All spontaneous reactions result in an increase in the entropy... but

[PocketRocket]

Isn't the freezing of water spontaneous in cold temperatures leading to a decrease in entropy?

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[DAT Destroyer]

Isn't the freezing of water spontaneous in cold temperatures leading to a decrease in entropy?

The freezing of water is an exothermic process,,,,,,and we see a decrease in entropy......therefore, BOTH ENTHALPY and ENTROPY. terms are negative. Since both are negative, it will only be spontaneous at LOW temperatures . How low ? Simply set the Delta G equal to zero.. Plug in your numerical values for entropy and enthalpy .....get a value. This value is the equilibrium temperature. Any temperature BELOW this will make the reaction spontaneous.

Hope this helps.

Dr. Romano

 

[PocketRocket]

Thank you orgoman22 for taking the time to answer. But I think I didn't ask what I meant. I am referring to the 2nd law of thermodynamics which is "All spontaneous reactions result in an increase in the entropy of the Universe". If that is so than how come at low temperatures we see a decrease of entropy during a spontaneous event?

I hope I am making sense, again, thank you for taking the time to answer.

 

[FeralisExtremum]

All spontaneous reactions do result in an increase in entropy...for the universe. This does not necessarily mean that the system you are observing will increase in entropy for a spontaneous reaction, but that the universe overall (system + surroundings) will have an increase in entropy.

Another way to look at it: ΔSsystem + ΔSsurroundings > 0 (the universe is the system we are looking at, and all its surroundings). Either component can be negative but sum of the two has to be positive. So for a spontaneous process, even if the entropy of the system has decreased, the entropy of the surroundings has sufficiently increased to compensate for it.

 

[DAT Destroyer]

Thank you orgoman22 for taking the time to answer. But I think I didn't ask what I meant. I am referring to the 2nd law of thermodynamics which is "All spontaneous reactions result in an increase in the entropy of the Universe". If that is so than how come at low temperatures we see a decrease of entropy during a spontaneous event?

I hope I am making sense, again, thank you for taking the time to answer.

A spontaneous reaction can be driven by entropy , enthalpy or BOTH. A favorable reaction for spontaneity implies a negative delta H and a positive delta S. At low temp.....the entropy term is not favorable...but the Enthalpy term sure is. Hope this helps.

 

[greyworm]

A spontaneous reaction can be driven by entropy , enthalpy or BOTH. A favorable reaction for spontaneity implies a negative delta H and a positive delta S. At low temp.....the entropy term is not favorable...but the Enthalpy term sure is. Hope this helps.

what about when ∆H is (+) and ∆S is (+) at high temps? Does this mean that the high entropy is so favorable that the endothemic process is negligible?

 

[greyworm]

Almost. If you look at these problems from the perspective of Gibbs free energy, which reflects a balance between these two forces, then they'll be easier to understand.
Remember that you have T (temperature) in the equation coupled to ∆S, so under high temperature, -T∆S will be negative and its value > ∆H, which causes the overall ∆G to be negative. Hence the reaction occurs spontaneously, because a high temperature coupled with a positive entropy drives the reaction.

o wow, that makes sense, thank you