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mckeonj
74317.  Tue Jun 13, 2006 2:31 pm Reply with quote

Entropy is a big fat juicy topic which must be explored. By way of illustration, see post 74160 in Quite Interesting, topic Time.

 
jimmyc99
74910.  Thu Jun 15, 2006 3:53 pm Reply with quote

I never did understand the difference between Entropy and Enthalpy....

 
QI Individual
74914.  Thu Jun 15, 2006 4:22 pm Reply with quote

These definitions might provide a helpful distinction although it's not an explanation.

Entropy is a thermodynamic quantity representing the amount of energy in a (closed thermodynamic) system that is not/no longer available for doing mechanical work.

Enthalpy is the amount of energy in a system capable of doing mechanical work (a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure).

 
grizzly
74915.  Thu Jun 15, 2006 4:26 pm Reply with quote

Can anyone explain this in much simpler terms? Thermodynamics are not my expertise...

 
jimmyc99
74926.  Thu Jun 15, 2006 5:04 pm Reply with quote

I seem to recall in my dim distant past, my Thermodynamics lecturer explaining that one of them was a "measure of how mixed up" something was - i.e. a measure of chaos within the system.

However, from QI Individual's definitions I am still not sure to which this definition pertains - I DID say it was a dim past...

 
Celebaelin
74956.  Thu Jun 15, 2006 6:23 pm Reply with quote

This is the stuff!

In biological thermodynamics enthalpy is signified by ΔH and is a concept used in relation to the energy advantage gained in the catalysis of reactions by enzymes versus uncatalysed reactions.

This image gives some indication of the process,



the ΔG references indicate Gibbs free energy but ΔH is an alternative way of expressing a related concept. There is a better diagram showing a reaction with three transition states here

http://www.lsbu.ac.uk/biology/enztech/mechan.html

Quote:
The [catalyst] catalyzes the reaction. There are several things to consider:

* The reaction rate is increased, by lowering of the overall activation energy, but the energy difference between the reactants and products has not been altered
* The reaction requires specific bonds to be broken (H2) and this requires the input of energy. The formation of specific bonds (C-H) is exothermic and releases energy.
o The difference between the two processes is the overall reaction enthalpy (which a catalyst does not change)
o If the catalyst makes it easier to break a specific bond, then to maintain the overall reaction enthalpy, the formation of the appropriate new bonds must not result in the release of as much energy (as in the absence of the catalyst). In other words, if it takes less energy to break a bond, then conversely, not as much energy is released when forming a new bond.

http://wine1.sb.fsu.edu/chm1046/notes/Kinetics/Catalyst/Catalyst.htm

The difference between the energy of the reactants and the products gives the enthalpy for the reaction. Exothermic reactions have an enthalpy, a measure of the energy released from the reaction. The activation energy, the energy demand for initiation of the reaction is sometimes (wrongly in my view) termed the activation entropy ΔSact. This corresponds to that portion of the first diagram above which is labelled as ΔGáhyd.

The more general use of the term entropy is explained in different terms

Quote:
The Second Law of Thermodynamics or the Law of Entropy sates that the amount of disorder in the Universe increases: Every energy transfer or transformation increases the entropy of the Universe.

Entropy is a measure of disorder or randomness. To maintain any ordered structure such as a cell a continual input of energy is required because any ordered structure naturally becomes more disordered over time.

Every chemical reaction increases the amount of entropy in the universe because heat is released in the reaction*. Heat is a low quality form of energy because it is the uncoordinated random movement of molecules and so cannot easily be harnessed to perform work.

Although cells (and other living systems) are more organized (have less entropy) than their surroundings they do not violate the 2nd Law of Thermodynamics because there is a net increase in the amount of entropy in the universe because you have to include the increase in entropy caused by the conversion of some of the sunís energy from an ordered form (light) to a less ordered form (heat).

http://faculty.plattsburgh.edu/neil.buckley/Concepts%20in%20Biology/2003Lecture3.htm

* Not strictly true but there are always losses from the process even if it is endothermic so entropy in the general sense does increase. Conventional thermodynamicists will be pleased to hear I'm not arguing with the Second Law, I can hear the sighs of relief as I type!

The biological meaning of enthalpy seems to dictate that the entropy of the reaction is at least partially a related quantity to the enthalpy as biologists view it. For myself I would say that there would be a component of entropy from all changes in 'total energy' as time progresses and we move from reactants to transition state(s) and finally products.

There is some related debate here for interested parties

http://www.bio.net/bionet/mm/btk-mca/1998-May/001800.html
http://www.bio.net/bionet/mm/btk-mca/1998-May/001797.html
http://www.bio.net/bionet/mm/btk-mca/1998-May/001799.html

Athel Cornish-Bowden (2002) EnthalpyĖEntropy Compensation: a Phantom Phenomenon Journal of Biosciences 27: 121Ė126.
http://www.ias.ac.in/jbiosci/mar2002/121.pdf

Quote:
Protein Science (2001), 10:1769-1774.
Copyright © 2001 The Protein Society
Rational design of enantioselective enzymes requires considerations of entropy
Jenny Ottosson, Johanna C. Rotticci-Mulder, Didier Rotticci,1 and Karl Hult

Department of Biotechnology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden

Reprint requests to: Karl Hult, Department of Biotechnology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden; e-mail: kalle@biochem.kth.se; fax: +46-8-224601.

Entropy was shown to play an equally important role as enthalpy for how enantioselectivity changes when redesigning an enzyme. By studying the temperature dependence of the enantiomeric ratio E of an enantioselective enzyme, its differential activation enthalpy (ΔR-SΔHá) and entropy (ΔR-SΔSá) components can be determined. This was done for the resolution of 3-methyl-2-butanol catalyzed by Candida antarctica lipase B and five variants with one or two point mutations. ΔR-SΔSá was in all cases equally significant as ΔR-SΔHá to E. One variant, T103G, displayed an increase in E, the others a decrease. The altered enantioselectivities of the variants were all related to simultaneous changes in ΔR-SΔHá and ΔR-SΔSá. Although the changes in ΔR-SΔHá and ΔR-SΔSá were of a compensatory nature the compensation was not perfect, thereby allowing modifications of E. Both the W104H and the T103G variants displayed larger ΔR-SΔHá than wild type but exhibited a decrease or increase, respectively, in E due to their different relative increase in ΔR-SΔSá.

Keywords: Enthalpy; entropy; enantiomeric ratio; lipase; Candida antarctica; site-directed mutagenesis

http://www.proteinscience.org/cgi/content/abstract/10/9/1769

Whilst I feel the need to post this last quote I must say that although I understand the intention the phraseology is not, I suspect, the literal truth as intended but is rather cart before horse and terminologically inexact.

Quote:
Entropy:

In a seminal paper Page and Jencks showed that the loss in entropy in going from a bimolecular to a unimolecular reaction, i.e. E + S <=> ES, resulting in the loss of translational, rotational and vibrational degrees of freedom, could account for as much as 10^8 of the observed rate enhancement. In other words, this much free energy would come from the intrinsic binding energy.

http://www.chemistry.ucsc.edu/~fink/231/lecture21.htm

Which I guess was meant to communicate that the enzyme-substrate binding reduced the magnitude of ΔG in the activation step by an amount sufficient to explain an increase in rate of reaction of 100 million fold.

 
gerontius grumpus
75185.  Fri Jun 16, 2006 8:06 pm Reply with quote

QI Individual wrote:
These definitions might provide a helpful distinction although it's not an explanation.

Entropy is a thermodynamic quantity representing the amount of energy in a (closed thermodynamic) system that is not/no longer available for doing mechanical work.

Enthalpy is the amount of energy in a system capable of doing mechanical work (a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure).



You've just summed up the entire 1977 Chemistry A level (London Board, nuffield) cuirriculum.

 
mckeonj
75215.  Sat Jun 17, 2006 4:50 am Reply with quote

Something vaguely floated into my mind, something about "the heat death of the universe".

 
QI Individual
75323.  Sun Jun 18, 2006 3:29 am Reply with quote

"Entopic Doom" is the phrase you're looking for I believe.

The second law of thermodynamics 'causing' the universe to 'run down'.

 
Jenny
75407.  Sun Jun 18, 2006 4:38 pm Reply with quote

I posted this the last time the subject of thermodynamics came up, but I do think the elegant simplicity of Flanders and Swann's version sums the subject up beautifully. If you want to listen to an mp3 version of this, go to http://www.haverford.edu/physics-astro/songs/links.html and scroll about halfway down the page.

Quote:
Going back to first principles, very briefly, thermodynamics is of course derived from two Greek words: thermos, meaning hot, if you don't drop it, and dinamiks, meaning dynamic, work; and thermodynamics is simply the science of heat and work and the relationships between the two, as laid down in the Laws of Thermodynamics, which may be expressed in the following simple terms...

After me...

The First Law of Thermodymamics:
Heat is work and work is heat
Heat is work and work is heat
Very good!
The Second Law of Thermodymamics:
Heat cannot of itself pass from one body to a hotter body
(scat music starts)
Heat cannot of itself pass from one body to a hotter body
Heat won't pass from a cooler to a hotter
Heat won't pass from a cooler to a hotter
You can try it if you like but you far better notter
You can try it if you like but you far better notter
'Cos the cold in the cooler with get hotter as a ruler
'Cos the cold in the cooler with get hotter as a ruler
'Cos the hotter body's heat will pass to the cooler
'Cos the hotter body's heat will pass to the cooler

First Law:
Heat is work and work is heat and work is heat and heat is work
Heat will pass by conduction
Heat will pass by conduction
Heat will pass by convection
Heat will pass by convection
Heat will pass by radiation
Heat will pass by radiation
And that's a physical law

Heat is work and work's a curse
And all the heat in the Universe
Is gonna cooool down 'cos it can't increase
Then there'll be no more work and there'll be perfect peace
Really?
Yeah - that's entropy, man!

And all because of the Second Law of Thermodynamics, which lays down:

That you can't pass heat from the cooler to the hotter
Try it if you like but you far better notter
'Cos the cold in the cooler will get hotter as a ruler
'Cos the hotter body's heat will pass to the cooler
Oh, you can't pass heat from the cooler to the hotter
You can try it if you like but you'll only look a fooler
'Cos the cold in the cooler will get hotter as a ruler
That's a physical Law!

Oh, I'm hot!
Hot? That's because you've been working!
Oh, Beatles - nothing!
That's the First and Second Laws of Thermodynamics!

 
Jenny
75408.  Sun Jun 18, 2006 4:44 pm Reply with quote

Oh and there's this, which I think puts everything neatly into perspective:

Quote:
The Insignificance Song
adapted from The Meaning of Life by Monty Python

Whenever life gets you down, Mrs. Brown
And things seem sad or tough
And people are useless, or obnoxious, or daft,
And you feel that you've had quite enough...

Just remember that you're
standing on a planet that's
evolving and revolving at 900 miles an hour.
That's orbiting at 19 miles a second,
so it's reckoned,
a sun that is the source of all our power.

The sun and you and me, and all the stars we see
Are moving at a million miles a day
In an outer spiral arm, at 40,000 miles an hour,
of the galaxy we call the Milky Way.

Our Galaxy itself contains 100
billion stars
It's 100,000 light-years side to side,
It bulges in the middle, 16,000 light-years thick,
But out by us it's just 3,000 light-years wide.

We're 30,000 light-years from galactic central point
we go 'round every 200 million years
and our galaxy is only one
of millions and billions
In this amazing and expanding universe.

The universe itself
keeps expanding and expanding
in all directions
it can whizz as fast as it can go,
the speed of light, you know,
12 million miles a minute, and that's the fastest speed there is.

So remember when you are feeling
small and insecure
how amazingly unlikely is your birth
and pray that there's intelligent life
somewhere up in space,
because there's bugger all down here on Earth.


http://hea-www.harvard.edu/%7Epgreen/educ/python.html

 
Celebaelin
75427.  Sun Jun 18, 2006 5:19 pm Reply with quote

Celebaelin wrote:
In biological thermodynamics enthalpy is signified by ΔH and is a concept used in relation to the energy advantage gained in the catalysis of reactions by enzymes versus uncatalysed reactions.
//
The difference between the energy of the reactants and the products gives the enthalpy for the reaction.

This is an absolute for the reaction whether it is catalysed or uncatalysed but the RATE of reaction, and thus the energy advantage over a biologicaly relevant time-span (ie short) is very much increased by the presence of an enzyme. This is the main point regarding the life-process concept of enthalpy. It is impossible to circumvent the Second Law of Thermodynamics but given an energy input such as an adjacent star it is possible for the natural process of evolution to take advantage of local conditions and for life to occur in seeming defiance of fundamental laws of the universe as we recognise them. That much is certain in terms of our (or rather my) understanding.

We're hitching a ride on a downhill process between what I would say are two extremes of an ordered state: matter/energy being entirely 'localised' and matter/energy being uniformly distributed (assuming that would ever happen).

 
swot
487132.  Fri Jan 23, 2009 5:50 am Reply with quote

*bump*

We're doing entropy in my chemistry class at the moment. Or rather, not doing it because the teacher's not all that good at explaining things like that. He's spent the lesson explaining previuos assignments to people in class because of various mistakes on them so the rest of us have been quite bored and wondering why we've not ahd a lesson (again). I shall be directing my class to this page and asking a couple of quesitons of the learned people who grace these forums if you don't mind. that way, we might actually get a lesson on entropy one of these days.
*sigh*

 
dr.bob
487374.  Fri Jan 23, 2009 11:31 am Reply with quote

I can think of no better way to learn than reading this website*

Although I can't help thinking, if we're now making up for the deficiencies of the state school system, shouldn't we qualify for a government grant or something? :)


*This statement may or may not be true.

 
swot
488119.  Sat Jan 24, 2009 11:00 am Reply with quote

Calm down Dr B, I only meant we needed a few pointers to get us going. Websites and people who have an idea of what we're learning about and so on.
I'll be directing my fellow accessers to wiki too, obviously.

 

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