Topics for Today Greenhouse Gases: How do they keep us warm?

Readings for Today 3.4 Vibrating Molecules and the Greenhouse Effect

Topics for Friday Climate Change: A Global Problem? Quiz #7 (of 11)!!

Readings for Friday 3.8 Methane and other greenhouse gases 3.10 Responding to science with policy changes 3.11 The Kyoto Protocol on Climate Change 3.12 Global warming and ozone depletion May even bleed into Monday.

Announcements!!!

Bring back your radon disks!!

Announcements! Nice, warm weather… Please remember: Do not wear shorts or sandals to lab!!

Exam Regrading … Same process as Exam 1.

Topics for Today Greenhouse Gases: How do they keep us warm?

Review from Monday…

Review… The Greenhouse Effect

Review from ‘a long time ago.’ Lecture 15. ( (

Gamma rays X-rays May ionize the molecule What happens when α molecule absorbs light?

Gamma rays X-rays May ionize the molecule UV light May break bonds in the molecule

Gamma rays X-rays May ionize the molecule UV light May break bonds in the molecule Visible light Usually doesn’t do much w/out help

Gamma rays X-rays May ionize the molecule UV light May break bonds in the molecule Visible light Usually doesn’t do much w/out help Infrared Important for climate change… coming up!!

Gamma rays X-rays May ionize the molecule UV light May break bonds in the molecule Visible light Usually doesn’t do much w/out help Infrared Important for climate change… coming up!! What happens to a molecule when it absorbs IR?

Figure 3.17 Among other motions, this includes stretching and bending!

Gamma rays X-rays May ionize the molecule UV lightMay break bonds in the molecule Visible lightUsually doesn’t do much w/out help InfraredVibrates Molecule What happens to a molecule when it absorbs IR?

Q: When will IR cause a molecule to vibrate? A: When there is a dipole moment!

Dipole Moment: Unequal distribution of charge! Where have we seen dipole moments before?

Polar Bonds: Unequal sharing of electrons in a chemical bond. Still a covalent bond! RESULTS IN AN UNEQUAL DISTRIBUTION OF CHARGE!! From Lecture 19 or so…

HCl δ-δ- δ+δ+ H H H N δ-δ-δ-δ- δ+δ+δ+δ+ δ+δ+δ+δ+ δ+δ+δ+δ+ δ-δ-δ-δ- δ+δ+δ+δ+ δ+δ+δ+δ+ O HH A bunch of polar molecules That is, they have dipoles

What about some other molecules? OCO..

OCO Carbon Dioxide Polar or nonpolar bonds?

OCO.. Carbon Dioxide Polar bonds δ-δ-δ-δ- δ-δ-δ-δ- δ+δ+δ+δ+

Carbon Dioxide Polar or nonpolar molecule? OCO.. δ-δ-δ-δ- δ-δ-δ-δ- δ+δ+δ+δ+

Carbon Dioxide Nonpolar Molecule So, no overall dipole moment… But CO 2 is a greenhouse gas! Why does it absorb IR radiation? OCO.. δ-δ-δ-δ- δ-δ-δ-δ- δ+δ+δ+δ+

The IR can cause (or induce) a dipole moment!

Possible Vibrations for CO 2 Think ELECTRONEGATIVITY!!!

“Symmetric Stretch” Both out, both in Keeps linear shape

“Symmetric Stretch” Both out, both in Keeps linear shape

“Symmetric Stretch” Both out, both in Keeps linear shape Does this induce a dipole moment?

“Asymmetric Stretch” One in and one out Keeps linear shape

“Asymmetric Stretch” One in and one out Keeps linear shape

“Asymmetric Stretch” One in and one out Keeps linear shape Does this induce a dipole moment?

“Bend” Bends in one plane Loses linear shape

“Bend” Bends in one plane Loses linear shape

“Bend” Bends in one plane Loses linear shape Does this induce a dipole moment?

“Bend” Bends in another plane Loses linear shape Same as the other bend, just in a different plane…

Figure 3.17 IR vibrations for a linear 3 atom molecule

Bending in each plane

CO 2 bending and stretching… Summary Symmetric Stretch Asymmetric Stretch Bend 1 Bend 2 Which of these is able to produce a dipole moment? CO 2 has THREE possible ways to absorb IR!! How do we measure this?

Spectrophotometry Sort of…

Figure 3.15 in your Text What are “wavenumbers? ”

4 μm = 4000 nm! What is transmittance ?

The opposite of absorbance! Absorbance

Review… The Greenhouse Effect

Where is visible light? Higher Energy Lower Energy

What happens when a molecule absorbs light? It depends on the wavelength

Gamma rays X-rays May ionize the molecule UV lightMay break bonds in the molecule Visible lightUsually doesn’t do much w/out help InfraredVibrates Molecule What happens to a molecule when it absorbs IR?

What happens when a molecule absorbs light? AND on the molecule. It depends on the wavelength

Summary… For a molecule to absorb IR radiation, it must 1) Permanently have an uneven distribution of charge (dipole)

HCl δ-δ- δ+δ+ H H H N δ-δ-δ-δ- δ+δ+δ+δ+ δ+δ+δ+δ+ δ+δ+δ+δ+ δ-δ-δ-δ- δ+δ+δ+δ+ δ+δ+δ+δ+ O HH Polar molecules have dipole moments

Summary… For a molecule to absorb IR radiation, it must 1) Permanently have an uneven distribution of charge (dipole) OR 2) Vibrate in a way that can CREATE an uneven distribution of charge (dipole) Nonpolar molecules MAY or MAY NOT absorb IR radiation…

CO 2 bending and stretching… Summary Symmetric Stretch Asymmetric Stretch Bend 1 Bend 2

Summary… For a molecule to absorb IR radiation, it must 1) Permanently have an uneven distribution of charge (dipole) OR 2) Vibrate in a way that can CREATE an uneven distribution of charge Nonpolar molecules MAY or MAY NOT absorb IR radiation…

Do N 2, O 2, or Ar absorb IR radiation?

O O.. NN 1) Polar or nonpolar molecules? 2) Can we vibrate these to produce a dipole moment? Ar

These molecules / atoms are NOT GHGs. 1) They are nonpolar 2) They cannot vibrate to create an unequal distribution of charge.

Summary… For a molecule to absorb IR radiation, it must 1) Permanently have an uneven distribution of charge (dipole) OR 2) Vibrate in a way that can CREATE an uneven distribution of charge -A molecule with 3+ atoms

How about water? H H O.. Is water polar? But it can also vibrate!!

Water vapor is an extremely strong absorber of IR And a very potent GHG!!

Water Vapor Figure 3.16

Figure 3.15 in your Text CO 2

Water Vapor Figure 3.16 CO 2 absorbs here

Water Vapor Figure 3.16 CO 2 absorbs here Different molecules absorb different wavelengths!

Does an increase in water vapor lead to an increase in Earth’s temperature?

On one hand…

Water Vapor Figure 3.16

On one hand…On the other hand…

Interactions with Clouds Clouds – Condensed water on a “seed” particle Reflects solar radiation

Review… What is the Greenhouse Effect?

The Greenhouse Effect Solar radiation Long-wave radiation Greenhouse Gases Greenhouse gases are transparent to incoming solar radiationGreenhouse gases are transparent to incoming solar radiation Greenhouse gases absorb the infrared light which is emitted from the earthGreenhouse gases absorb the infrared light which is emitted from the earth This traps energy in the earth’s atmosphereThis traps energy in the earth’s atmosphere

About 80% of the heat emitted from the surface is trapped in the atmosphere

The Earth’s atmosphere is about 380 ppm CO 2 The average temperature is about 59 o F

What would the average temperature of Earth be with no CO 2 ? About -4 o F!

About 80% of the heat emitted from the surface is trapped in the atmosphere Any more than this 80% is called the “enhanced greenhouse effect.”

Some people are worried that this will lead to climate change (global warming)

Greenhouse gases CH 4 CO 2 H 2 O N 2 O (nitrous oxide) CFCs and replacements

Greenhouse gases CH 4 CO 2 H 2 O N 2 O (nitrous oxide) CFCs and replacements DONE!

Water Vapor Figure 3.16

Water Vapor Extremely strong dipole moment (Very good at absorbing IR) Most abundant greenhouse gas (varies from 0-4% of the atmosphere)

Some GHGs… CFCs HCFCs HFCs CO 2 CH 4 N2ON2O O3O3 H2OH2O

What makes greenhouse gases more potent? N2ON2O CFCs HCFCs HFCs CH 4 CO 2 CH 4 O3O3 H2OH2O

1) Atmospheric Lifetime

Methane Atmospheric lifetime about 10 years Removed by the hydroxyl radical… we will see this reaction Monday!

CFCs Atmospheric lifetime >> 100 years Destroyed in the stratosphere by UV-C

1) Atmospheric Lifetime 2) Atmospheric Concentration

Concentration of CO 2 is about 370 ppm.

CFC concentrations are around ppm

1) Atmospheric Lifetime 2) Atmospheric Concentration 3) How efficiently can the molecule absorb IR?

CO 2 bending and stretching… Summary Symmetric Stretch Asymmetric Stretch Bend 1 Bend 2 CO 2 has only 3 vibrations

CFCs CFCs have at least 10 possible vibrations!! CFCs are much more efficient at trapping IR than CO 2. This is due to the more complicated SHAPE