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Offline ILoveISO

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Thermodynamics Q
« on: April 22, 2010, 11:06:10 PM »
Dry ice sublimes at -78.5 degrees C under 1atm pressure. Indicate the signs of change in H, G and S for the reaction CO2(g) --> CO2(s) (1ATM)

Temp:                      H                    G              S

0 degree                  - (why neg?)    +               -   (why is this neg? shouldn't it positive at 0 degrees isn't it still a solid? so less random)

-78.5 degree            -                     0 (why zero...) -

-100 degree             -                     -               -


Basically the change in free energy column I got all wrong and don't get how to find change in G and the q's I put in brackets can anyone help

Offline Matias Ekstrand

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Re: Thermodynamics Q
« Reply #1 on: April 23, 2010, 12:56:30 AM »
At 0 degrees C you are at a temperature above the sublimation point. Does that mean the reaction should be shifted towards the left or right? How does this affect ΔG?

ΔH is negative because this change of physical state gives away heat (the opposite reaction requires heat if that is easier to understand).

Can you tell me what happens to entropy when performing the reaction that goes to the right?

ΔG is 0 at -78.5 degrees C because you will reach an equilibrium at this temperature where no net reaction takes place.
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Offline ILoveISO

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Re: Thermodynamics Q
« Reply #2 on: April 23, 2010, 01:10:19 AM »
But at 0 it hasn't sublimed yet so isn't it in a solid state still?

Also if at -78.5 G is 0 then why is at 0 degree it's positive and at -100 is negative? How do you determine G?

Offline Matias Ekstrand

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Re: Thermodynamics Q
« Reply #3 on: April 23, 2010, 01:19:16 AM »
0 > -78.5

G describes if the reaction to the right is spontaneous or not. Take a look at your reaction again, and note the physical states.
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Offline ILoveISO

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Re: Thermodynamics Q
« Reply #4 on: April 23, 2010, 01:31:27 AM »
Neg G = nonspont? and Pos G = spont?

Still don't get why H is - at 0 degrees...

Offline Matias Ekstrand

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Re: Thermodynamics Q
« Reply #5 on: April 23, 2010, 01:41:39 AM »
Neg G = nonspont? and Pos G = spont?

Other way around. A negative G means spontaneous reaction to the right.

Quote
Still don't get why H is - at 0 degrees...

ΔH does not depend on temperature.
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Offline MrTeo

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Re: Thermodynamics Q
« Reply #6 on: April 23, 2010, 11:16:15 AM »
Quote
Still don't get why H is - at 0 degrees...

ΔH does not depend on temperature.

I'm not very sure about that... we can only consider it constant for small temperature gaps... ::)
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Offline Matias Ekstrand

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Re: Thermodynamics Q
« Reply #7 on: April 23, 2010, 04:15:49 PM »
Quote
Still don't get why H is - at 0 degrees...

ΔH does not depend on temperature.

I'm not very sure about that... we can only consider it constant for small temperature gaps... ::)

Thank you for that clarification :)
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