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### Topic: Some entropy questions  (Read 12076 times)

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#### dalliance

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##### Some entropy questions
« on: September 29, 2005, 07:08:33 PM »
here are a bunch of questions I have problems with
It would be great if I could get help on some of them!

cheers!

1.Calculate work and delta E when one mole of liquid is vaporised at its boiling point at 80 degrees celcius and 1.00atm pressure. deltaHvaporisation for the liquid is 30.7 kj per mol at 80 degrees celcius

- i really have no idea how to do this one

2.Is the deltaS of fusion or delta S fof evaporation higher for a liquid?

- I reckon the delta S for evaporation would be higher, but I'm not sure how to explain that.

3. What types of experiments can be carried out to determine if a reaction is spontaneous? Does spontaneity have any relationship to the final equilibrium position of a reaction?

4. The synthesis of gluclseo directly from carbon dioxide and water and the ysntehsis of proteins directly from amino acids are both non-spontaneous procsses under standard conditions Yet these processes must occur for life to exist. in light of the second law of thermodynamics, how can life exist?

-The high entropy in the surroundings is enough to get a positive increase in the system??

5. Consider a mixture of air and gasoline vapor in a cylinder with a piston. The original volume is 40cm^3. If the combustion of this mixture releases 950 j of energy, to what volume will the gases expand against a constant pressure of 650 torr if all the energy of combustion is converted into work to push back the piston?

I used the w= -PdeltaV
converted 650 torr to -0.855 atm
950J = -0.855atm x delta V
so Delta V = 1111.1 cm3?
which is 11.0L?

thanks a big bunch!
« Last Edit: September 29, 2005, 07:13:09 PM by dalliance »

#### Donaldson Tan

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##### Re:Some entropy questions
« Reply #1 on: November 25, 2005, 12:32:10 PM »
1.Calculate work and delta E when one mole of liquid is vaporised at its boiling point at 80 degrees celcius and 1.00atm pressure. deltaHvaporisation for the liquid is 30.7 kj per mol at 80 degrees celcius
H = U + PV
dH = dU + P.dV
dU = dH - P.dV

Since Vvapour >> Vliquid, then
dV = Vvapour - Vliquid = Vvapour

Vvapour = RT/P (assume ideal gas)

work = - P.dV
deltaE = dU = dH - P.dV (dH is enthalpy of vapourisation)

2.Is the deltaS of fusion or delta S fof evaporation higher for a liquid?

Liquid->Gas
there is more disorder in gas than liquid, so dS > 0

Liquid->Solid
there is more disorder in liquid than solid, so dS < 0

3. What types of experiments can be carried out to determine if a reaction is spontaneous? Does spontaneity have any relationship to the final equilibrium position of a reaction?

you need to determine if dG < 0

4. The synthesis of gluclseo directly from carbon dioxide and water and the ysntehsis of proteins directly from amino acids are both non-spontaneous procsses under standard conditions Yet these processes must occur for life to exist. in light of the second law of thermodynamics, how can life exist?

do you think our body operating condition is 25C, 1atm, 1mol/dm3? our blood pressure is about  90-130mmHg (less than 1atm, ie. 760mmHg). The average body temperature is 36oC.

« Last Edit: November 25, 2005, 12:35:50 PM by geodome »
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