## Determining Spontaneity

$\Delta G^{\circ}= \Delta H^{\circ} - T \Delta S^{\circ}$

$\Delta G^{\circ}= -RT\ln K$

$\Delta G^{\circ}= \sum \Delta G_{f}^{\circ}(products) - \sum \Delta G_{f}^{\circ}(reactants)$

Michelle Luong 1H
Posts: 20
Joined: Fri Sep 25, 2015 3:00 am

### Determining Spontaneity

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

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
Posts: 60
Joined: Fri Sep 28, 2018 12:26 am

### Re: Determining Spontaneity

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

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

Here is a chart that helps understand the relationship

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

### Re: Determining Spontaneity

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

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

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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