Page 1 of 1

Test 2

Posted: Fri Mar 08, 2019 4:10 pm
by AngelaZ 1J
Can someone please explain why delta G is less than 0 for the following two processes?

The precipitation of snowflakes inside a cloud at -10 C and 0.839 atm.
The sublimation of dry ice in a warm room.

Re: Test 2

Posted: Fri Mar 08, 2019 4:56 pm
by Kathryn Wilhem 1I
For the snowflakes, delta H is negative because the snowflakes lose heat, and delta S is negative because going from a liquid to a solid is less chaotic. Negative delta H minus a negative delta S makes a negative delta G. For the sublimation, delta H is positive because heat is going in, and delta S is positive because going from a solid to a gas is more chaotic. Positive delta H minus a positive delta S makes a negative delta G.

Re: Test 2

Posted: Thu Mar 14, 2019 10:17 pm
by Joonsoo Kim 4L
Another (less exact, but a way for confirmation) approach for me was to think about whether or not the reaction would happen by itself or not. Both of these scenarios would probably occur spontaneously given the conditions, so it would make sense that the delta G is negative.

Re: Test 2

Posted: Thu Mar 14, 2019 10:40 pm
by Jennifer Su 2L
Kathryn Wilhem 1I wrote:For the snowflakes, delta H is negative because the snowflakes lose heat, and delta S is negative because going from a liquid to a solid is less chaotic. Negative delta H minus a negative delta S makes a negative delta G. For the sublimation, delta H is positive because heat is going in, and delta S is positive because going from a solid to a gas is more chaotic. Positive delta H minus a positive delta S makes a negative delta G.


I'm still a little confused. So deltaG=deltaH-T*deltaS
For the first scenario, you said that deltaH is negative and deltaS is also negative. So deltaG= (negative)-(negative) or (negative)+(positive). How do we know if T*S is smaller than deltaH?

For the second scenario, you said that deltaH is positive and deltaS is also positive. So deltaG= (positive) -(positive). How do we know that T*S is greater than deltaH?

Re: Test 2

Posted: Thu Mar 14, 2019 11:28 pm
by Yvonne Du
Jennifer Su 2L wrote:
Kathryn Wilhem 1I wrote:For the snowflakes, delta H is negative because the snowflakes lose heat, and delta S is negative because going from a liquid to a solid is less chaotic. Negative delta H minus a negative delta S makes a negative delta G. For the sublimation, delta H is positive because heat is going in, and delta S is positive because going from a solid to a gas is more chaotic. Positive delta H minus a positive delta S makes a negative delta G.


I'm still a little confused. So deltaG=deltaH-T*deltaS
For the first scenario, you said that deltaH is negative and deltaS is also negative. So deltaG= (negative)-(negative) or (negative)+(positive). How do we know if T*S is smaller than deltaH?

For the second scenario, you said that deltaH is positive and deltaS is also positive. So deltaG= (positive) -(positive). How do we know that T*S is greater than deltaH?


I might be wrong, but I think since you need a very low temperature for snow to precipitate, TdeltaS would be very small. The same concept goes with the second scenario. Think about the temperature that you work with for each case.

Re: Test 2

Posted: Fri Mar 15, 2019 6:15 pm
by Xinyi Zeng 4C
Yvonne Du wrote:
Jennifer Su 2L wrote:
Kathryn Wilhem 1I wrote:For the snowflakes, delta H is negative because the snowflakes lose heat, and delta S is negative because going from a liquid to a solid is less chaotic. Negative delta H minus a negative delta S makes a negative delta G. For the sublimation, delta H is positive because heat is going in, and delta S is positive because going from a solid to a gas is more chaotic. Positive delta H minus a positive delta S makes a negative delta G.


I'm still a little confused. So deltaG=deltaH-T*deltaS
For the first scenario, you said that deltaH is negative and deltaS is also negative. So deltaG= (negative)-(negative) or (negative)+(positive). How do we know if T*S is smaller than deltaH?

For the second scenario, you said that deltaH is positive and deltaS is also positive. So deltaG= (positive) -(positive). How do we know that T*S is greater than deltaH?


I might be wrong, but I think since you need a very low temperature for snow to precipitate, TdeltaS would be very small. The same concept goes with the second scenario. Think about the temperature that you work with for each case.



I agree with you, as when both delta H and delta S are positive, whether delta G is positive or negative really depends on the temperature at that particular moment!

Re: Test 2

Posted: Fri Mar 15, 2019 6:32 pm
by Celio_G_Dis2C
the reactions occur by themselves therefore they occur spontaneously