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### Determining Spontaneity

Posted: 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?

### Re: Determining Spontaneity

Posted: 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.

### Re: Determining Spontaneity

Posted: 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.

### Re: Determining Spontaneity

Posted: Tue Mar 05, 2019 7:10 pm
Spontaneity is dependent on delta S and delta H so you cant assume the rxn is spontaneous

### Re: Determining Spontaneity

Posted: Tue Mar 05, 2019 7:15 pm
Here is a chart that helps understand the relationship

### Re: Determining Spontaneity

Posted: Tue Mar 05, 2019 7:18 pm
in general, you shouldn't assume anything in terms of spontaneity since it fluctuates with each reaction

### Re: Determining Spontaneity

Posted: 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.

### Re: Determining Spontaneity

Posted: 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.