Pub #brownchem: @alpha137
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alpha137: hello
LizK: Hello,
LizK: I looked at the test
Link to the test
alpha137: Good
alpha137: We can discuss it in a minute
LizK: It was hard, but at least there were only 4 questions, and you
could work with 4 people
alpha137: Right, the way this test works is that it is supposed to be qualitative
and it is OK to "cheat"-that is look stuff up. We can discuss the different
parts however
LizK: Thats really nice
alpha137: First, we are thinking of the atmosphere being bathed by sunlight.
LizK: It would be really hard if you couldn't look stuff up, or talk
to peol
LizK: BUt back to the atmospohere
alpha137: OK, sun light is made up of all different wavelengths of light,
right?
LizK: Yes
alpha137: There is a relationship between the frequency of light and its
wavelength. Know what
it is?
LizK: Yes, the wavelength equals the speed of light divided by the frequency
LizK: or maybe its the other way aourng
LizK: around
It is the speed of light divided by wavelength.In other words, frequency time wavelength = speed of light in
the case of phtons.
LizK: But also, its an inverse relationship
alpha137: Right. Now there is a relationship between the energy that a photon
of light contains and its frequency. Know what that is?
LizK: Oh, doesn't that have to do with Plank's constant, let me find
it
alpha137: It may be called the Bohr relation. and it does have Planck's
constant in it.
LizK: Yes, I have it E = -2.18 * 10^-18J (z^2/n^2)
alpha137: This is the Bohr relation for atomic hydrogen. I guess I was thinking
Einstein relation!!
alpha137: Save the Bohr relation for a bit.
LizK: Od
LizK: od
LizK: ok
alpha137: Einstein relation is between energy and frequency of light E =
planck's constant times frequency E = h*nu
LizK: But one question - Einstein's Equation like e = mc^2?
LizK: I thought that was planks equation, but maybe not
alpha137: There are many relations named for Einstein!
LizK: Ok, anyhow
alpha137: It could be named after Planck.
alpha137: Point is that a light particle carries with it an energy of h*frequency
= h*c/wavelength
LizK: could you hold on like two seconds there is someone at my door
alpha137: OK
LizK: Sorry, it was a package for my mom
alpha137: A present?
LizK: No, I think it was something for work, Like a document
alpha137: The Bohr relation for atomic hydrogen gives a series of energy
states for a given value of n. Each n corresponds to different electron configurations.
LizK: Yes
alpha137: Corresponding to each n is a value of orbital motion designated
by L the orbital angular momentum quantum number.
LizK: Yes
alpha137: And for each permissible L there are 2L + 1 "magnetic"
quantum numbers, m.
LizK: oh, yes, I remember this
alpha137: Forgetting electron spin which has its own quantum numbers the
energy states permitted for atomic hydrogen may be designated by n,L,m
LizK: ok
alpha137: In the ideal case of atomic hydrogen each n value corresponds
to a particular energy given by that Bohr formula.
LizK: ok
alpha137: There is nothing in between. That is, these are the only permitted
energy states for hydrogen.
LizK: ok
alpha137: If hydrogen absorbs a photon and the hydrogen is in energy designated
by n what happens?
LizK: Won't it jump to another energy level?
alpha137: Yes.
alpha137: n -> n + 1
LizK: Yes
LizK: I see
alpha137: Nothing else.
alpha137: Now you have hydrogen in an excited, higher, energy state n +
1 what do you think will happen next?
LizK: It falls back to the original energy level
alpha137: Yes, and what happens to the energy it had?
LizK: IT is released, as heat I think
alpha137: It is released as photons.
LizK: Oh, that was my other guess
alpha137: If there is a collision then the excited hydrogen atom will transfer
its energy in the form of heat and go back to its lower state.
LizK: Because can't you even calculate the wavelength of the photons
released
alpha137: Both things happen.
LizK: ok
alpha137: You can caluclate the wavelength of the PHOTONS released, but
the heat is a bit more difficult.
LizK: ok
alpha137: The photons are released in all directions-throughout a sphere.
LizK: I see
alpha137: Only a part of them go towards earth.
LizK: ok
alpha137: We were using hydrogen as an example because you had that formula
for the energy
states, but we are thinking molecules. There are molecular energy states as well.
LizK: Do the released ones have enough energy to break apart an O_2
molecule?
alpha137: This is our question. What photon energy is required to break
the O_2 molecular bond?
alpha137: Does the sunlight contain that photon frequency (wavelength)?
LizK: It was on the test, let me go see
alpha137: Question 4
LizK: I thought it did, because it said that sunlight contains the energy
between 60 and 600, and it takes 490 kJ/mol to break apart O_2
alpha137: We know that sunlight contains the photons with energy needed
to break oxygen bonds. How do we compute it?
alpha137: The Einstein formula is for one molecule
LizK: ok, well cwhat about Bohrs equation based on one MOle?
alpha137: The 490.1 is kJ/mole
alpha137: Yes, we could multiply that equation by Avagadro's number, or
we could divide the dissociation energy by N_A
LizK: So divide 490.1 bu avogadro's number
alpha137: Sounds good.
LizK: What is N_A
LizK: ?
alpha137: N_A is Avagadro's number
LizK: oh, I thought it might be
alpha137: 490100/6.02x10^23 = 8.1 x 10^-19 joules
LizK: ok, and then we can use that in the equation
alpha137: Right
alpha137: I get 245 nanometers. You can check this number later, OK.
LizK: Ok,
alpha137: Now, if you take something and heat it with a flame it will glow.
Right?
LizK: yes
alpha137: That is called black body radiation.
LizK: ok,
alpha137: The amount of the glow will depend on how hot the body is.
LizK: that makes sense
alpha137: The sun is like a black body with a temperature of 5783 K.
LizK: ok
alpha137: In question 4 there is a curve showing the wavelengths from the
sun as a black body.
LizK: Yes
alpha137: Any wavelength of 245 nm or shorter will dissociate molecular
oxygen.
alpha137: Why?
LizK: ok
LizK: Because it will have the energy to break the bond
alpha137: Right. The table in question 4 shows a bunch of molecular dissociations
and to answer the question we have do to for each one what we just did for oxygen.
LizK: ok
LizK: That makes sense
alpha137: Do you think nitrogen will be affected much by sunlight?
alpha137: How about ozone?
LizK: I don't think N_2 will be
alpha137: Right, about nitrogen.
LizK: ozone would probably be affecteed
Here is a link to some info on ozone
alpha137: Of course, because you already know there is an ozone problem.
alpha137: And the numbers in the table bear this out.
LizK: Yes
LizK: I thought that question was one of the simpler ones
alpha137: Lets discuss question 1, then.
LizK: ok,
alpha137: What answer would you give, nonsense, or true?
LizK: I was not sure what the answer would be, it almost depended upon
how you looked at the problem, but I think I would have said true
alpha137: No, it is nonsense.
LizK: Why? (I thought it might be nonsense because it said that the
temp. increases in the mezosphere soemwhere in the book)
alpha137: Well, it is nonsense because by temperature we mean molecular
motion.
LizK: ok
alpha137: If the temperature is high it means that the kinetic energy of
the molecules is high too.
alpha137: It means that the velocity of the molecules is high.
LizK: Yes, that makes sense,
alpha137: Remember the Maxwell distribution we slugged through at the beginning?
LizK: so molecular activity would be high as well
LizK: yes
alpha137: Yes, molecular activity would be high.
alpha137: I am assuming that the words "molecular activity" means
stuff other than chemical reactivity!
LizK: ok
alpha137: We have a rough relationship between kinetic energy and temperature.
Know what it is?
LizK: isn;t it a direct relationship the higher the temp the more kinetic
energy so the more pressure
alpha137: Forget pressure.
LizK: ok,
alpha137: Kinetic Energy = (1/2)m v^2 = k_B T where k_B = Boltzmann's constant
LizK: Ok, I have never heard of that,
alpha137: But it is true nevertheless
LizK: But I don't doubt that its true
LizK: ok
alpha137: OK, the Maxwellian velocity distribution max is represented by
a relationship like that one.
LizK: I see
alpha137: Anyway, you have all the elements to describe what is happening
when sunlight bathes the atmosphere. There is a heating effect due to collisions
of excited molecules and there are dissociations and then reactions. There is absorption
of
alpha137: light and reemission of light.
LizK: Ok
alpha137: You can even put in some numbers here and there!
LizK: ok
alpha137: By the way, I think the engineers at Indiana State do not know
what temperature is by the way they made the statement in question 1.
LizK: I was wondering about that, it seemed a little strange the way
it was phrased
alpha137: Right, it was nonsense.
alpha137: You understand the primary processes of the absorption and emission
of light by molecules in the atmosphere. You can explain the green house effect too.
LizK: But I thought that maybe if you assumed that there was a linear
relationshiop between dist. from the sun and temp. that the temp would be high,
but in reality it was low,
LizK: But I guess not
LizK: THat is sort of a round about rationalization
alpha137: The sunlight is heating up the atmosphere, but it is complicated
because of molecular energy levels.
One of the questions in the Concept test is asking about
the atmospheric temperature and how it is that it drops and then increases with altitude.
This is called inversion and is like the inversion over cities that trap pollution.
LizK: yes
alpha137: I have to go now. Think about question 2 for tomarrow. OK?
LizK: Ok, but I can only meet at 1:30 to 2:30 or after 5:00
alpha137: Do you think that the NO emitted in the stratosphere from jet
engines would affect ozone levels?
LizK: I think so
alpha137: Why don't we meet Monday instead?
LizK: Thats fine with me - maybe the NO wouldn't - it looks like it
wouldn't be affected by the sun;s radiation, but in the book it mentioned something
about how ith would be dangerous
LizK: Monday at 9:30?
alpha137: Yes, and I have a little story about the NO stratosphere problem.
See you Monday.
alpha137: bye
LizK: ok
LizK: thanks and bye
*** Signoff: LizK (Leaving)