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Lab Partner:
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Date:
PhET Simulation: Molecules and Light
Chemistry in Context
Today you are going to use a sim to explore how light interacts with molecules in our atmosphere.
Light sometimes behaves as waves, and sometimes as particles
(photons). In today’s sim, light is represented as particles so we can
differentiate between low intensity light and high intensity light.
Intensity/brightness
control
1.
Do not think of photons like gas molecules—photons are bundles of
energy and, unlike gas molecules, photons have no mass, so they don’t
collide with or “push” other molecules out of the way.
Explore the sim. Change the light intensity. Change the target molecule.
2. Below are scenes, in sequence, that you might see when infrared light is focused on a water molecule.
1
2
3
4
5
6
Reproduce these scenes. Slow down the rate of photons -- step through the sim (use the ❙▶ button) so
you can see individual “frames” as in the above scenes.
In the “movie” sequence above, what happened to the photon between panel 1 and 2?
Where did the photon in panel 6 come from?
Compare the energy of the photon in panel 6 with the photon in panel 1. Explain.
3. Conduct experiments to test what happens to each molecule with each type of radiation. Make sure to
vary light brightness as well as wavelength. Use the table to record your observations for each
combination. You may use the key below. If any molecules fragment (fall to pieces), record the fragments.
Light Absorption Behavior Key
NA = no absorption
G = absorb, glow, emit
B = absorb, bend, emit
R = absorb, rotate, emit
BB = absorb, break bond, emit S = absorb, stretch, emit
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DATA: Interactions between Molecules and Light
Light Absorption Behavior Key
NA = no absorption
G = absorb, glow, emit
B = absorb, bend, emit
R = absorb, rotate, emit
BB = absorb, break bond, emit S = absorb, stretch, emit
Note: for any molecules that fragment (fall apart into pieces), record what the fragments are
Microwave
IR
Visible
UV
Carbon
monoxide, CO
Nitrogen, N2
Oxygen, O2
Carbon
dioxide
Water
Nitrogen
dioxide
Ozone
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ANALYSIS
1. Arrange these regions of the electromagnetic spectrum from lowest to highest energy: IR, microwave, UV,
visible.
Lowest energy
Highest energy
2. How does increasing the light intensity affect the type of behavior observed with a given molecule?
3. Some of the processes you observed required more energy than others. Arrange the following processes
from lowest energy to highest energy:
a. bond breaking
b. bond bending/stretching (both are forms of vibrational energy)
c. molecule rotating
Takes the least energy
takes the most energy
4. Compare the light absorption characteristics of the homonuclear diatomic molecules with the
heteronuclear diatomics.
Predict what you would observe for interaction of chlorine gas, Cl2, with microwaves.
5. Some triatomic molecules have linear geometry while others are bent. Compare the microwave
absorption of linear vs. bent triatomic molecules.
Predict what you would observe for the interaction of carbon disulfide, CS2, with microwaves. CS2 has this
geometry:
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Predict what you would observe for the interaction of sulfur dioxide, SO2, with microwaves. SO2 has this
geometry:
6. Is/are any of the gases colored? Explain how you know.
7. Write a balanced equation for the UV-induced fragmentation of nitrogen dioxide.
8. For each of these 4 types of radiation, give at least one example of how the radiation is relevant to your
everyday life.
Microwaves:
Infrared:
Visible:
Ultraviolet:
9. If you were trying to explain to a middle school student, in one sentence, about molecules and light
absorption, what would you say?
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