Pub #brownchem: @alpha137
*** #brownchem :End of /NAMES list.
*** Mode is +
alpha137: hello
*** LizK (gloworm4@204-185-143.ipt.aol.com) has joined channel #brownchem
alpha137: hello
LizK: Hello
alpha137: Were you able to figure out the problem?
LizK: Ok, I attempted the homework but I'm not sure exactly where you were going.
alpha137: Let us recap.
LizK: I assumed that since there were no quantities you didn;t want hard numbers so I was thinking of a formula like thing
alpha137: Yes, a forumla.
LizK: All I could find that fit, sort of was that S = W(the w in Q2-Q1=w)/T1
alpha137: We are trying to compare the efficiency from a fuel cell to that of burning methane.
LizK: But that doesn't relate to the S1+S2=0
LizK: Yes
alpha137: No, that does not relate quite, but close.
LizK: Ok
alpha137: S1 = Q1/T1 and S2 = Q2/T2 so Q1/T1 = Q2/T2
LizK: Oh, that makes sense, but how did you get that?
alpha137: Then you have w = Q2 - Q1 and plug in.
alpha137: I got that from the fact that for a reversible, equilibrium, process the entropy change (total that is) is zero.
alpha137: That is S_1 + S_2 = 0
LizK: I had no idea how the S=Q/T related to S1+S2=0 let alone how they related to Q1, Q2, T1, and T2
alpha137: The definition of entropy, S, is the heat reversibly added (or subtracted) divided by the temperature, hence S = Q/T
LizK: yes, I got that far
alpha137: For part 1, the fridg, S1 = Q1/T1 and for part 2, the room, S2 = Q2/T2.
LizK: Ok
alpha137: Then we have the Second Law of Thermo which states that the total entropy change is either zero (for equibrium-reversible) or positive.
alpha137: Since our compressor is ideal it is running reversibly and the S change for
alpha137: it will be zero.
alpha137: We put the bookeeping for energy and work and heat together with the total entropy change.
alpha137: Plug it all together.
LizK: I didn't know to make the assumption that S1 related to th frige and S2 related to the room, i would have gotten it if I had known to make those assumption
LizK: s
LizK: I get the rest of it, and it makes sense now,
alpha137: OK. part 1 could be the fridg, or it could be our auto engine!
LizK: And part two could be the fuel cell?
alpha137: What you should get is that w/q = (T_2 - T_1)/T_1
LizK: Ok,
alpha137: The signs will depend if you are getting work out of the heat transfer or requiring work to DO the heat transfer.
LizK: ok
alpha137: You can assign 1 and 2 anyway you want depending on the problem. The signs should take care of themselves.
LizK: ok, I see
alpha137: Now, if you were burning methane in an engine there would be a certain enthalpy. Remember what it was?
LizK: yes, do you want the number?
alpha137: No, I have the number.
alpha137: Now the work delevered by that enthalpy is our w.
alpha137: Assume that the temperature of the combustion is 800K
alpha137: That is one of the T's
LizK: Thats what I thought, that the change in H = w = Q2-Q1
alpha137: Assume that by some feat of engineering the exhaust temperature is 300K
LizK: ok
alpha137: Compute the w we can get from the 892 kJ/mol (or whatever it was)
LizK: I don't know
LizK: I'm sorry
LizK: How does this relate to the 800K and the 300K to the 892kJ/mole?
alpha137: w = (heat from enthalpy)(300 - 800)/800
alpha137: w is the work we can get.
LizK: oh yes, that equation, I see
alpha137: This equation is fundamental to all processes where heat is transfered.
LizK: ok, I seee
alpha137: A car, a power plant, or whatever.
alpha137: We are taking hot stuff and cooling it and extracting useful work.
LizK: Yes
alpha137: Could be an air conditioner, or a fridge.
alpha137: This is the very best would could to in a perfect world!
LizK: Ok,
alpha137: So, w = (890)(500/800) = 556 kJ/mol about.
alpha137: Compare this figure with the free energy change for the combustion of methane.
LizK: The free energy change was
LizK: -818kJ/mol
LizK: about that
alpha137: Right. If we do electrical work we get all of that cause DG = -nFV
LizK: yes
alpha137: where F = 96.5 kCoul/mol
LizK: Ok
LizK: I remember that equation
alpha137: There are losses due to entropy. Any idea what the magnetude is?
alpha137: Remember G = H - TS
Pub #brownchem: alpha137 LizK
*** #brownchem :End of /NAMES list.
*** Mode is +
*** Channel started at Wednesday, July 15, 1998 1:17:02 PM LizK:
Hello, what happened?
The computer here crashed. Probably because someone loged
onto the web page and was trying to submit something and my submit thing is broken
for the moment. So the machine, doing what is does best, crashed.
alpha137: As the guy in the movie said, I'm back.
alpha137: The point of all this is to say that if we can get work through the free energy we will get more of it than if we just get work from transfering heat.
LizK: yes, that makes sense
alpha137: This is handy if we are doing electric cars.
LizK: Ok,
alpha137: If you compute the efficiency for our methane thing you should find 92% for the free energy thing and 63% for the burning methane process.
LizK: WOW!, thats a big difference
LizK: !
alpha137: Won't that help not only in air pollution, but global warming as well?
LizK: And that means that so much less fuel would have to be used, yes, it would help both pollution and glolbal warming!
LizK: Thats incredible, but I
alpha137: When I did a search of the NY Times articles on fuel cells I found more than 11,000!
LizK: have always thought that burning stuff to get the heat and using the heat to fuel a car seemed like a round about way to run a car
alpha137: The fuel cell, electric car, seems like a better idea.
LizK: 11,000! thats a lot, but if they are so much more efficient, and not terrible complicated to build why don't we have any yet?
alpha137: We have discussed, quantitatively, the chemical principles for these two methods of extracting energy.
LizK: Yes, we have
alpha137: You ought to be able to see the usefulness in understanding entropy, and the relationships between heat, work and internal energy.
alpha137: Now, however, we have to design the fuel cell!
LizK: Yes, I see now, I 'm feeling better about all that
alpha137: I am not suggesting we do that here.
LizK: Thats a problem
LizK: I didn't think you were!
alpha137: But we need some kind of membrane that would into which we feed the methane and another one into which we feed the oxygen.
alpha137: So knowing about membranes is useful.
LizK: Ok, and would the membrade be permeable so that the two gases could go back and forth?
alpha137: We also need a way of putting in electrods to grab those electrons in the half reactions!
alpha137: Yes, the membrane must be permeable.
alpha137: Perhaps is can be a plastic of some kind.
LizK: Yes, I thought it would, but I wasn't sure, I thought of plastic too, but it would have to be able to withstand a certain amount of heat, and be permeab;e which some plastics are not
alpha137: Right. This is obviously a research and development problem.
alpha137: We need some venture capital for this!
alpha137: The pay off is great, but so is the risk.
LizK: Would the electrodes be a type of metal like they are in the normal battery, or would they be different because there are such diff. materials involved- I'm just curious
alpha137: I am not sure what the electrodes should be.
alpha137: I would guess they would be dispersed in the semiperably membrane.
LizK: OH, that makes sense,
alpha137: We also need to figure how much methane (or other fuel) we need and at what rate it is going to be consumed.
LizK: That is true, and very important too
alpha137: We can do this, we know the chemistry. The R&D is a big problem however.
LizK: Doesn't that quantity and rate depend upon the efficiency of the cell
alpha137: Yes, the quantity and rate depends on the efficiency of the cell.
alpha137: And our cell is better than burning gas in a car.
LizK: Yes, by fal
LizK: far
alpha137: But there are details. You know, they say the devil is in the details.
LizK: Exactly, thats probably why the Rand D are such a big problem, because the details are so important
alpha137: Well, we have made a start on taking care of one part of the air pollution problem in the troposphere. Lets go to the stratosphere, OK?
LizK: or at least part of the reason why R&D is hard
LizK: Ok,
alpha137: You looked at Chap 2, right?
LizK: Yup, I read it all
alpha137: Most of the discussion is about ozone.
LizK: YEs, and how the ozone layer is affected by CFC's and stuff like that
alpha137: Right, but first, prior to CFC's, there was ozone.
alpha137: Where did it come from?
LizK: Yes, there was ozone
LizK: It is made- there is a four step process someone came up with when sunlight breaks apart a molecule of O_2 and the other O atoms combine with two other O_2 moleules to form O_3
LizK: And then it talks about how it breaks down and goes into more detail
alpha137: There are several chemical principles here. First, the absorption of light by molecules.
LizK: Its called the Chapman Cycle and the book says that in nature the concentration of ozone stays the same becaues molecules are constantly being broken up and put back together
alpha137: This is called photochemistry.
LizK: Yes, absorbtion of light
LizK: Ok, makes sense
alpha137: We need to know how molecules absorb light. Why different molecules absorb light differently.
LizK: Ok
alpha137: Then, we need to know that light is energy and that a molecule absorbing light is absorbing energy. If this energy is great enough it may break a molecular bond.
LizK: I am going to guess that how molecules absorb light is related to their electorn configuration
LizK: Maybe not,
alpha137: Yes, how molecule absorb light is directly connected to their electron configuration.
alpha137: If light is absorbed by a molecule it excites the electrons in the molecule into different configurations. Conf one -> conf two.
LizK: Yes, I get it
alpha137: So the energy difference E(conf 2) - E(conf 1) = light energy.
LizK: Oh, but how would you find the Energy of a given configuration?
alpha137: Actually, we find the energy of a given configuration by taking light and seeing how it is absorbed by the molecule!
LizK: I see
alpha137: We make a study of different molecules in this way and compile the results.
alpha137: This is call spectroscopy and people who do this are called spectroscopists.
alpha137: This is a branch of chemistry, usually called physical chemistry.
LizK: Yes, I remember that, we did a lab on it during the year, and we had to know all these bands of light
alpha137: Modern physical chemists use lasers to do this.
LizK: Yes
alpha137: Anyway, we study ozone in the lab and determine which wavelengths of light it absorbs.
LizK: Yes
alpha137: From this we know it absorbs in the ultraviolet.
LizK: Oh, and that is why now with less ozone etc. everyone is saying how dangerous the UV rays are
alpha137: Right.
LizK: And the fact that they discoverd that UV rays cause cancer
alpha137: Right.
alpha137: The sun emits light of many colors, among them is ultraviolet.
LizK: Yes
alpha137: Ozone filters out the uv.
LizK: That's good for life
alpha137: On the web page, in the central frame and lower on that frame, is a Concept test on the atmosphere. Take a look sometime.
LizK: Ok, do you want me to take it? or just look?
alpha137: Anyway, the sunlight breaks up oxygen into atomic oxygen (atoms) which react in the Chapman cycle producing ozone. We have some chemical kinetics here.
alpha137: No, just look at the Concept Test for the time being.
LizK: Yes, I understand that much
alpha137: The effectiveness of all this depends on the speed of these reactions in the upper atmosphere.
alpha137: CFC's are also affected by sunlight, right?
LizK: And CFC's speed up the fourth reaction which normally is really slow, and since the CFC isn't used up it causes problems
alpha137: Right.
LizK: Yes they are affected by sunlight- they form different compounds
alpha137: To convince world leaders that this is not hokem someone needs to know the numbers.
LizK: True,
alpha137: Also, one of the important intermediates in the ozone distruction was the chlorine monoxide molecule.
LizK: Oh, yeah, it said that in the book
alpha137: It was thought that if this molecule could be found in the stratosphere this would convince world leaders that CFC's were a real problem
LizK: And was it found?
alpha137: Yes, Jim Anderson at Harvard, sent up a balloon to very high altitudes with an instrument pod that measured ClO in the stratosphere.
alpha137: Later, using a U2 spy plane with an instrument pod mounted on one wing, he also found ClO.
LizK: And they measured the concentrations and they were the highest near the holes in the ozone layer
alpha137: Correct.
LizK: THere was a diagram of that in the book
alpha137: Other workers, it is a big project, have measured stuff too.
LizK: It was a perfect inverse relationship
alpha137: This is an example of the use of science to prove the environmentalists conjecture.
LizK: and that's why we cut back on CFC's
alpha137: And that is why we cut back on CFC's. Of course, the US was the largest manufacturer of CFC's so other countries did not have much of a problem agreeing.
LizK: True
LizK: But there are still lots of CFC's being used
LizK: Today
alpha137: Next time lets be a bit more quantitative about the absorption of light by ozone, OK?
LizK: Ok,
alpha137: And there is a reservior of CFC's in the atmosphere that will take years to get rid of.
LizK: Do I have a homework probem?
LizK: If i don't that is perfectly fine with me
alpha137: No, just take a look at the Concept Test when I get the web page back on line, and you can consider how you might understand absorption of light by ozone.
LizK: Ok, I'll look at it
alpha137: Lets quit for now. See you Thurs, same time?
LizK: Do you want me to listen to one of the lectures?
LizK: Yes, see you tommorow same time
LizK: Bye, and thank you, I'm sorry I was so slow today, I'm not feeling very well
alpha137: I am not sure the real audio server is working right. You can try and listen to Flanders & Swan on thermo.
LizK: Ok, I'll try.
alpha137: Hope you get better.
alpha137: bye
LizK: Thanks, its not a big thing
LizK: Bye
*** Signoff: LizK (Leaving)