act• A 1970 federal law regulating emissions

• Amendments passed in 1977 make regulation more enforceable

• Overall pollution in urban areas is less even though there are more vehicles.

• Vehicles today emit 60–80% fewer pollutants than those built in the 1960s.

• Ground-level ozone is a persistent problem.

• Additional Clean Air Act amendments were passed in 1990.

The Clean Air ActNumber of days in which the National Air Quality Standard was not attained for at least one pollutant in selected cities from 2001–2008

The Clean Air Act• Ozone, even at low concentrations, can significantly reduce

lung function. It also damages rubber, agricultural crops, and trees.

Ozone Depletion:Problems in the Stratosphere• Two types of UV light : UV-A and UV-B

• UV-B is shorter in wavelength and higher in energy.

• Excessive exposure to UV-B increases risks of skin cancer, cataracts, and a weakened immune system.

• Chemical reactions produce and use ozone

• Ozone absorbs UV-B

O3 + UV light O2 + O

O2 + O O3 + heat

Chlorofluorocarbons

• A triumph of chemistry: colorless, odorless, nontoxic, chemically inert, inexpensive

• Used in refrigeration systems, as propellants, as foaming agents, and as industrial solvents

Inertness of CFCs• Allows them to persist into the stratosphere

• UV causes photodecomposition to atomic chlorine, which reacts with ozone.– ONE Cl atom destroys TWO ozone molecules and is

regenerated in the process.

O3 + UV light O2 + O

CF2Cl2 + UV light Cl + CF2Cl

Cl + O3 ClO + O2

ClO + O O2 + Cl

atomic chlorine consumed

atomic chlorine regenerated

ozone destroyed

atomic chlorine produced

Polar Regions

• CFCs are relatively uniformly distributed globally.

• Why then is there a transient ozone hole only over Antarctica?

Temperature and PSCs• The ice crystals in polar stratospheric clouds act as

catalysts for the production of free chlorine gas.

• The polar vortex breaks down in the Antarctic summer and the hole “recovers.”

Global Ozone Depletion

• Global atmospheric ozone levels have also fallen.

• Evidence of ozone depletion has brought about world-wide cooperation for the phase-out of CFCs.

The Montreal Protocol

• Called for a 50% decrease in CFC usage worldwide by 2000

• Updated to call for complete phase-out by 2000

• Then-president George H. Bush pushed the deadline closer, up to January 1, 1996.

• Some products are still transitioning however.

• The ozone depletion has decreased 6% since 1979.

Ozone Depletion Myths

• Myth 1: CFCs are heavier than air and do not rise into the stratosphere.– Our atmosphere is a turbulent system that

quickly mixes the gases that compose it.

• Myth 2: Natural sources, such as volcanoes, add much more chlorine to the stratosphere than CFCs.– Very little of the HCl emitted by volcanoes gets

beyond the troposphere.– HCl is very reactive and water soluble.

Chapter Summary

Molecular Concept

• Boyle’s Law

• Charles’ Law

• The air we breathe

• The ozone

Societal Impact

• The effects of polluted air have prompted our society to pass legislation (the Clean Air Act).

• CFCs harm the ozone layer in the stratosphere.