Pub #brownchem: @alpha137

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*** Channel started at Tuesday, July 7, 1998 1:14:14 PM***
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LizK (gloworm4@171-6-5.ipt.aol.com) has joined channel #brownchem

alpha137: Hello

LizK: Hello

alpha137: Do you have Adobe Acrobat?

alpha137: Do you know what Adobe Acrobat is?

LizK: No and No

alpha137: OK then on the web page there is a link to Adobe.com and you can down load a free Adobe Acrobat reader.

LizK: Ok, I'll do that

alpha137: Adobe Acrobat is a cross computer (platform) document format called whatever.pdf

alpha137: In the reading list are some pdf files indicated by a red symbol. One of them is about fuel cells. This is something we want to discuss later.

alpha137: Do you know why I chose alpha137 as my irc name?

In physics alpha is a dimensionless constant that is a measure of the strength of electric charge. It is
the ratio of e^2/c to planck's constant, where c is the speed of light and e is the charge of the electron. alpha has a value very close to 1/137. It was thought at one time that on further measurment it would be exactly alpha = 1/137, but it was not to be. This alpha is a fundamental constant in physics and is used in theories of how electrical charges interact with each other and with light. I once did some work on this and hence chose alpha137 as a nick.

LizK: I think I tried to download it once before and it would n't let me, it said "Error" and I tried it again and it didn't work- said the file wansn't availiable, and no to your last stmmt

alpha137: Your browser should have an Adobe Acrobat plugin, but it not the download should do the trick. In order to read the pdf files you need this Adobe Reader.

LizK: how do I get that?

LizK: I keep getting the error message

alpha137: On the web page under chem F1 in the frame on the left there is a link to Adobe.

LizK: I know but when I click on that I get an error message

alpha137: Woops, I will take care of that right now.
This is done

LizK: Thanks

alpha137: Let me make this available after we are through. There is a file I have lost for the moment.

LizK: Ok, thats fine

LizK: That would explain the Error messages

alpha137: Your ppm analysis is OK. The general formula is micrograms/m^3 = 1000 x Mw x ppm/RT where at T = 273 RT = 22.4. Note that the Mw is in there.

alpha137: See if you can square your analysis with this formula.

alpha137: Now you have a way of converting ppm to weight per cubic meter and a way of finding out how much junk gets into lungs during some time period.

alpha137: Also, the breathing rate depends on exercise so you can see that with a high ozone pollution day people are advised to take it easy!

LizK: Oh, I understand, I didn't realize that RT would equal 22.4- that makes a lot more sense and my work does fit, but i did it in a round about way

alpha137: Your way was OK.

LizK: About the high ozone day- Those code red days- we have them during the summer

LizK: And the people in Florida too- all the smoke!

alpha137: Now where to these "criteria" pollutants come from? That is, TSP, SO2, CO, NO2, O3 and hydrocarbons?

alpha137: TSP means total suspended particulates-dust.

LizK: Machines and cars and burning fossil fuels- combustion reactions mostly

alpha137: I thought that if you had octane, say, that it went to CO_2 and H_2O.

alpha137: Does it?

LizK: IT does, but people don't normally drive on pure octane- there is other stuff in therie

LizK: there

alpha137: Well, suspose they did drive on pure octane. Would CO_2 and H_2O be all that came out the tail pipe?

LizK: Not unless all of the octane burned completely, and it would probably pick up some residue of something on its way from the engine to the tailpipe

alpha137: Lets forget the residue.

alpha137: If there is incomplete combustion what would probably be emitted from the tail pipe?

LizK: Ok, but it wouldn't all burn completely right?

alpha137: Right, incomplete.

LizK: CO and H_2, and maybe some of the orig. hydrocarbon

LizK: i MEANT H_2O

alpha137: Obviously, we are going to get lots of carbon dioxide and water and, yes, some hydrocarbon and some carbon monoxide.

LizK: Yes

alpha137: We are also going to get some oxides of nitrogen like NO and NO_2 and what is called NO_x to stand for all the oxides of nitrogen. Where do these come from?

alpha137: They come right out the tail pipe. Why?

LizK: I have no idea- from the air?

alpha137: Right, there is nitrogen in the air.
There might be nitrogen in some of the compounds in the gasoline and this would also be a source of nitrogen oxides.

alpha137: And there is oxygen too.

alpha137: So what do you think would cause inert nitrogen and oxygen to combine to form NO_x?

LizK: But mostly nitrogen- so thats how there is a lot of those NO_x

LizK: THe heat

alpha137: Right as rain, the heat.

alpha137: The internal combustion engine is hot and the temperature allows for the combination of nitrogen with oxygen to from NO_x.
The chemistry would be N_2 + O_2 -> 2NO; with further oxidation of the NO.

alpha137: If we have an engine we have heat and if we have heat we have NO_x.

LizK: Yes, so thats a problem- but it has to be hot to do its job

alpha137: Do jet airplanes produce NO in the stratosphere?

LizK: Yes, probably- they must

alpha137: Of course.

alpha137: Now there are a couple of things here. First, the combustion of octane produces heat and from this we can get some work and make our automobiles go.

alpha137: We want to know the relationships that allow us to know (measure) this heat.

LizK: Heat can be measured in Joules or Calories

alpha137: Second, there is some temperature effect on the speed of a chemical reaction. We need to understand that.

LizK: WE wouldfind a change in enthropy

LizK: of the two sides of the reaction

alpha137: Right, heat can be measured in Joules or Cal and there will be entropy changes.

LizK: No, but wouldn't you measure the change in heat by measuring the enthropy?

LizK: I don't know why i said no, i meant yes

alpha137: If you take 78% nitrogen and 21% oxygen plus other stuff and put it in a container at room temperature there is not production of NO_x.

alpha137: By the way, heat and entropy are not the same thing!!

alpha137: Entropy is defined as the heat added reversibly to the system divided by the absolute temperature S = Q/T.

alpha137: Heat is defined by the First Law of Thermodynamics.

LizK: I was talking about enthropy- the first law of thermo- change in H- not S

LizK: Enthalpy is what i was thinking

alpha137: In the following way: internal energy of the system = heat added to the system + work done on the system by the surroundings.

alpha137: Enthalpy would be correct.

LizK: Ok, I was really confused for a second

alpha137: The First Law is "energy" conservation. A relationship between energy, heat and work.

LizK: Ok, I remember this now

alpha137: E = Q + W, or some say E = Q - W. With the + sign W is work done ON the sytem and with the - W is work done BY the system ON the surroundings.

alpha137: By the way, gasoline contains more energy per unit weight than does octane.

alpha137: We want to know how much work we can get out of a gallon of gas.

LizK: HOw do we know how much energy gas contains- am i supposed to know that?

alpha137: We know that the internal combustion engine is polluting the atmosphere, right?

LizK: Yes

alpha137: You cannot know how much energy a gallon of gas contains, but someone has measured this for us and we can look it up.

LizK: Ok,

LizK: Where?

alpha137: We can then understand the effiency of a car and perhaps we can find something that is more efficient.

LizK: That makes sense

alpha137: The chapter on energy has a discussion of this, but we can develop it here to some extent. I will post a table of these "energies"-rather these enthalpies.

LizK: Thank you

alpha137: There are two chemical driving forces. The tendency towards lower "energy" and the tendency towards higher disorder (greater entropy).

LizK: And which one "wins" determines the endo- or exo- thermicness of the reaction

alpha137: We have to measure the enthalpies in some standard manner. Then we have to define entropy (did all ready) and measure entropies for compounds as well.

alpha137: Yes, which one wins determines endo- or exo-thermicity.

LizK: They have tables for that don't they?

alpha137: Yes they do.

alpha137: Here is the way we can do this.

LizK: I have a copy of a table for change in H, S, and G already

alpha137: We take 12 grams of pure carbon place it in a container with excess oxygen and place the container in a known weight of water. The container is sealed and has the equivalent of a spark plug in it.

alpha137: We strike the spark-that is, combust the carbon-and measure the temperature rise of the water. We therefore know the Joules released for C + O_2 -> CO_2.

LizK: That makes sense- I was thinking you

alpha137: This is called the heat of combustion of carbon and it is also the enthalpy of formation of carbon dioxide. Look up the value.

LizK: would measure the temp change of watere

LizK: -393.5

alpha137: Right.

LizK: kJ/mol-1

LizK: mol^-1

alpha137: Now, if we DEFINE the enthalpy H for carbon (all the elements in their normal state) at 298K to be zero then we have the enthalpy of formation of carbon dioxide -393.5 kJ/mol

LizK: why 298K?

alpha137: So, here is a problem for you. Take the enthalpies of formation for methane, for water and for carbon dioxide and find the enthalpy change for the combustion of methane.

alpha137: The 298K is because this is 25C which is room temperature. We have to agree on standard temperatures and pressures and this is the practical temperature.

alpha137: Naturally, we will pick 1 atm as the standard pressure.

LizK: ok,i was just wondering

alpha137: So, those tables are for T = 298K and P= 1 atm.

alpha137: If we have a reaction at a different temperature we may have to do something.

alpha137: You can treat a chemical reaction equation like an algebraic equation.

alpha137: CH_4 + O_2 -> CO_2 + H_2O you need to combine the enthalpies of formation for each of these so that when you add them up you get this over all equation.

alpha137: In your tables do you have the enthalpy of formation for octain?

LizK: I get that it is an exothermic reaction and that

LizK: The totalchange in H is -1040.15kJ/mole

alpha137: That is for methane?

LizK: Yes, but i thought that it was 2H_2O and 2O_2 to make the equation balanced and that is why the numbers are bigger

alpha137: I did not give you a balanced chemical equation.

LizK: But don't you need a balanced one?

alpha137: You obviously understand that you must have the same numbers of atoms on each side of the equation. Right?

LizK: I don't have the numbers for octane

LizK: Yes

alpha137: I do. the enthalpy of formation for octane = -249.9 kJ/mol

alpha137: C_8H_18 is octane.

LizK: I remembered that- the -ane, -ene, -yne thing

alpha137: We should compare the enthalpies of combustion for these two.

LizK: Was I wrong with methane?

alpha137: Later we want to understand how we get useful work from these reactions.

alpha137: Well, I think so. I think the enthalpy of combustion of methane = -890 kJ/mol

alpha137: That would be for one mole of methane. We should also compute the "enthalpy density" for octane and methane as combustion sources. More on that later.

alpha137: This is a homework problem to do these two enthalpy of combustion calculations.

LizK: Ok, I don't know- I could have added numbers wrong- but i think i had the process right-

alpha137: Where we are going is that because the auto is the source of so much pollution if we had a more efficient energy source we could cut that pollution down.

LizK: yes, but just a question on more efficient cars etc. once one is developped how long will it take before a significant amount of people actually are driving it

LizK: ?

alpha137: The fuel cell is one such idea. First we have to understand ordinary sources of energy like the combustion of octane (or gasoline) and methane.

LizK: Ok

LizK: THat makes sense

alpha137: That business of having numbers of people driving is a political one, and an economic one. In europe and japan gasoline is taxed heavily. In europe this money is used to fund social programs, etc. Here in the USA we have very cheap gas.

alpha137: On the other hand, US has fairly strict environmental regulations.

alpha137: To meet these environmental regulations we need a less polluting auto.

alpha137: A less polluting auto must mean a more efficient auto, or one that does not burn gas in the usual way. Right?

LizK: Yes

alpha137: This motivates us to understand the enthalpy of combustion thing.

LizK: I see

alpha137: There are two other factors of basic chemistry.

alpha137: First, is the question of order/disorder (entropy) in a chemical reaction.

alpha137: In you tables there is a column of entropies I would guess.

LizK: Yes

LizK: and free energies too

alpha137: Do you know what Free Energies are?

LizK: Yes, thats the third law of thermodynamics, it depends on the change in heat and entropy

LizK: IT is the energy availiable to do work

alpha137: The free energy of formation of octane = + 6.4 kJ/mol and the entropy = 361.1 J/K

alpha137: You can compute the enthalpy of combustion for methane and for octane. Also, compute the free energy change for these two reactions and the entropy change as well. OK?

LizK: Ok

alpha137: I mentioned a fuel cell. This is based on getting useful work out of the free energy. It should be more efficient than just getting work out of the flow of heat. That is where we are going.

LizK: I see, is the fuel cell related to that electrochemistry stuff? and redox equations?

alpha137: G = H - TS is the relationship between the free energy change the enthalpy change and the entropy change at a constant temperature, here T = 298K.
The temperature does not have to be constant. In our use it is easier to consider T = const. Then we can write dG = dH - TdS where the d means difference. A property of these E, G, H, S things is that they are independent of the path and depend only on the initial and final states. How we get to the final temperature conditions does not matter when we express the difference, say, in Free Energy.

alpha137: When you make your calculations see if this relationship is valid.

alpha137: Yes, the fuel cell is related to that electro chem stuff.

alpha137: I think we should stop now. You have some problems and I have some other work to do. By the way, I am enjoying this.

LizK: Doesn't that = H-TS help to find the different changes- a shortcup>

LizK: Me too,

LizK: shortcut

alpha137: The point is that Delta G = change in Free energy = Delta H - T Delta S
dG = Delta G = G_2 - G_1 = G(final) - G(initial) = dH - TdS at T = const.

alpha137: are all connected. If you know the G's and the H's you get the S's, etc.

LizK: yes, thats what i thought, I'll see you tommorow

alpha137: Anyway, have a nice day. Bye See you Wed at 9:30 am?

LizK: Thats fine with me

LizK: thank you

*** Signoff: LizK (Leaving)