Determining Spontaneity






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Michelle Luong 1H
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Joined: Fri Sep 25, 2015 3:00 am

Determining Spontaneity

Postby Michelle Luong 1H » Mon Jan 25, 2016 12:23 am

So a system is spontaneous when DeltaG is negative and also when Delta S total is positive. But why can't we tell the spontaneity of a system by looking at Delta H? I know when Delta H is negative, it's exothermic, but why can't we tell if the reaction is spontaneous just by looking at Delta H?

Shah Jafri 4k
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

Re: Determining Spontaneity

Postby Shah Jafri 4k » Mon Jan 25, 2016 12:30 am

spontaneity is a concept that does't just depend on deltaH. It is when delta G is negative. Delta G depends on delta H, temperature, and delta S. Just because a reaction is exothermic, it is not necessarily spontaneous.

Cameron_Greenberg_3C
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Joined: Fri Sep 28, 2018 12:26 am

Re: Determining Spontaneity

Postby Cameron_Greenberg_3C » Wed Feb 27, 2019 8:31 pm

While exothermic reactions release energy, they can't be assumed to be spontaneous because a low entropy value could counter a negative enthalpy value ad make delta G positive, making it non-spontaneous.

Destiny Diaz 4D
Posts: 51
Joined: Fri Sep 28, 2018 12:28 am

Re: Determining Spontaneity

Postby Destiny Diaz 4D » Tue Mar 05, 2019 7:10 pm

Spontaneity is dependent on delta S and delta H so you cant assume the rxn is spontaneous

Destiny Diaz 4D
Posts: 51
Joined: Fri Sep 28, 2018 12:28 am

Re: Determining Spontaneity

Postby Destiny Diaz 4D » Tue Mar 05, 2019 7:15 pm

Here is a chart that helps understand the relationship
WIN_20190305_19_14_07_Pro.jpg

LeannaPhan14BDis1D
Posts: 57
Joined: Fri Sep 28, 2018 12:16 am

Re: Determining Spontaneity

Postby LeannaPhan14BDis1D » Tue Mar 05, 2019 7:18 pm

in general, you shouldn't assume anything in terms of spontaneity since it fluctuates with each reaction

Sapna Ramappa 1J
Posts: 63
Joined: Fri Sep 28, 2018 12:24 am

Re: Determining Spontaneity

Postby Sapna Ramappa 1J » Sat Mar 09, 2019 8:40 am

Spontaneity is associated with the delta G values. If you are given delta H and you want to find if something is spontaneous or not, plug in values for T and delta S.

Pritish Patil 1K
Posts: 60
Joined: Fri Sep 28, 2018 12:24 am

Re: Determining Spontaneity

Postby Pritish Patil 1K » Tue Mar 12, 2019 1:31 pm

Because if delta S is small enough (negative enough = large negative number), then even if the reaction was slightly exothermic (delta H < 0), then delta G could be greater than 0, making it nonspontaneous.


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