Pub #brownchem: @alpha137

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*** LizK (gloworm4@203-203-166.ipt.aol.com) has joined channel #brownchem

LizK: Hello

alpha137: hello

alpha137: Figure out the enthalpy stuff?

LizK: Yes, b ut I don;t know that the numbers are right- especially the entropy ones.

alpha137: G = H - TS where T = 298K and you get a G from the tables

For a reaction, we will say the change in G is G(final) - G(initial) = G_2 - G_1 = Delta G = Delta H - T Delta S, at T = constant.

alpha137: This should agree with this formula when you plug in the H and the S you calculate.

LizK: For the burning of Methane I got that the change in heat was -890.452, the entropy was -242.68 and the free energy was -818.01

LizK: But I didn't check the formula on them.

alpha137: In anycase for the combustion of methane the enthalpy change = -890 kJ/mol

alpha137: You are correct and the formula checks.

alpha137: A thing to remember is that the H is in kJ/mol while the S is in J/mol-K

LizK: I was a little confused because the entropy increased and that didn't sound right, but since there was energy given off I thought it was ok.

alpha137: When you form T*S the units are J/mol and you need to divide by 1,000.

alpha137: There are two factors in a reaction. First, is the enthalpy change.

alpha137: Second is the entropy change.

alpha137: These are independent, physical things-the way I like to put it.

LizK: For the octane, I got -109424 kJ/mol as the enthalpy change

alpha137: Looking at the combustion reaction CH4 + 2O_2 -> CO_2 + 2H_2O

alpha137: Is it balanced?

LizK: Yes

alpha137: OK, which side would tend to be more disordered?

alpha137: And why?

LizK: I don't know, I would probably guess the left.

alpha137: Well, from the value of the entropy change you computed you know it is the left.

LizK: Yes, but I don

LizK: t know why

alpha137: It is not so easy to tell in this case, but we are looking for the side with the greatest disorder.

alpha137: Where would this disorder arrise?

alpha137: From what physical things?

LizK: Does it have anything to do with resonance structures- if they are bouncing back and forth or there is lots of varition that wouldn't be terribly orginized

alpha137: Not resonance structures.

LizK: I didn;t think so

alpha137: That has to do with stuff like benzene and its bonds. There would be entropy effect there, but I am looking for more dynamical, simple things.

alpha137: This methane is a gas.

LizK: oh, and so is O2 and water is a liquid and therefore more orginized

LizK: I see

alpha137: Right. I am looking for translation (they are moving around in the gas) and vibration (the molecules have different kinds of vibration) and rotation.

alpha137: The resonance stuff is a higher order kind of thing here.

LizK: Ok, it was just a guess, I remembered it with all the different hydrocarbons and thought maybe it was somehow related

alpha137: When I wrote the methane combustion equation I did not specify the state of the molecules, i.e. whether liquid or gas and the pressure and the T.

alpha137: Do we need to do that and why?

LizK: No because at 298K methane, O2 and CO2 are gases and H_20 is a liquid

alpha137: Oh, contrare. I HAVE to specify the state of each compound in the equation.

alpha137: What if the water produced were a gas?

LizK: But we already assumed a set of conditions,

alpha137: OK, OK, we said T = 298K and P = 1 atm, but what if water is a gas?

LizK: That is true, but still, if it were produced as a gas, wouldn't it condense into water because of the temperature

alpha137: The reaction H_2O(gas) -> H_2O(lig) has enthalpy and entropy changes.

LizK: Ok, I see, you can't just assume that it is one or the other state

alpha137: Here is a homework problem. Compute the enthalpy, entropy and free energy (G) changes for this reaction from your tables.

LizK: Which reaction?

alpha137: Tell me tomarrow what the ans is: H_2O(gas) -> H_2O(liq)

alpha137: Remember, this is at P = 1 atm and T = 298K

LizK: Ok, will do

alpha137: Before you do this calculation from some boring tables tell me the sign of

alpha137: the S change and the H change.

alpha137: Then do the calculation. Tell me tomarrow.

LizK: Ok

alpha137: In other words, before you compute something try and figure out roughly how it is supposed to be. That is, understand qualitatively what is going on.

alpha137: Now back to gasoline and cars and air pollution.

LizK: Yes, I get that, then it is sort of a rough way to check the answer too

LizK: Alright, cars and pollution etc.

alpha137: Another homework problem:

LizK: Ok

alpha137: What is the enthalpy change per gram for the heat of combustion of methane and for the enthalpy change for the combustion of octane?

alpha137: Per gram of compound!

alpha137: The heat of combustion for octane is - 5471 kJ/mol C_8H_18

alpha137: Now some questions.

alpha137: Why is gasoline "good", or useful?

alpha137: What is the "good" side?

LizK: It is good because it produces a lot of energy to run the car, and for people in the USA, it is cheap

alpha137: OK. What else?

LizK: IT is also convenient- you just have to fill up and then go- no bother like recharging batteries

alpha137: OK. It is a liquid and easily transported and pumped.

alpha137: You say "produces lots of energy." Relative to what?

LizK: I just meant that it makes the car go

alpha137: Yes, but we are scientists here and "makes the car go" does not cut it.

LizK: OK, so it produces enough energy during combustion to make the engine run which in turn allows the car to move

LizK: But that is the same thing just in better words

alpha137: What would be a more quantitive way to put this so we could compare with other energy sources.

LizK: YOu could compare calories or Joules produced

alpha137: Right, per gram or unit weight, or by volume, or by mol.

alpha137: That is why I asked you to calc the kJ/gram for methane (a gas) and octane (a liquid).

LizK: So is that what the scientists do when they are testing the alternative fuels? Compare energy by specific units?

alpha137: Part of it.

alpha137: There are questions of efficiency too.

alpha137: Gasoline is convenient to deal with physically, to store, to pump and it has a reasonable "energy density" (kJ/gram, or kJ/unit volume).

alpha137: What is the "down" side of gasoline?

LizK: The pollution, maybe that it is so combustible and will burst into flames so easily- but you need it to burn to produce energy

LizK: IT is also made from a limited supply of nat. resources

alpha137: Right to all these.

alpha137: The burning is contained so not so bad.

alpha137: You have read the chapter, so what are the pollutants from a car?

LizK: But if something goes wrong in the car was what I was thinking- then it would just blow up- but there are lots of ways you can die in a car

alpha137: The biggest way to die in a car is to be a teenager. Statistics prove that.

LizK: Pollutants from a car, CO, N0_x, Maybe some ozone from the oxygen too,

alpha137: To be a teenager and to drink and drive.

alpha137: Where does the ozone come from? How does it come to be? Is it bad?

LizK: Ozone from the Oxygen- the heat could make an O2 molecule break apart and then the single O atom would bond with another O2 molec ule. Bad because it is another pollutant, but

LizK: good up in the atmosphere

alpha137: Right.

alpha137: The formation of ozone in the troposphere is complicated chemistry involving oxides of nitrogen and sunlight and hydrocarbons, etc. Can you find that in the text?

LizK: Its in chapter 2-I've got it

alpha137: OK.

LizK: THere is this a set of 4 reactions which can take place

alpha137: The obvious question: How can we control the ozone down at street level?

LizK: and CFC's are bad because the Cl is a catalyst for this forth reaction which changes O3 back to O2

alpha137: This last refers to the stratosphere.

alpha137: We are still on the ground wanting to get to the stratosphere.

alpha137: Ozone is a highly reactive molecule and in our lungs does lots of damage.

LizK: Ok, but haven't they started putting little filters on the exhaust pipes which trap some of these pollutants

alpha137: Right, we have to control the pollutants from the automobile.

LizK: But then it talks about a catalyst that will convert CO and O3 to CO2 and O2

LizK: That would be a good thing

alpha137: Right, we have to spend some $ on a catalytic converter to convert unburned hydrocarbons to CO_2 and H_2O.

alpha137: And so forth.

alpha137: The O3 is not produced IN the car engine.

LizK: Ok, and don't most cars now have catalytic converters?

alpha137: Yes, all cars have catalytic converters.

LizK: So where is is produced? outside the car? Inside the car but outside the engine?

alpha137: This is a price we pay to use gasoline and reduce health risks.

LizK: Yes, thats what I thought, but the book talked as if they were a new invention

alpha137: Ozone is produced in the lower atmosphere (I call it "street level") by reactions between unburned hydrocarbons, sunlight, oxides of nitrogen and oxygen itself.

LizK: Ok, so outside the car- so we can't just filter it out

alpha137: A new invention? Must be quite old people who wrote that book

alpha137: Older than me, and I am quite old myself.

LizK: I don't know that was just the impression I got

alpha137: The important point is that we need to control the emissions from cars for health reasons (not to mentions plants, etc.)

alpha137: What is a catalyst?

LizK: Yes

LizK: A substance that speeds up a reaction with out getting used up

alpha137: Right.

alpha137: Here is another point. We are using basic chemical principles to help solve environmental problems.

LizK: Yes, thats pretty neat. Except that it will be a big political problem to actually implement any of the poss. sol.

LizK: But we don't need to worry about that now

alpha137: That is too true.

alpha137: Voluntary health organizations like Am Cancer Soc, Am Heart Assoc. and Am Lung Assoc. lobby congress and educate the public. This has had a positive effect.

LizK: Yes, that is good

alpha137: Lets take stock of where we are and what we are doing and where we are going with this. Give me your take.

LizK: Well, we are talking about the chemical priniciples and processes which occur and how those affect the environment. IT is important to understand these chemical processes because nothing just appears, it must come from somewhere and it would be harder to find any sort of solution with out understanding how these pollutants got here

alpha137: Right.

LizK: I think we are going to look at the various environmental problems and what has been done already to try to soleve them and what still is needed

alpha137: If we are dealing with energy, to drive cars for one example, we need to understand in a quantitative way various energy sources and how they pollute.

alpha137: Right to your last statement.

LizK: Yes to what you said about understanding quantitatively

alpha137: Here are two chemical principles: the conservation of energy and the relationship between heat, work and internal energy

alpha137: Second, the idea that things naturally tend to become more disordered.

LizK: Ok, this all relates back to the HW problems

alpha137: Yes.

alpha137: The energy thing is written as internal energy = heat added to system less the work done by the system on the surroundings. In other words, if you had a beaker (or an engine) and you added heat to it, its internal energy would increase. Right?

alpha137: Still there?

LizK: Ys

LizK: Yes

alpha137: The statement is written in short hand as E = Q - W where W is the work done

alpha137: by the system on the surrounds.

LizK: That makes sense

alpha137: So if heat is added E increases if no work is done. What happend to this heat? What form does it take in this beaker?

alpha137: What happens to the molecule?

alpha137: molecules?

LizK: It is reflected in the KE of the molecules

LizK: They move faster

alpha137: Right. And if the molecules do work against the atmosphere the internal energy is reduced.

LizK: OK, I see

alpha137: This is a quantative relationship defining internal energy, and/or the heat. Work we think we have defined as W = -P*volume change for PV work, or it could be electrical work.

alpha137: OK?

LizK: Ok\

alpha137: Now we (or they) can define a new quantity the enthalpy, H, as the internal energy plus the pressure - volume work. PV is a work term.

alpha137: So in equation form H = E + PV OK?

LizK: When you say PV work do you mean the change in pressure caused by an increase in heat?

LizK: Ok

alpha137: No, I mean at a constant pressure and a volume change then work has been done.

alpha137: It may be that heat has caused the volume change.

LizK: BUt what is the work if the pressure and vol. are constant

LizK: OR are they changing at a constant rate?

alpha137: If there is a pressure change at a constant volume that has units same as work, but is NOT work.

alpha137: If the pressure and the volume are constant there is no work.

LizK: Thats what I thought

alpha137: Work is a force through a distance. That means that pressure (the force) through a distance (the volume change).

LizK: So they both have to change?

alpha137: The relation H = E + PV is a definition that is convenient for us.

LizK: OK thats fine

alpha137: NO, pressure is constant and only the volume changes for work to occur.

LizK: Why can vol change and that is work but when pressure changes it isn't work?

alpha137: Because work is defined in physics as a force through a distance. The force is constant throughout the distance to which it is applied.

LizK: I've never had physics so this is a bit new, but I'll think about it and just take the formula

alpha137: The force is the impact each molecule makes with the walls of the container. There is a momentum change as the molecule hits the wall and reverses direction.

alpha137: If the wall expands then it does work.

alpha137: So, work = pressure (a constant) times a volume change.

LizK: Ok, if the wall expands then what does work? the wall? or the stuff inside?

alpha137: The wall does the work and the stuff inside was responsible.

LizK: OK

alpha137: If we work at constant pressure (1 atm for example) then our definition is nice.

LizK: That's fine with me

alpha137: Enthalpy (H) change = Internal energy (E) change plus work.

alpha137: work done by our system on the surroundings.

LizK: eg. by the inside on the wall

alpha137: Next we have the other interesting chemical drive, entropy, or disorder.

alpha137: Yes, the wall moves out and does work on the atmosphere!

LizK: Yes, I understand that

LizK: So it is like a whole spiral

alpha137: Entropy is most easily visulized by thinking of molecules and order/disorder, but entropy can be defined as the heat added to the system in a reversible way at a temperature.

alpha137: S = Q(revesible)/T.

LizK: Ok

alpha137: Q, or heat, has the units of energy (Joules).

alpha137: Tell me what are the units of entropy?

LizK: Aren't they Joules too? like J/moleK? But then there is that whole reaction rate and order part which has to do with entropyu

alpha137: No, you are making this much too complicated.

LizK: Ok, it isn't joules?

alpha137: No, not Joules.

alpha137: You had it in all that complicated stuff.

LizK: I was thinking maybe the order of the reaction?

alpha137: No.

alpha137: S = Q/T

LizK: But that isn't units

LizK: its a formula

alpha137: Q has units and T has units.

LizK: oh, the J/moleK

LizK: ?

alpha137: Right. A heat unit.

alpha137: So what are the units of TS?

LizK: J/mole?

alpha137: Right.

alpha137: If we were going to define one thing that would take into account the two major chemical driving forces, the lowering of the enthalpy, and the tendency to increas disorder, what might you choose?

LizK: THis isn't a definition but more of an ex. Doesn't it all go back to the atoms trying to fullfill their octet shells? Once they are fullfilled they are more stable because they are at a lower energy( heat ) and

LizK: but then they become more orderly so never mind

alpha137: Right for atoms and molecules, but we are after something that does not depend on the microscopic view of matter.

alpha137: The two quantities we have been discussing are H and S and if we where going to combine these two how might we do it so that units are right?

LizK: Like and example of someones room it takes energy to clean up and put stuff away so it become s more disorderly

alpha137: Right to the last statement. Good example.

In my experience, with my daughter and son, the room tended to become more disordered, naturally. It seemed to follow a law of nature and it must have been the entropy principle!

LizK: Does this go back to the free energy equation?

alpha137: Yes, exactly

LizK: Thats what I thought in the beginning but I thought that was too easy

alpha137: G = H - TS and the units are kJ/mol

alpha137: Take the easy way first.

LizK: BUt where is the heat unit?

alpha137: H is measured in heat units and T * S is a heat unit.

LizK: I see

alpha137: That is why we put into this definition of G the TS term.

LizK: That makes sense

alpha137: Why do we use a minus sign as in -TS

alpha137: You can think about this last and refer to G = H -TS

alpha137: Here is where I am going. There is a relation between the change in the free energy, G, and the equilibrium constant for a reaction. G = -RT ln(K)

LizK: Because it is comparing the change in heat with the change in entropy and that determines wheter or not the reaction will run

LizK: Ok, equilibrium constants

alpha137: There is also a relationship between the change in the free eneryg and electrical work as in G = -n*(96,000 Coulombs)*(electrical potential)

alpha137: We can use all this. The second one will relate to fuel cells which we may want to use for environmental reasons.

LizK: But isn't the G=H-TS just determining which one "wins" ? ( if one is unfavorable towards running a reaction)

alpha137: Equilibrium constant, right.

alpha137: Yes, G change determines which one wins. Or which one wins determins the sign of G.

LizK: Ok,

LizK: So thats how they are related and why you subtract the TS?

alpha137: So it is G changes that we want to get at. The G changes will tell us the maximum work we can get from a chemical reaction.

alpha137: Think about all this.

LizK: Yes, I undetstand that

alpha137: We have to quit now. How about tomarrow at 9:3) am?

alpha137: 9:30 am

LizK: That sounds fine with me

alpha137: OK, bye and have a great dau.

LizK: Talk to you then, and thank you

LizK: Hav a nice day too

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