Negative Entropy in Chemical Reactions
Date: 08/20/2001 at 12:45:32 From: Joseph Subject: Negative entropy in chemical reactions... Hello! I am doing some research on entropy and the Second Law of Thermodynamics. Perhaps you can help me with a question that I have? The Second Law of Thermodynamics states that entropy (disorder) in the universe must always either increase or remain constant. However, although UNIVERSAL entropy cannot decrease, INDIVIDUAL entropy can. There are many examples of this; one would be the freezing of water. This process causes water to go from a "less ordered" state to a "more ordered" state. Thus, there has been a decrease of entropy in the water. HOWEVER, during the freezing process, water also gives off energy into its surroundings, warming them and causing them to INCREASE in disorder. Consequently, there is no change in UNIVERSAL entropy; just a move from disorder in the water to disorder in the surroundings; and the Second Law of Thermodynamics is not broken. Now, to my question... Could you name some more chemical reactions in which the disorder of the chemicals involved decreases as a result of the reaction? Thanks so much! Sincerely, Joseph Dugan
Date: 08/21/2001 at 11:44:31 From: Doctor Achilles Subject: Re: Negative entropy in chemical reactions... Hi Joseph, Thanks for writing to Dr. Math. There are countless examples of reactions that give off heat in exchange for decreasing local entropy. Rather than just listing some of them, I'll give you a couple of main CLASSES (types) of reactions and you can figure out specific examples: 1) Phase changes. Water freezing is an example. Matter basically exists in three states: solid, liquid, and gas. As things move from gas to liquid or from liquid to solid, they lose entropy and give off heat. Even without changing phase, cooling things down decreases their local entropy and necessarily heats their surroundings. The reason is that molecules are moving faster (and thus have more randomness or entropy) when they are warmer. A phase change is just a dramatic example of this: gas molecules bounce around freely, liquid molecules are stuck together but move around one another, solid molecules just vibrate in place but don't move around. 2) Phase changes are often considered PHYSICAL reactions, because no molecules actually change; they just associate in different ways. Chemical reactions (where molecules interact and become other molecules) have entropy changes associated with them as well. There are published tables of the relative entropy of different molecules. So for example if you have two molecule A's reacting with one molecule B to make two molecule X's: 2A + B -> 2X Then you can find the change in entropy by adding the entropy of the products (2X) and subtracting the entropy of the reactants (2A+B). Fortunately, you don't HAVE to look relative entropies to make a good guess. In the reaction above, we started with three molecules and ended up with two. As a general rule, if you end up with fewer molecules than you started with, then entropy decreased (you've crammed the same number of atoms into a smaller number of molecules, so they are more ordered). HOWEVER, if there are phase changes involved, for example three liquid molecules becoming two gas molecules, then the general rule about counting molecules is no longer valid. Most general chemistry will focus on simple chemical reactions, such as the generation of water vapor from hydrogen and oxygen gas. My favorite subset of chemical reactions (and the topic of much chemical research these days) is biomolecules: proteins, fats, sugars, etc. Building proteins out of amino acids and building fats out of simple carbon compounds are processes that decrease local entropy substantially: many of these large biomolecules (especially proteins) are very carefully ordered to achieve a specific function. Cells expend a lot of energy and generate a lot of heat in the process of making these large molecules. When plants make sugar, they take the energy neatly packaged in individual particles of light and turn it into heat. Life is very highly structured, and living things are constantly fighting against the second law of thermodynamics to stay structured. In the process, they must use a lot of energy, which is released as heat. Hope this is helpful. Good luck coming up with examples, if you'd like to talk about this some more, or if you have other questions, please write back. - Doctor Achilles, The Math Forum http://mathforum.org/dr.math/
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