Post on 29-Dec-2015
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.
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Global Warming – An Environmental Issue Involving Carbon Dioxide
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7
FIGURE 7-22•World primary energy consumption by energy source
petroleum
coal
natural gaswind
nuclear
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!)
Slide 4 of 57
The “greenhouse” effectFIGURE 7-23
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7
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.
Slide 6 of 57Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7
Global Average
Global average atmospheric carbon dioxide level
Actual and predicted CO2 emissions
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/climate-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
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 8 Slide 14 of 50
FIGURE 8-1•The simplest wave motion – traveling wave in a rope
Electric and magnetic fields propagate as waves move through empty space or through a medium.
A wave transmits energy.
Electromagnetic wavesFIGURE 8-2
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 8 Slide 15 of 50
Low
High
Frequency, Wavelength and Speed of Electromagnetic Radiation
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 8 Slide 16 of 50
• Frequency () in Hertz—Hz or s-1.• Wavelength (λ) in meters—m.
• cm m nm Å pm
(10-2 m) (10-6 m) (10-9 m) (10-10 m)(10-12 m)
• Velocity (c)—2.99792458 108 m s-1.
c = λ λ = c/ = c/λ
The electromagnetic spectrumFIGURE 8-3
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 8 Slide 17 of 50
Refraction of lightFIGURE 8-6
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 8 Slide 18 of 50
(a) (b) (c) (d) (e)
Sources for light emissionFIGURE 8-8
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 8 Slide 19 of 50
The atomic, or line, spectrum of heliumFIGURE 8-9
Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 8 Slide 20 of 50
The Balmer series for hydrogen atoms – a line spectrum
One portion of the emission spectrum of atomic hydrogen.
Copyright © 2011 Pearson Canada Inc. 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)
EPhoton
νPhoton (s-1)
Slope = Planck’s Constant = h = 6.626 x 10-34 J s ∙
PABAs ?
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 10∙ 9 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
Energy perAtom (J)
EHIGH
ELOW
ΔEAbsorption 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.