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Offline JohnDenver

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Spontaneous reactions
« on: March 28, 2015, 04:13:22 AM »
What would be a classic example of a reaction that is spontaneous at all temperatures? Do such reactions fall under a certain category?

I've noticed a couple of combustion reactions that have ΔH<0 and ΔS<0 so they are spontaneous at low but not at high temperatures. Is that generally the case?

Offline magician4

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Re: Spontaneous reactions
« Reply #1 on: March 28, 2015, 06:21:43 AM »
Quote
I've noticed a couple of combustion reactions that have ΔH<0 and ΔS<0 so they are spontaneous at low but not at high temperatures.

 :rarrow: define "spontaneous"

what you've given here are thermodynamic parameters (and those are dependent of T , by the way: they're not valid "at all temperatures"), and thermodynamics doesn't tell you nothing 'bout kinetics (take the system "gasoline + air" as an example)
= a negative ΔG doesn't tell you nothing about wether the reaction will take place spontaneously

 :rarrow: define "all temperatures"

to the best of my knowledge, no reaction will take place at or near 0 K. neither will any chemical bond withstand temperatures above ~ 6000 K (hydrogen H2 maybe ? ... not sure.. but even this molecule will give up very soon thereafter)

 :rarrow: identify parameters that might tell you something about kinetics

you might wish to take a look at the Arrhenius equation for this, and in esp. try to figure out what the parameter " Ea" might stand for

regards

Ingo
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Offline JohnDenver

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Re: Spontaneous reactions
« Reply #2 on: March 28, 2015, 08:03:40 AM »
This is the table from my text:


The two middle cases, where the signs of ΔH and ΔS are either both positive or both negative, have easy examples which come to mind. Condensation of water is spontaneous at low temperatures, and evaporation of water is spontaneous at higher temperatures.

I want to know of an example of a reaction for which ΔH<0 and ΔS>0, i.e., a reaction that is always spontaneous. In other words, a reaction for which ΔG<0, regardless of the value of T.

Offline magician4

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Re: Spontaneous reactions
« Reply #3 on: March 28, 2015, 08:37:10 AM »
with all due respect, I don't agree with your textbook's definition of "spontaneity"

as I already pointed out , there's an abundance of situations (i.e. "systems") where ΔG is negative, though nothing happens at all

consider a coal mine, air: this is a stable situation, has been so for hundreds of millions of years, though there's a huge negative ΔG

in fact, even your life depends on this "kinetic hindrance" , as the system "human being, air" is of negative ΔG , too.


btw.: reactions with a negative ΔG will be called "exergonic", those with a positive ΔG endergonic

Quote
I want to know of an example of a reaction for which ΔH<0 and ΔS>0, i.e., a reaction that is always spontaneous. In other words, a reaction for which ΔG<0, regardless of the value of T.
there is no such thing as a complete independence of T , as I already pointed out earlier

however, talking of ambient temperatures, one of many examples that comes to mind where ΔH is negative, ΔS is positive  over a broad temperature range would be the formation of Al2O3 from the elements :
4 Al + 3 O2  :rarrow: 2 Al2O3 ; ΔHof −1669,8 kJ/mol , ΔSof 52.3 J/molK

pls. also note, that this reaction by no means is spontaneous



regards

Ingo
« Last Edit: March 28, 2015, 09:20:15 AM by magician4 »
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Offline JohnDenver

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Re: Spontaneous reactions
« Reply #4 on: March 28, 2015, 09:43:57 AM »
Let me try to be clearer. I'm looking for a reaction for which ΔH°rxn<0 and ΔS°rxn>0, i.e., an exothermic reaction in which the entropy of the system increases.

I calculated the reaction you indicated, and it doesn't meet the conditions. For your reaction, we have (at 298 K):

ΔH°rxn=-3351.4 kJ/mol
ΔS°rxn=-626.4 J/molK

Your reaction is exothermic, but the entropy of the system decreases.

Offline Borek

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Re: Spontaneous reactions
« Reply #5 on: March 28, 2015, 10:26:13 AM »
an exothermic reaction in which the entropy of the system increases.

Trivial - you want to produce a lot of gases. Many combustion reactions will do that.
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Offline magician4

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Re: Spontaneous reactions
« Reply #6 on: March 28, 2015, 10:29:37 AM »
I see..

so let's better take this one:  formation of carbondioxide from the elements

oxygen O2 : Δ Hof = 0 kJ/mol; ΔSof = 205.0 J/molK
carbon C : Δ Hof = 0 kJ/mol ; ΔSof = 5.7 J/molK

 :rarrow: Σ ΔSof 210.7 J/molK

carbondioxide  : Δ Hof = -393.5 kJ/mol ; ΔSof = 213.6 J/molK

 :rarrow:  ΔHR =  -393.5 kJ/mol , ΔSR= + 2.9 J/molK

and again: not spontaneous

regards

Ingo

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Offline Dan

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Re: Spontaneous reactions
« Reply #7 on: March 28, 2015, 11:34:02 AM »
with all due respect, I don't agree with your textbook's definition of "spontaneity"

Well it is the textbook definition, not just the definition in that particular textbook.

A spontaneous process is any that releases free energy, and is not related to rate - Arrhenius is irrelevant. A reaction with ΔG<0 is spontaneous by definition, whether it takes a <1 femtosecond or >100 million years is irrelevant.
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Offline Borek

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Re: Spontaneous reactions
« Reply #8 on: March 28, 2015, 12:19:34 PM »
I see..

so let's better take this one:  formation of carbondioxide from the elements

(...)

and again: not spontaneous

I never said ALL combustion reactions are like that. I said "many".
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Offline JohnDenver

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Re: Spontaneous reactions
« Reply #9 on: March 28, 2015, 06:14:23 PM »
an exothermic reaction in which the entropy of the system increases.

Trivial - you want to produce a lot of gases. Many combustion reactions will do that.

This (and magician's example of formation of CO2) answered my question. The combustion reactions of solid carbon and n-octane (L) are both reactions of the sort I was looking for. Thank you! :)

Offline magician4

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Re: Spontaneous reactions
« Reply #10 on: March 28, 2015, 06:46:56 PM »
(...)I never said ALL combustion reactions are like that. I said "many".
we've got a case of crossing over with respect to the answers here:
"is see" was related to John's post, where he pointed out that my example didn't match his requirements completely.

Well it is the textbook definition, not just the definition in that particular textbook. (...)
I'm aware of at least three different definitions of "spontaneous":
- "thermodynamically spontaneous" ( meaning: ΔG negative)
- spontaneous with respect to second law of thermodynamics ( meaning: ΔS (universe) increases whilst process takes place)
- spontaneous with respect to kinetics ( meaning ΔG negative + Ea at room temperature ≈ fullfilled)

... the last one coming the closest to everydays use of "spontaneous"


that's why I was asking John which one he was referring to




regards

Ingo
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