Sunday, 30 September 2012

Second law of Thermodynamics in life

Energy in 3 consecutive forms: potential, kine...

Energy in 3 consecutive forms: potential, kinetic, internal (Photo credit: Wikipedia)

A Falling Rock

A rock has potential energy (PE) localized in it when you lift it up above the ground. The rock is the system; everything else it encounters is the surroundings. Drop the rock and its PE changes to kinetic energy (energy of movement, KE), pushing air aside as it falls (therefore spreading out the rock’s KE a bit) before it hits the ground, dispersing a tiny bit of sound energy (compressed air) and causing a little heating (molecular motion energy) of the ground it hits and in the rock itself. The rock is unchanged (after a minute when it disperses to the air the small amount of heat it got from hitting the ground). But the potential energy that your muscles localized in by lifting it up is now totally spread out and dispersed all over in a little air movement and a little heating of the air and ground.

A Hot Frying Pan

A hot frying pan? The iron atoms in a hot frying pan (system) in a room (surroundings) are vibrating very rapidly, like fast "dancing in place". Therefore, considering both the pan and the room, the motion energy in the hot pan is localized. That motion energy will disperse—if it is not hindered, according to the second law. Whenever the less rapidly moving molecules in the cooler air of the room hit the hot pan, the fast-vibrating iron atoms transfer some of their energy to the air molecules. The pan’s localized energy thus becomes dispersed, spread out more widely to molecules in the room air.

Some Rusting Iron

In a chemical reaction such as iron rusting, i.e., iron plus oxygen to form iron oxide (rust), the reactants of iron and oxygen don't have to be at a high temperature to have energy localized within them. Iron atoms (as -Fe-Fe-Fe-) plus oxygen molecules of the air (O-O) have more energy localized within their bonds than does the product of their reaction, iron rust (iron oxide).

That’s why iron reacts with oxygen—to release energy from their combined total of higher energy bonds and form the lower energy bonds in iron oxide. Then, all that difference in energy becomes dispersed to the surroundings as heat i.e., the reaction is exothermic and makes molecules in the surroundings move faster. But remember how chemical reactions occur! Remember that it requires energy to break bonds and therefore to start any reaction there must be some extra energy, an activation energy supplied somehow to break a bond or many bonds in the reacting substances. Then, if the bonds that are being formed in the product are much stronger than those being broken in the reactants, that difference in energy (which usually causes greater motion energy of all the molecules) can feed back to break more bonds in the reactants.

However, in the case of iron reacting with oxygen at normal room temperature around 298 K, the process is very slow because only a few oxygen atoms are moving exceptionally fast and hit the iron just right so an Fe-Fe bond and an O-O bond are broken and an Fe-O bond can form. There isn't enough heat (motion energy) localized in nearby iron atoms, and there are no other unusually fast-moving oxygen molecules. It's a slow process depending on collision of the small amount of fast moving oxygen atoms in the surroundings to make it happen.

Therefore, even in moist air (that speeds up another process yielding iron oxide), iron doesn't react very rapidly with oxygen but it steadily does so and in time, both the iron atoms and the oxygen molecule spread out to the surroundings the portion of their bond energy that iron oxide doesn't need for its existence at that temperature.

A Leaky Tire

Air in a tire is at a higher pressure than the atmosphere around it, so it shoots out even from a small hole.  (Every spontaneous physical or chemical process involves the second law!) Those nitrogen and oxygen molecules in the tire each have motion energy but it is far more localized, compressed in the small volume of the tire, than it would be in the huge volume of the atmosphere. Thus, the second law explains why punctures or blowouts occur: the motion energy of those localized molecules will become dispersed and spread out to the lower-pressure, larger-volume atmosphere if it is no longer hindered by the tire walls from becoming so.

A Melting Ice Cube

An ice cube melts in a big warm room. How can the melting of a little ice cube in a warm room maybe 200,000 times bigger than it is be an example of the second law? How could that possibly be a spreading out of energy? But the second law has to do with energy dispersal and there's a little spreading out in that 200,001st part of that total of system plus surroundings!

Lots of things are happening when molecules of the warm air disperse some of their energy to the molecules that are vibrating (like dancing rapidly in one place) in the ice cube. Right at the surface many hydrogen bonds between the water molecules of the ice are broken by the motion energy of the air molecules being transferred to the those surface molecules. (This doesn't change the amount of motion energy of those molecules and therefore their temperature doesn't change. They increase in potential energy due to the hydrogen-bond breaking.) Now, because the water molecules whose hydrogen bonds to other molecules in the rigid ice structure are broken, they are free to form hydrogen bonds to other water molecules that are liquid—they can exchange partners and move from one to another. The vibrational energy that allowed them to dance in place in the crystal is changed to translational energy in the liquid and the molecules can move just a bit.

Thus, although the true picture is just a bit more complex (i.e., it is the closer energy levels in translation than in solid vibration that make the energy far more dispersed in liquid than solid), we can sense that the movement of molecules in liquid water allows the energy to be more spread out than in crystalline ice, even at melting temperature. It is not a matter of order and "disorder"!

Order to Disorder

The second law tells us about energy dispersal, and entropy is the word for how that energy dispersal is measured—how spread out the energy becomes in a system, how much more dispersed it has become compared to how localized it was. Such energy changes and consequent entropy changes are the focus for understanding how and why spontaneous events occur in nature. Only sometimes do the structures or arrangements of molecules in an object help us to see greater or lesser localization of energy (that used to be called "order to disorder").

Now we can understand what scientists have been talking about the last century and a half when they spoke in apparently mysterious sentences like "The entropy of the universe increases toward a maximum." All they meant was simply that energy, everywhere, spreads out as much as it can (and that spreading out of energy is measured by entropy).

Enhanced by Zemanta
Technorati Tags: ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Blogger Labels: Second,Thermodynamics,life,Energy,Photo,Wikipedia,Rock,system,surroundings,Drop,movement,muscles,room,Some,Iron,chemical,reaction,temperature,product,difference,Remember,activation,collision,molecule,portion,existence,Leaky,Tire,atmosphere,Thus,walls,Cube,times,example,dispersal,Lots,crystal,translation,vibration,disorder,Order,events,nature,arrangements,universe,ScienceShot,Smallest,Possible,news,Largest,beginnings,Chemistry,Mind,Science,drug,children,Technologue,Oceans,Four,Covalent,Bond,Split,atoms,reactions,substances,blowouts,articles,kinetic,molecules,oxygen,oxide,reactants,doesn,hydrogen,entropy

No comments:

Post a Comment