Transcript Document

Global Energy Use – the Anthropocene
• Energy consumption is growing rapidly due to
a growing human population and changing
lifestyles (more cars, etc). Some energy
sources, and some other substances not
related to energy consumption, produce
volatile gases that trap heat (infrared
radiation) thus raising the surface
temperature of the planet – anthropogenic
global warming (AGW) climate change.
Global Warming – An Environmental Issue Involving Carbon
Dioxide
nuclear
FIGURE 7-22
•World primary energy consumption by energy source
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General Chemistry: Chapter 7
Slide 2 of 57
Conservation Anyone?
• A key component of energy use going forward
will be conservation – better home insulation
in colder regions, more efficient air
conditioning, cars with better gas mileage.
Some of the changes that are necessary fall
outside the rubric of an introductory
chemistry course! You’ll see these issues
discussed in higher level chemistry, economics
and engineering courses (and endlessly in the
news!)
FIGURE 7-23
The “greenhouse” effect
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General Chemistry: Chapter 7
Slide 4 of 57
Greenhouse Effect
• A detailed understanding of the greenhouse
effect requires a consideration of the power
spectra of light incident on the planet and
leaving the planet. Light leaving the planet is
much “richer” in infrared light. The growing
levels of some atmospheric gases are shown
on the next slide. A graph showing seasonal
variations in CO2(g) levels will be considered in
class.
Global average atmospheric
carbon dioxide level
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Actual and predicted
CO2 emissions
General Chemistry: Chapter 7
Slide 6 of 57
Ocean Acidification
• A second consequence of growing
atmospheric CO2(g) levels is increasing ocean
acidity. Atmospherics carbon dioxide and
dissolved (oceanic) carbon dioxide exist in
dynamic equilibrium.
• CO2(g,atmosphere) ↔ CO2(aq,ocean)
• CO2(aq) + 2H2O(aq) ↔ H3O+(aq) + HCO3-(aq)
• CO2 is much less acidic than some other
common non-metal oxides.
Climate Change Links
• NASA: http://climate.nasa.gov/
• Sydney Morning Herald (Australia):
• http://www.smh.com.au/environment/climat
e-change
• NSIDC: http://nsidc.org/
Atoms, Electrons and
Atomic/Electronic Energies
• Macroscopic objects and systems can be
adequately described using Newtonian
mechanics and thermodynamics. A train, a
hockey puck and an iceberg are all
macroscopic objects/systems. We might wish,
for example, to consider how the kinetic
energy of a hockey puck varies with velocity
(goalies do!) or to calculate how much
heat/energy is needed to melt an iceberg.
Pucks and Gas Molecules
• Hockey players who have studied physics know
that the kinetic energy of a puck varies with
velocity as described by the familiar equation.
• Ekinetic = ½ mv2
• During the course of a hockey game the puck will
move with a range of velocities and,
correspondingly, a range of kinetics energy
values. We assume that the kinetic energy of the
puck can be continuously varied. Why?
Gas Molecule Kinetic Energies are
Variable
• Gas discussion - the pressure exerted by a gas
arises because individual gas molecules have
high velocities. The rapid motion of gas
molecules gives gas molecules significant
kinetic energy. At a given T not all gas
molecules have the same velocity or kinetic
energy. A range of velocity/kinetic energy
values is seen for a gas at a particular T. As the
gas temperature increase the average kinetic
energy of gas molecules also increases.
Atomic and Molecular Energies
• Atoms and molecules can possess energy in
addition to translational kinetic energy. Two types
of energy – rotational and vibrational – will not
be considered in Chemistry 1050. Electronic
energies will be considered.
• The energies of individual atoms, electrons and
molecules are best studied experimentally using
spectroscopy (interaction of light with matter).
Key spectroscopic experiments tell us that, for
atoms and molecules, Newtonian mechanics
does not work! Ouch!
Atomic and Molecular Energies
• Spectroscopic experiments show us that atomic
and molecular energies are not continuously
variable.
• Experiments show us that atomic and molecular
energies are quantized – only a small number of
energy values are observed. In order to
understand spectroscopy the properties of light
will be reviewed briefly. The wave/particle “dual”
character of light is particularly important.
8-1 Electromagnetic Radiation
Electric and magnetic
fields propagate as waves
move through empty
space or through a
medium.
A wave transmits energy.
FIGURE 8-1
•The simplest wave motion – traveling wave in a rope
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General Chemistry: Chapter 8
Slide 14 of 50
Low 
High 
FIGURE 8-2
Electromagnetic waves
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General Chemistry: Chapter 8
Slide 15 of 50
Frequency, Wavelength and Speed of
Electromagnetic Radiation
• Frequency () in Hertz—Hz or s-1.
• Wavelength (λ) in meters—m.
• cm
m
nm
Å
(10-2 m)
(10-6 m) (10-9 m)
pm
(10-10 m) (10-12 m)
• Velocity (c)—2.99792458  108 m s-1.
c = λ
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λ = c/
General Chemistry: Chapter 8
 = c/λ
Slide 16 of 50
FIGURE 8-3
The electromagnetic spectrum
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General Chemistry: Chapter 8
Slide 17 of 50
FIGURE 8-6
Refraction of light
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General Chemistry: Chapter 8
Slide 18 of 50
(a)
(b)
(c)
(d)
(e)
FIGURE 8-8
Sources for light emission
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General Chemistry: Chapter 8
Slide 19 of 50
FIGURE 8-9
The atomic, or line, spectrum of helium
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General Chemistry: Chapter 8
Slide 20 of 50
One portion of the emission spectrum of atomic hydrogen.
The Balmer series for hydrogen atoms – a line
spectrum
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General Chemistry: Chapter 8
Slide 21 of 50
Quantized Atomic Energies
• The H atom electronic emission spectrum
shows a small number of “spectral lines” with
well defined (reproducible) frequencies (or,
wavelengths). Well defined frequencies, taken
together with the equation E = hν, means that
H atoms can possess only certain well defined
energies. We say that H atom energies are
quantized.
Musical Instruments – A Useful
Analogy to Quantized Energies?
• The idea of quantized atomic (and molecular!)
energies is a difficult one to accept. An
analogy that is somewhat useful is to consider
the finite number of frequencies produced by
a musical instrument. What are the limitations
of this analogy to musical instruments?
Guitars – a Finite Number of Notes
(Frequencies!)
Simple Frequency Wavelength
Conversion
• In spectroscopic experiments both frequency
and wavelength measurements are reported.
Conversions using the velocity of light are
needed. Example: What is the wavelength of
the 2.450 GHz radiation (light) used in a
typical microwave oven?
• Recognize Hz as equivalent to s-1.
• Then ν = 2.450 GHz = 2.450 x 109 s-1
Frequency and Wavelength
•
•
•
•
Use c = λν to get λ = c/λ
= 2.9979 x 108 m∙s-1/2.450 x 109 s-1)
= 0.1224 m = 12.24 cm (mention quarter-wave plates?)
Higher frequency light is more energetic than lower
frequency light. We know that infrared light makes us
feel warm. Visible light and UV light can cause serious
sunburn. The energy transported per photon of light is
proportional to the frequency of light. The relationship
between light frequency and energy was studied by
Max Planck. (Pre-procreation graph? Why?)
Photon Frequency (s-1) and Energy (J)
PABAs ?
EPhoton
Slope = Planck’s Constant = h
= 6.626 x 10-34 J∙s
νPhoton (s-1)
Energy per Atom and per Mole
• Example: How much energy is possessed by (a) 1
photon and (b) NA photons of 16.6 GHz
electromagnetic radiation?
• EPhoton = hν = (6.626 x 10-34 J∙s)*(16.6 x 109 s-1)
• = 1.10 x 10-23 J
• For one mole of photons
• (1.10 x 10-23 J/photon)*(6.022 x 1023 photons/mol)
• = 6.62 J
• This is a small amount of energy. It takes a lot of
microwave photons to heat up a cold cup of coffee! We
could calculate the number of photons needed to heat a
cup of coffee. How?
Hot Atoms Can Emit Energy!
• Highly energetic/hot objects have a tendency to
lose energy/heat to the surroundings. Atoms are
no exception. Very hot atoms emit only a few
frequencies of light. These so-called line spectra
(together with E = hν and sometimes c = λν)
indicate that atomic/electronic energies are
quantized (not continuously variable). “Cold”
atoms readily absorb light – as long as the energy
of the photon corresponds to the difference in
energy between two energy levels of the atom.
Wavelength and Energy
• Calculate the amount of energy released by
(a) one H atom and (b) one mole of H atoms
emitting light with a wavelength of 434.0 nm.
(Spectrum on previous slide.)
Simple Two Energy Level Atomic or
Molecular System
EHIGH
Energy
per
Atom (J)
ΔEAbsorption
ELOW
ΔEEmission
Energy Conservation Still Applies!
• Previous slide: Conservation of energy dictates
that
• ΔEAbsorption + ΔEEmission = 0
• Careful measurements of light frequencies (or
wavelengths) absorbed or emitted enables a
“pattern” of atomic or molecular energy levels to
be determined. In a few cases the energy level
pattern and corresponding absorption/emission
spectra can be described using simple equations.
Atoms Losing Energy!
Molecules Losing Energy?
• Simple combustions reactions can give us both
heat and light.