gchem qs

[zriceman]

prob 173 from destroyer:

N2h4(l) ------> N2(g) + 2H2(g)

change in H is -13 kcal.


The solution states reaction is spontaneous at all temperature.


I though reaction would not be spontaneous if temperature was too low with the hypothetical numbers for T and S:

G = H - TS
= -13 - (-25)(10)
= +237

so why would the reaction be spontaneous at ALL temps?

---

Members don't see this ad.

 

[Chga]

isnt T supposed to be in Kalvin???
which would make T positive at all temprature

 

[zriceman]

isnt T supposed to be in Kalvin???
which would make T positive at all temprature

well suppose T was -25 Kelvin, the G would still be a positive number so how would G still be spontaneous at all T???

maybe i m getting something confused here, cuz i doubt the answer from Destroyer is wrong....

 

[gogobruin]

Since liquid----> gases, it means it would be less order, so entropy will be positive. Enthalpy is also positive, thus, the reaction will be spontaneous! Hope this help

 

[zriceman]

Since liquid----> gases, it means it would be less order, so entropy will be positive. Enthalpy is also positive, thus, the reaction will be spontaneous! Hope this help

The problem had set the enthalpy to -13 Kcal. So G has to be negative inorder for the reaction to spontaneous.

So i randomly assigned -25 Kevlin as the Temp and randomly assigned +10 for Entropy. The result is a positive G, which means the reaction under these assumed temp and entropy is not spontaneous.

But the answer states reaction is spontaneous at ALL temps. I m confused about the reasoning behind "all" temps.

Any other suggestions>??

 

[gogobruin]

Hi,
It's hard to get negative kelvin in the real world, becuase it will be -273c!!!!!!!!

 

[saltlake007]

Change in G = Change in H - T*Change in S

If you have a negative Change in H and Positive change in S then it is spontaneous for all temp.

If you have a positive chage in H and a negative change in S then it is nonspontaneous for all temp.

If you have negative change in H and negative change in S..then it is spontaneous at low temp.

If you have positive change in H and positive change in S...the it's spontaneous at at high temp.

 

[klutzy1987]

Delta S is positive when you go from solid to liquid to gas. Therefore since you go from one equavalent of liquid to 2 equivalents of gas your entropy is very positive. If the Delta H is negative and the Entropy is positive then a negative-positive is equal to negative. therefore your Delta S is negative andf therefore its spontaneous at all temperatures Because negative temperatures are impossible.

 

[zriceman]

Delta S is positive when you go from solid to liquid to gas. Therefore since you go from one equavalent of liquid to 2 equivalents of gas your entropy is very positive. If the Delta H is negative and the Entropy is positive then a negative-positive is equal to negative. therefore your Delta S is negative andf therefore its spontaneous at all temperatures Because negative temperatures are impossible.

Gotcha, now it makes sense. Apparently Lowest kelvin will be 0 degrees.

thanks.

 

[smile101]

Gotcha, now it makes sense. Apparently Lowest kelvin will be 0 degrees.

thanks.

you are right!!!
-25K would be -298C and i don't think the reaction would be feasible at that temp....gotta think within reasons...don't take it to extremes!!!

 

[klutzy1987]

there is no such think as negative kelvin values. O degree kelvin is absolute cant go any lower.