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How can you tell if a reaction is spontaneous?

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How can you tell if a reaction is spontaneous?

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In order to tell if a reaction is spontaneous you must ask one simple question:

"Does the entropy of the system increase?"

All processes seek to have an entropy INCREASE, therefore if there is an entropy decrease the theoretical process cannot happen in the real universe (although I've been told theres some "odd" physicist acceptions, but these really arent applicable here)

Mathematically however this is expressed as:

[tex]\Delta G = \Delta H - T\Delta S [/tex]

Where:

[tex]\Delta G[/tex] - The gibbs free energy, this is a measure of the overall energy change for the reaction (and therefore entropy), a negative gibbs free energy corresponds to an increase in the entropy of the system and therefore a spontaneous reaction

[tex]\Delta H[/tex] - The enthalpy change for the process in question, be it a reaction enthalpy or otherwise. This is the energy released to/absorbed by the reaction and is a gauge of the overall entropy change of the system, if energy is released to the surroundings it can be said that the entropy of the surroundings is increasing.

[tex]T\Delta S[/tex] - The entropy change of the process in question.

A sometimes convinient way of viewing this equation (even though its a gauge of free energy) is:

[tex]\Delta S_{Total} = \Delta S_{Surroundings} + \Delta S_{System}[/tex]

"Does the entropy of the system increase?"

All processes seek to have an entropy INCREASE, therefore if there is an entropy decrease the theoretical process cannot happen in the real universe (although I've been told theres some "odd" physicist acceptions, but these really arent applicable here)

Mathematically however this is expressed as:

[tex]\Delta G = \Delta H - T\Delta S [/tex]

Where:

[tex]\Delta G[/tex] - The gibbs free energy, this is a measure of the overall energy change for the reaction (and therefore entropy), a negative gibbs free energy corresponds to an increase in the entropy of the system and therefore a spontaneous reaction

[tex]\Delta H[/tex] - The enthalpy change for the process in question, be it a reaction enthalpy or otherwise. This is the energy released to/absorbed by the reaction and is a gauge of the overall entropy change of the system, if energy is released to the surroundings it can be said that the entropy of the surroundings is increasing.

[tex]T\Delta S[/tex] - The entropy change of the process in question.

A sometimes convinient way of viewing this equation (even though its a gauge of free energy) is:

[tex]\Delta S_{Total} = \Delta S_{Surroundings} + \Delta S_{System}[/tex]

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- #3

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In order to tell if a reaction is spontaneous you must ask one simple question:

"Does the entropy of the

[tex]\Delta S_{Total} = \Delta S_{Surroundings} + \Delta S_{System} > 0[/tex]?

"The entropy of the universe strives toward a maximum"; therefore,if there is a decrease in the entropy of the universe, the process cannot happen.

Is there a state function of the

can occur spontaneously? There are, in fact, several. If the process occurs at constant temperature and volume, the Helmholz free energy should be used;

if the conditions are constant temperature and pressure, then the Gibbs free energy may be used. For the latter case,

[tex]\Delta G = \Delta H - T\Delta S [/tex],

where G, H, T and S are properties of the system. If the change in the

Gibbs free energy is negative, then the process in question can occur

spontaneously.

Not necessarily. The entropy of the system can in fact decrease as long...a negative gibbs free energy corresponds to an increase in the entropy of the system

as the enthalpy change compensates for this---that is what the equation says! In many exothermic chemical reactions, the change in the enthalpy is huge in comparison with the entropy term. If all of this energy were released as heat, then there would be a massive increase in the entropy of the universe. Instead of letting the energy be released as heat, one can harness the process to produce useful work. The change in the free energy function tells us how much useful work can be extracted from the process.

- #4

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KISS

Calculate the change in gibbs free energy for the reaction that you want to examine. You should have tables of Gibbs energies in the back of your textbook.

If it is negative then it is spontaneous. Simple as that.

Calculate the change in gibbs free energy for the reaction that you want to examine. You should have tables of Gibbs energies in the back of your textbook.

If it is negative then it is spontaneous. Simple as that.

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- #5

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A process is spontaneous if the Gibbs free energy is negative.

- #6

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A process is spontaneous if the Gibbs free energy is negative.

A process is spontaneous if the Gibbs free energy variation is negative.

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Is a correct statement, because the enthalpy releases energy to the surroundings that then become "The system" in order to compensate for the fact that the "universe" is a closed system in itself. I guess the wording whas slightly off i'm not seeing why I had my post rehashed and shot down when the content was basically exactly the same, but the general math was put in front of the qualitative entropy stuff, because he's probably asking it for a homework Q.

- #8

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a negative gibbs free energy change corresponds to an increase in the entropy of the system

Is a correct statement... Q.

"Shooting down" your post was not at all my intention. After all, you did

mention

put it at the very end and then in the form of a footnote, as it were? I

did, however, intend to strongly

there are fundamental errors in it and 2)several things in it

are very confusing, at least to me. If you feel hurt by my wording, I

apologise for that but I do not wish to retract my criticism.

The first error is your main statement:

This is notYou must ask one simple

question: Does the entropy of the system increase?

the relevant question but rather "Does the entropy of the

increase?". Quoting from Atkins, "The

spontaneous change in thermodynamics is the increase in total entropy of

the universe" By universe is meant the system plus the surroundings. The

entropy of the

decrease (The existence of the birds and the bees and the flowers and the

trees bears witness to this).

This criterion fully answers the question posed in the original post. And

it is not "qualitative entropy stuff"; it can be made quantitative and

precise and

derived from it

of the enthalpy H and entropy S, state functions

You failed to mention, however, that the particular criterion for

spontaneous change, a decrease in the Gibbs free energy change, only

applies to reactions which occur under the restraint of constant

temperature and pressure. You are in excellent company, for Andy_Resnick

and Lightarrow also leave out this important qualifier. They thereby

ascribe to the Gibbs free energy criterion a generality that it does not

possess: for reactions restrained to occur at constant temperature and

volume, for example, it is the Helmholz free energy that one must use.

Granted, most chemical reactions are carried out under conditions of

constant temperature and pressure, but not all by any means. So why appeal

to a criterion that only applies some of the time when a general criterion

is available? Why negotiate with the monkey when the organ grinder is

present?

Now to the confusion. Why do you insist that

? You could say that an increase in the entropy contributesA negative gibbs free

energy change corresponds to an increase in the entropy of the

system

to decrease in G. True, but G could still increase if the enthalpy

increased sufficiently. And conversely, G could decrease even if S

decreased, as long as H decreased sufficiently. So although G depends on

S, it does not

leads to a lot of confusion.

Thermodynamics is exquisitely precise but one must be precise in talking

about it. Consider this:

Whatbecause the enthalpy releases energy to

the surroundings that then become "The system" in order to compensate for

the fact that the "universe" is a closed system in itself.

does this mean? You have to be quite clear about what is the system and

what are the surroundings; you cannot willy nilly change them about. In

talking about the enthalpy you say

. Since when is the enthalpy a gauge of the overallThis is the energy released

to/absorbed by the reaction and is a gauge of the overall entropy change of

the system,

entropy change of the system? This is just plain wrong. Also, you mean

"the energy released to/absorbed by the

following in your remarks about the free energy change:

. What precisely is "overall energy change"? And why doThe gibbs

free energy, this is a measure of the overall energy change for the

reaction (and therefore entropy), a negative gibbs free energy corresponds

to an increase in the entropy of the system and therefore a spontaneous

reaction

you add "(and therefore entropy)"? Are energy and entropy being equated

here or what? Again "...an increase in the entropy of the system and

therefore a spontaneous reaction." Incorrect. There is, however, a true

statement hiding in all this: "a negative gibbs free energy (change)

corresponds to...a spontaneous reaction", at least at constant temperature

and pressure!

I'm sorry but I am hopelessly confused by such statements. You probably

want to throw bricks at me, and I understand that, but I think that a

reply to a question should be as clear as possible.

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[...]

You failed to mention, however, that the particular criterion for

spontaneous change, a decrease in the Gibbs free energy change, only

applies to reactions which occur under the restraint of constant

temperature and pressure. You are in excellent company, for Andy_Resnick and Lightarrow also leave out this important qualifier.

[...]

for reactions restrained to occur at constant temperature and

volume, for example, it is the Helmholz free energy that one must use.

That's correct. We are too much used to constant T and P processes and we can easily forget the others.

I assume that, e.g., for an enginer working on spark ignition internal combustion engines, thinking about constant volume processes is more usual.

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- #10

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- #11

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Certainly, but you know that spontaneity in chemical physics is a different concept from kinetics.

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However I do personally use the system/surroundings convention, just I see gibbs free energy as a measure of the total entropy change of a reaction, and it constitutes both the entropy change of the reaction and the entropy change of the surroundings, but I do agree that it's better to explain it in terms of the universe, just I am used to trying to explain it to people who are convinced that AHMAHGAHD it must be a change in the system! you cant talk about the surroundings! its a closed system! etc etc.

Also I wasnt aware of the helmholtz being its own equation , for constant volume but varying pressure I just tended to use the [tex]\Delta H = \Delta U + p\Delta v[/tex] relationship but it's nice to know theres a name for the constant volume case. Thanks for the clarification of the matter Pkleinod and I'll try not to do it again, after all I'm here to learn as much as the next guy :).

- #13

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spontaneity in chemical physics is a different concept from kinetics

This is exactly what I was trying to point out.

Thermodynamics (Gibbs, Helmholtz, universe entropy criteria, ...) will tell you what can be spontaneous and what won't. But even when is thermodynamically established that a reaction is spontaneous it might be that on human time scale the reaction does not evolve at all. If a reaction is thermodynamically established to not to be spontaneous, it cannot be spontaneous independently of time scale.

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- #15

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A process is spontaneous if the Gibbs free energy variation is negative.

Doh! right. Thanks.

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