Pub #brownchem: @alpha137
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LizK: Hello
alpha137: Hello and good morning
alpha137: Get a chance to work the problems and think of the questions?
LizK: I have some anwers to the questions-whether or not they are right, I don't know
alpha137: Lets find out
. LizK: First, I guessed that both the heat and entropy would decrease when gas H_20 goes to liquid H_20
LizK: and the numbers were change in H=-44.1 kJ/mol amd the change in entropy was -118.74 J/K mol
LizK: For the second question, I found that the enthalpy change per gram of methane was -55.7kJ/gm
LizK: And for octane it was -48kJ/gm
I looks like, on a weight basis, methane is better than
octane for energy per unit weight.
alpha137: Looks right.
LizK: Oh good
alpha137: Why is the entropy change negative?
alpha137: For water.
LizK: Because it is in a more orginized state, liquid has more orginization than gas, and ice even more than liquid
LizK: And that makes sense, they are less free to move around
alpha137: Right. Why is the change in enthalpy negative?
LizK: because it is becoming colder and so giving off some heat and I knew entropy was going to go down so it was obvous enthalpy had to decrease or the reaction would have been non-spontaneous, and this is definately a spontaneous reaction
LizK: Spontaneous at some temperatures
alpha137: We know that at 25 C = 298 K water vapor would condense to form the liquid, right?
LizK: yes
alpha137: If you compute the change in the free energy for this what do you get?
alpha137: Change in G = Change in H - (298)* Change in S
alpha137: Get everything in kJ/mol
LizK: I get 35340.4kJ/mol
alpha137: I get - 8.6 kJ/mol
LizK: because i forgot to convert the change in entropy to kJ/mole
alpha137: Remember that S is in J/mol-K so you have to divide by 1000 to get kJ
alpha137: Right
alpha137: OK, so -8.6 kJ/mol
LizK: Yes, I thought that was a strange number to get
alpha137: Now this was a spontaneous reaction because at room temp water condenses.
LizK: Yes
alpha137: G is looking at both the heat and the disorder thing. We get heat out of the water because the change in H is negative.
LizK: Yes
alpha137: But we do the unnatural thing and create more order by forming liquid water and so the entropy part of the G is a positive.
alpha137: When we add them up the net is negative.
LizK: I understand
LizK: And negative G is good
alpha137: This negative G means a spontaneous reaction
LizK: Yes, because energy would be released
alpha137: It could be that the entropy term wins out under some conditions.
LizK: like if it were a lot colder
LizK: i meant a lot hotter
LizK: I was thinking of the flip
alpha137: Do this: Assume that the change in the free energy is zero. That is G = 0 = H - TS and caluculate T.
alpha137: For this water thing.
alpha137: Assume that H and S are constant with T.
LizK: I get 371.4 degrees K
alpha137: Now that is pretty close to the boiling point of water right?
LizK: yes
LizK: It makes sense- if it is 200 degrees C or something water won't condense
alpha137: In fact, since we assumed H and S were independent of T, that is the boiling point of water.
alpha137: At the boiling point of water, at 1 atm pressure, are water vapor and liquid water in equilibrium or not?
LizK: That makes even more sense, in fact i was thinking that it probably was the boiling pt. of water
LizK: Yes, during that flat phase on the graph of temperature vs. time
LizK: as temperature increases there are two plateaus where water is changing state, so temp is constant but state is changeing
LizK: Yes it is in equilibrium
alpha137: OK, the point is that G is a measure of whether a reaction is going to favor the products or the reactants and G tells us if we have equilibrium, G = O.
LizK: I see
alpha137: If G change is negative then the products are favored and if G change is positive then reactants are favored.
LizK: I get that much
alpha137: I want to get back to pollution, but first discuss where this is going.
LizK: OK,
alpha137: When the energy conservation/definition of heat and work was discussed we reallly stated the First Law of Thermodynamics E = Q - W (work done by system on surroundings)
LizK: Yes,
alpha137: This was used in defining the enthalpy H = E + PV, right?
LizK: Yes
alpha137: So if we have other work terms, such as electrical work, this would go into the first law equation right?
LizK: I think so
alpha137: Sure it would, because E = heat added - any work done on the surroundings
alpha137: It could be any kind of work.
alpha137: Why?
alpha137: What are we doing here?
LizK: Because all work is fundamentally the same thing?
alpha137: Right and we want to keep track of all of it so that energy is conserved.
alpha137: So if this other work gets into E it gets into H, right?
LizK: Yes
alpha137: And since WE defined G = H - TS if this other work gets into H it gets into G, right?
LizK: Yes
alpha137: And therefore, if we have electrochemical work (as in a battery) that gets into G?
LizK: Yes
alpha137: Or if we have the transport of electrons across a cell membrane that takes work and that will have a G change, right?
LizK: Yes
LizK: So that is how photosynthesis really works
alpha137: If we have an electrochemical reaction (that battery again) and we get work from it that will be reflected in a change in G, right?
alpha137: Yes, that is a part of the description of photosynthesis.
LizK: Yes, and we can use that G to do work like run a car, right?
alpha137: This G change will tell us the maximum useful work we can get from a process.
LizK: I see
alpha137: If we just burn hydrocarbons the way we extract the energy is simply to transport heat from a hot place to a colder place.
alpha137: Right?
LizK: Yes
alpha137: But if we can do something that will utilize the G change we may be able to extract a greater amount of work. That is the way it turns out.
LizK: I see
alpha137: If that work is electrochemical in nature, we are talking fuel cell now, then we can extract electrical chemical work and that will be the maximum work.
LizK: Ok
alpha137: Fuel cells are going to be more efficient than just burning hydrocarbons.
alpha137: That is where we are going.
LizK: I see,
alpha137: Now, if you had that battery I mentioned and you put a volt meter across the plus and minus terminals you would measure a voltage, right?
LizK: Yes
alpha137: You could use that voltage to drive an electric motor?
LizK: Why does a fuel cell maximize the energy change?
LizK: Yes, you could use the voltage to drive electrric motpr
LizK: motor
alpha137: We have to do some math with G and S to see how we get max work from the free energy thing. Later.
LizK: Ok
alpha137: And how this beats the simple transfer of heat.
LizK: I see
alpha137: Anyway, you have this volt meter attached to the batter and you have the battery driving the electric motor. Like the bunny you just keep on going. What eventually happens?
LizK: Well, if the battery runs out of energy then the motor will stop
alpha137: Even for the battery the bunny uses!
LizK: Yes, i think so
alpha137: Right and what voltage will you measure?
LizK: O?
LizK: 0
alpha137: Right.
LizK: Ok, I don'
alpha137: The voltmeter is a free energy meter for the battery.
LizK: t understand how a battery can run out of reaction, what has happened -I'm talking about the real batterys not like little ones you use every day
alpha137: A battery is nothing more than a chemical reaction.
LizK: I mean, do the actual liquid and solid parts of the battery, or paste and solid as the case may be change into something else so that they can
LizK: cant react in the same way?
alpha137: It is a chemical reaction that has been constructed in a way that allows one to get electrons (and electrical work) out.
LizK: Ok,
alpha137: There are different constructions of batteries depending on the particular chemical reaction.
LizK: Yes
alpha137: When the battery is new do you think there are more reactants present or more products. A stupid question really.
LizK: More reactants
LizK: And so when the battery dies it means that there are more products, so the reactants cant react because they are not there anymore
alpha137: Right. And at the start (starting concentrations of reactants, etc.) do you think the free energy change for the reaction is positive or negative?
LizK: It would be negative
alpha137: Right.
LizK: Now I understand it
alpha137: When the battery reaction is at equilibrium the free energy change for the reaction (with appropriate concentrations) will be zero.
LizK: Yes
alpha137: The battery cannot do useful work at equilibrium.
LizK: Ok
alpha137: The voltage is zero and so is the free energy change.
LizK: ok
LizK: That makes sense
alpha137: If we have a suitable chemistry we may be able to recharge the battery.
LizK: Ok
alpha137: What is the charger doing?
LizK: Adding electrons?
LizK: OR at least energy
alpha137: That, but in a more macro sense.
LizK: reactants
alpha137: OK, but Free Energy, G.
alpha137: Yes, the charger is driving the reaction backwards from products to reactants.
LizK: Ok, i see where you were going
LizK: another question, if you recharge batteries, their "life" is shorter than the original one, right?
alpha137: The battery charger is pushing free energy into the reaction to do this. It is overcomming the G of the reaction with the electical voltage from the charger.
LizK: I understand
alpha137: The battery life becomes shorter because life is not fair.
LizK: Ok,
alpha137: That is, we are considering ideal situations and they are rarely achieved.
LizK: I see, it can't fully change all the products back to reactants
alpha137: The other day I put out a formula relating G change and electrochemical work.
LizK: Yes, I remember that, it had a really long number in it
alpha137: I said that Delta G = - n(number of moles of electrons) F(number of Coulombs/mole of e's) V (electrical potential.
alpha137: G = -nFV
LizK: Ok, so maybe I remembered a different equation with a long number
in it
No, you had it right because the F, or Faraday, is F = 96,500
Coulombs/mol e's.
alpha137: I am using V because I already used E for internal energy.
LizK: ok
alpha137: No, you are remembering another formula that connects the Free
Energy change with the equilibrium constant for a reaction, G = -RT ln(K)
Well, you remembered the right formula with the long number
and here is another one. Note that RT has energy units and LnK is dimensionless.
G has energy units.
alpha137: Both these are OK and in fact we could equate them.
LizK: oh, I see
alpha137: What do you get if you do that?
LizK: That -nFV=-RT ln(K)
alpha137: Right. This connects the equilibrium constant with the electrochemical potential for a reaction.
LizK: ok
alpha137: If you measure the V then you can compute the K.
LizK: oh, and you could use that to find the max amount of energy
alpha137: Think about all this. We are really down to fundamentals here.
alpha137: Yes, the max useable eneryg.
LizK: or temperature at wich the reaction should be run
alpha137: That too.
LizK: This makes sense
alpha137: So, we can use this later to compare the usable work from a fuel cell (a chemical reaction from which electrical work is obtained) to burning stuff.
LizK: Oh, I see
LizK: So that is how you could compare the amount of energy in different fuels
alpha137: I want to put this on hold for a bit so that you can digest it and so I can think as well. In the text (I have an older version) there is a box 1.10 do you have it? around page 19
alpha137: Yes, about the fuels.
LizK: Yes, I have it
alpha137: This box is talking about the clean air act and asks some opinion.
LizK: ok, maybe I don't- is it a "Your Turn" or a "Consider This"?
alpha137: OK, think about this and lets discuss it tomarrow. I had lunch with some economists yesterday and we discussed growth and environment.
alpha137: It is a Consider This.
LizK: Well, I'll try to find it
alpha137: I will put it in the mIRC part of the web page so that we can discuss it.
LizK: Oh, thank you, I don't see it here
alpha137: It is the economists that I really want to talk about and it has also to do with the china article - the one where the author says their growth rate is off set by environmental costs.
alpha137: I have to go now. Bye and have a great day again. We will talk Monday at the same time?
LizK: Yes, and there was another artical talking about how biologists and economists theorized the same tthings
alpha137: That is what I want to discuss.
LizK: Actually, I have a problem with Monday
alpha137: OK when is a good time?
LizK: I can talk between 9-10 or between 1 and 2 in the afternoon
LizK: which ever is best for you
alpha137: On Monday? I would perfer 9 - 10
alpha137: am that is.
LizK: Ok, thats fine, talk to you then, have a nice weekend,
alpha137: Could be earlier.
alpha137: OK 9 - 10
alpha137: bye
LizK: or 8:45 is fine
alpha137: 8:45 it is
LizK: ok, see you then, thank you, bye
*** Signoff: LizK (Leaving)