The Second Law of Thermodynamics!
Q: Who cares about the second law of thermodynamics?
A: Anyone who wonders how the material world -- our world of energy and matter --works.
Q: Big deal?
A: The biggest, most powerful, most general idea in all of science. Why paper, trees, coal, gas and all things like them burn (and why people "should" spontaneously catch fire in air), why sand and dry ice even in pure oxygen can't ever burn, why the sun will eventually cool down, why iron rusts (but not why it rusts faster nearer the ocean), why there are hurricanes or any weather at all on earth, what makes things break, why houses get torn apart in tornadoes or explosions, why everything living tends to die.
That's for starters.
Q: Just STARTERS? OK, OK, I'm impressed. So, what IS the second law of thermodynamics? Well, wait a minute, what's the first law?
A: The first law is very simple and important but pretty dull: You can't create or destroy energy.
Q: Sounds fair. I'm listening.
A: Look at the direction that energy flows in any happening or process or event. That is the first step to understanding what the second law of thermodynamics is and what it applies to.
Q: Come on. All this build up for that dumb example?
A: Don't put me down. I could have snowed you with differential equations and diagrams instead of what you see everyday. We're being practical and visual rather than going the math route, essential as that is in chemistry.
The big deal is that all types of energy spread out like the energy in that hot pan does (unless somehow they're hindered from doing so) They don't tend to stay concentrated in a small space; they flow toward becoming dispersed if they can -- like electricity in a battery or a power line or lightning, wind from a high pressure weather system or air compressed in a tire, all heated objects, loud sounds, water or boulders that are high up on a mountain, your car's kinetic energy when you take your foot off the gas. All these different kinds of energy spread out if there's a way they can do so.
Get the picture? The second law of thermodynamics summarizes that totally different events involving all kinds of energy have a common cause. A blowout in a tire or a car battery shorting out or slowly running down -- what could seem to be more unlike than those! Yet the reason for their occurring is the same, the tendency for concentrated energy not to stay localized, to disperse if it has a chance and isn't hindered somehow.
A major goal in life is to find true BIG ideas that describe how the world works, to understand why and how things happen around us in terms of a small number of basic principles. Principles that can be tested and checked. You can't do better than the second law of thermodynamics. And the direction of energy flow is just a tip of the iceberg of that law.
Q : Iceberg? Titanic iceberg?
A: Come on now. You know that's just a figure of speech to give a feeling for the size of this principle. But... OK, let's get literal: Run that Titanic movie as the ship hits the iceberg. See those steel plates ripped open and the ship begin to sink. Realistic, right? Can you imagine a real happening in which the reverse occurs? A sinking ship whose steel side heals up as it comes up out of the water and floats? Ridiculous. Too stupid to think about. But why is it stupid? Because it is so improbable from your and my experience. Only a movie run backward would show that kind of unrealistic fantasy. The second law isn't some weird scientific idea. It fits with everything common happening that we know.
In a video that is run backward, you may have laughed at some diver who zooms up from the water to a ten-meter diving board, but you're never fooled that the video is going forward, i.e., that you are seeing an event as it actually happened in real time. Unconsciously, you are mentally comparing what you see now with your past practical experience -- and that has all followed the second law. Even though you may never have heard of the law before, in the years of your everyday experience you have seen thousands and thousands of examples of energy flowing from being concentrated to becoming diffused.
A swimmer doesn't come shooting up out of the water to the diving board, rocks in a valley don't suddenly roll up a mountain, outside air doesn't rush into a flat tire, batteries don't get charged by sitting around. Those events all would have energy spontaneously become more concentrated, the opposite of energy spreading out. We sense that videos or movies are shown in the right direction of showing time going forward only if the events in them agree with our lifetime experience about the direction of energy flow: concentrated to dispersed and spread out. The second law points the direction of how we feel time goes.
Q: So that's why people call the second law "time's arrow"?
A: You got it. Now let's clear up those tricky words, "spontaneously" and "tends" in the statement of the second law, "Energy spontaneously tends to flow only from being concentrated in one place to becoming diffused and spread out."
Q: Why bother? "Spontaneously" means fast, quick, ad lib .......
A: Hold it! That may apply to people, but we're talking about how things and chemicals behave. In the second law "spontaneously" means only that any energy which is available in the object or substance for diffusing will spread out from it -- if given a chance. It doesn't have anything to do with how fast or slow that occurs after the dispersal of energy starts, or even when it might start. That's why "tends" is so important to understand as part of the second law.
The energy available in a hot frying pan or in a loud BOOM from a drum immediately and rapidly begins to spread out to their environments. Nothing hinders that from happening. Lots of ordinary and also unhappy events are like that. But there are an enormous number of "energy diffusing" second-law happenings that are hindered so they don't occur right away. Here's a simple illustration: If I hold a half-pound rock in my fingers so it is ready to fall, it has potential energy concentrated in it because it is up above the ground. If the second law is so great and powerful, why doesn't the energy that has been concentrated in the rock spread out? Obviously, it can't do that because my fingers are "bonding" to it, keeping it from falling. The second law isn't violated. That rock tends to fall and diffuse its energy to the air and to the ground as it hits -- and it will do so spontaneously by itself, without any help -- the second I open my fingers and "unbond" the rock.
Q: Is that really so important?
A: Yes, it is. Many philosophers and novelists learned about the second law only from physicists.
Unfortunately, physics emphasizes what happens in a closed or isolated system of tiny particles rather than in our real open-flow world of trees, shiny steel, sunshine, rocks and people, the world of sun energy and things made out of chemicals. Thus, many readers of popular philosophy articles and recent novels have been misled and frightened by talk about the second law as a fast-approaching doomsday. The writers pass too quickly over the fact that it is a tendency rather than a prediction of what will happen right away.
In many real-world chemicals and things the second law can be obstructed or hindered for millions of years. Certainly, the mountains of the world haven't all slid down to sea level in the last several hundred centuries! Similar to my fingers holding the small rock (but millions of times more tightly), even overhanging stone in cliffs or mountains is bonded, chemically bonded, to adjacent atoms in the stone and so the stone can't obey the second law tendency for it to fall to a lower level. Here, as in countless other examples, the second law is blocked by the strength of chemical bonds. It takes a huge number of repetitions of outside energy input like freezing and thawing and earthquakes and windy rainstorms to break the bonds along even a weak bond-line, make a crack, and free particles or pebbles or rocks so they can follow the second law by falling to a lower level. (But even then, they may just fall into a mile-high valley and be kept from dropping any closer to sea level; so here in a different way the second law is further hindered.)
Blockage of the second law is absolutely necessary for us to be alive and happy. Not one of the complex chemical substances in our body and few in the things we enjoy would exist for a microsecond if the second law wasn't obstructed. Its tendency is never eliminated but, fortunately for us, there are a huge number of compounds in which it is blocked for our lifetimes and even far longer.
Q: About time we got to something human. Are we through with rocks?
A: Yes, but don't forget what they have illustrated.
I think it is helpful to see some of the ways the second law works in the ordinary dusty world of actual objects before looking at its relation to pure substances, the chemicals that make up those objects. Chem profs approach the second law the other way around, starting with atoms and molecules first. And that's certainly OK. Professors rightfully avoid much talk about the behavior of big visible things at all. In the limited time of a chemistry course they can only develop the nature of atoms and molecules and of chemical substances. Objects made from chemicals like a gear or a bridge or a wooden house or a book or a bone just have to be assumed to behave like their constituent substances. What they do because they are big things composed of tiny molecules just has to be left for other courses or our future experience.
We'll go further to find how the second law affects common objects and how it is really the mother of all serious Murphy's Laws that apply to things. (We'll omit the zillions of humorous or stupid variations about the way fallible humans behave, as well as all the problems surrounding computers and programs!)
Q: The second law and Murphy's Law -- they're related?
A: Yes, but I shouldn't have mentioned it. First, we need to know more about bad things that can happen to us because of the behavior of chemicals in the objects around us. For a grim example, wooden houses burn down with disastrous financial loss even when people aren't killed. What's going on here in terms of energy flow?
Wood and paper are both primarily cellulose. Paper is easier to experiment with so let's think about its burning in air. When paper catches fire and burns, there's a lot of energy given out as heat and some as yellow light. It's well known now that the products of the combustion of cellulose with the oxygen of the air are carbon dioxide and water. (The slight amount of black ash is due to the clay that was on the paper adsorbing a small amount of carbon.) Once started, the burning is spontaneous --i.e., the process goes on by itself without any further help after a match starts it -- and also the burning is really fast. Now, if all that energy is flowing out in this reaction of paper with oxygen, the paper and oxygen must have had a lot more energy inside them before the reaction than do the carbon dioxide and water after the reaction -- as shown in the diagram above.
What's happening here is a beautiful illustration of the predictions of the second law. Systems (groups) of chemicals -- or objects made from them (like sheets of paper or houses) -- tend to react if they have more energy bound inside their molecules than do the reaction products that they can form. [[Note for Advanced students: "Energy change here means
G, not just H".]] Thus, when systems react spontaneously, their reactant molecules are spreading out their internal energy in two ways: 1) part to the bonds in each molecule of the products because each of those has less energy concentrated in it than was in the molecules of the starting materials, and 2) a part to making all those product molecules move much faster (i.e., they have more kinetic energy than the original cellulose and oxygen). These faster molecules show a high temperature on a thermometer. We say they are hot, not because heat is a "something" but because heat is the process of energy transfer from one kind of matter to another -- from fast molecules of gas to the thermometer bulb or to one's hand if you get it near the flame.
Q: Wait a minute! Wait a minute!! You had to put a match to that paper to start it burning! What's spontaneous or second law about that??
A: Wait a minute yourself. Have you already forgotten that essential word "tends" in the second law?
All the paper and wood and things made from them in the entire world tend right now to react with the oxygen in the air and form one gigantic fireball. Why don't they? Well, why don't all the mountains on earth spread out the potential energy in their high stone cliffs this second and collapse into spread out much-lower mounds of sandy particles? It's the strength of the chemical bonds (between silicon, oxygen, potassium, aluminum and other atoms and ions) that holds stone together and acts as an obstacle to the second law's immediate execution. The potential energy of high rocks/mountains is hindered from spreading out instantly.
Just so, the strength of the chemical bonds (between carbon, hydrogen and oxygen) in cellulose holds it together and obstructs the instant spreading out of the energy inside the cellulose by reacting with the oxygen of the air. This strength prevents oxygen from instantly breaking into the cellulose molecules to form the even stronger bonds (of carbon dioxide and water) and to release large amounts of energy. However, it takes just a little extra push of energy from the hot match flame to begin to break a few quadrillion bonds in the cellulose of paper or wood so that oxygen molecules also striking those 'breaking bonds' can complete the breaks and make the first 'gazillion' CO2 and H2O molecules.
Q: Aha. I remember that in the Malibu fires a few years ago some houses started to blaze from the inside because heat from the nearby burning trees and brush ignited the cloth drapes inside the picture windows. Then there were others with big windows that didn't catch fire because they had aluminum blinds which were closed. That involved activation energy, right? Cotton cloth is cellulose, isn't it?
A: Yes to both questions. First of all, the glass of the windows probably got extremely hot, both from the heated air of the fire and the fire's infrared radiation. In addition, as you suggest, the intense IR radiation went right through the windows and heated the fabric drapes even more -- enough to exceed their activation energy that normally hinders their oxidation in air. They began to burn and this gave out enough energy to ignite the whole interior -- by exceeding the activation energy of oxidation of all the other flammable materials in the house.
Just as does every idea that we've been talking about, the concept of activation energies gives us tremendous power in understanding how the world works, even in unusual events.
For instance, you've heard about the dangers of nitroglycerin, a liquid that explodes violently just from being shaken hard or jarred sharply. Do you think that its energy diagram would look like the one for cellulose above? Of course not. It must have a very low activation energy, Ea. That leads to an extremely fast formation of hot gaseous products, an explosion (despite the relatively smaller difference in energy between "nitro" and the products). Explosives form hot gases so rapidly because they all have oxygen atoms as part of their molecules. Thus, those molecules don't need to wait until some molecules of atmospheric oxygen happen to hit them -- the way most substances have to do. Alfred Nobel was driven to invent a safer explosive when four workers and his brother were killed in the family nitroglycerin plant. He made what he called "dynamite" when he mixed oily nitroglycerin with some powdery silica material to form a seemingly dry solid that could be pressed into stick shape. They didn't detonate just from being hit or dropped. Obviously, therefore, an energy diagram for dynamite must look like the dotted line, a considerably higher Ea indicating that more energy must be put in, e.g., by a blasting cap, to initiate the spontaneous decomposition of the nitroglycerin. (TNT, used in armor piercing shells, is about six times more resistant to shock than nitroglycerin. Thus, you can guess at TNT's activation energy for reaction.) Dynamite has been mainly replaced by other explosives for excavation, etc., today.
There. We've seen some substances with low activation energies but we don't often handle nitro or TNT! How about a more important problem to many of us, rusted iron, the result of iron reacting with oxygen to form iron oxide. Of course, I'm running the risk here of opening the whole can of worms about human activity and the second law.
Q: What do you mean, "can of worms about human activity and the second law"? What has that got to do with simple old rusting iron?
A: Mainly that we usually don't want it -- except for iron ore (which is a mixture of dirt or rock that has a lot of iron rust , i.e., iron oxide, in it). We really like millions and millions of tons of that because it's worth millions and millions of dollars! But before we start digging in an iron mine, let's look at how we humans use the second law for our purposes. Whenever we run a truck or any kind of engine, we're using the second law for our benefit:
[Note! From here on when I write "the second law", it is using those words as a code phrase or shorthand for "what the second law describes", namely: "some process in which energy disperses or spreads out"]
Q: OK, I get it. Every time I start our car, I'll think of the energy the gasoline's giving out by reacting with oxygen, making hot gases, pushing those pistons and turning that crankshaft….
A: And every time you breathe, don't forget the oxygen going all over your body and …well, let me do some more summarizing before talking about that biochemistry angle a little more:
This minute all around the world there are tens of thousands of people who are "using" (transforming to mechanical work, losing some to waste heat spread out to the environment) the concentrated energy of a mixture of oxygen with coal, oil or gas to dig up the iron ore with giant scoops and transport it via trucks, trains, and ships from different mines to steel mills. Then, more energy is used by more thousands of people to change it into iron and finally to shiny steel...What a long parade of actions based on using the second law to get what we want!
Every step from the original rusty dirt in the ground requires transformation of concentrated energy (of oxygen plus coal, oil, gas) to do a lot of mechanical work (along with that dispersing of less concentrated energy in the hot exhaust gases of CO2 and water). Then bringing together thousands and thousands of tons of ore, coal and limestone to one place, the steel mill, is another enormous expenditure of concentrated energy in fuels (not counting the human effort in muscle and brain). Next, a totally different variety of energy transformation is done, changing the iron (oxide) ore to almost pure iron metal that has a larger internal energy content in its bonds than does the iron oxide. Wait a minute! Doesn't it seem against the second law to force a dispersed-energy chemical like iron oxide to change into a concentrated-energy chemical like nearly pure iron? Sure it is, but there's no problem. Just as in running all those truck, train and ship engines, we can take energy flow from a spontaneous process (here in this case, from two chemical processes):
The spontaneous reaction of carbon from coal with a little oxygen to form CO whose molecules are moving very fast (i.e., are very hot), followed by
the spontaneous reaction of CO with iron oxide to form fast moving CO2 molecules plus pure iron and cause the nonspontaneous change of iron oxide to iron. Of course, in doing that we will lose a large flow of energy as waste heat. To give an idea of the size of it in iron making, a ton of near-pure carbon (coke from coal) reacts with four tons of air at around 1000 C in a blast furnace to form a ton of pig iron from two tons of ore. The energy price is six tons of hot flue gas that the process spews out, some of which isn't available for more changing of iron oxide to iron. Pretty big operation.
Did we beat the second law? No way. But by using the second law (taking the energy from two spontaneous "downhill" reactions and transferring much of it to force a nonspontaneous process to go "uphill" energy-wise and make something), just as we take gasoline energy and change some of its energy into mechanical energy (to make pistons, crankshafts, and driveshafts turn the car wheels), we got what wanted: iron from which we can produce steel, the structural material for a near-infinite number of useful objects. Better than rusty dirt, right?
Q: Are you trying to make an iron man out of me?
A: No, no. Stick to the triathlon for that. The reason I went so long on that kick of ore to iron is that it's a perfect summary of the tremendous variety of what humans can do with the aid of the second law.
We gather objects and mixed-up raw materials from all over the world. Just bringing stuff of all sorts from so many widely separated places to one spot as in iron and steel making is certainly not a probable occurrence in inanimate nature! It's a human act, especially when you consider the further elaborate arrangements that we make with all varieties of matter, from lining up botanicals in a National Arboretum in Washington, DC to joining metal and many other kinds of materials into building a skyscraper in Chicago or a Getty Center in LA: Those are big things.
Equally as spectacular are the human actions in smaller things, bringing together the materials and fabricating a Boeing 747 or a jet engine with so much power that a couple of them could move a Titanic. Gathering, arranging, building, fabricating -- in all of these we use (what we can of) the directional energy flow from spontaneous chemical reactions such as the oxidation of petroleum and coal.
Let’s finish this recap of human use of the second-law energy flow: Besides making concentrated-energy chemicals like pure metals -- iron, copper, chromium and silver -- from their diffused-energy ores (and innumerable objects from them), we make thousands of other high energy substances for our pleasure or our needs. Minor things like flavors for foods. Important pharmaceuticals that save millions of lives. It may take dozens of reactions (milder than that violent one for iron from iron oxide!) to change starting materials stepwise to the final chemical product, but the overall process involves diverting energy from spontaneous 'downhill' reactions to make the 'uphill', more concentrated-energy substance that we want.
Of course, this is the kind of coupled process (i.e., a spontaneous + a non-spontaneous) that nature uses – taking a tiny bit of sunlight energy and, with the aid of extremely complex processes in organisms like plants, changing lower-energy carbon dioxide and water and traces of minerals into thousands of higher-energy substances. But don’t think that "natural" or "from natural materials" has something to do with good or harmless! There are hundreds of harmful or even poisonous chemicals in nature – from strychnine to the extremely deadly compound in simple castor beans. (Also usually omitted when someone extols the beneficial qualities of everything "natural" is the fact that all terribly toxic viruses and bacteria are totally natural!)
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Q: Who cares about the second law of thermodynamics?
A: Anyone who wonders how the material world -- our world of energy and matter --works.
Q: Big deal? A: The biggest, most powerful, most general idea in all of science. Why paper, trees, coal, gas and all things like them burn (and why people "should" spontaneously catch fire in air), why sand and dry ice even in pure oxygen can't ever burn, why the sun will eventually cool down, why iron rusts (but not why it rusts faster nearer the ocean), why there are hurricanes or any weather at all on earth, what makes things break, why houses get torn apart in tornadoes or explosions, why everything living tends to die.
That's for starters.
Q: Just STARTERS? OK, OK, I'm impressed. So, what IS the second law of thermodynamics? Well, wait a minute, what's the first law?
A: The first law is very simple and important but pretty dull: You can't create or destroy energy.
The second law of thermodynamics looks mathematically simple but it has so many subtle and complex implications that it makes most chem majors sweat a lot before (and after) they graduate. Fortunately its practical, down-to-earth applications are easy and crystal clear. These are what we'll talk about. From them we'll get to very sophisticated conclusions about how material substances and objects affect our lives.You can just change it from one form to another, for example, electricity to heat, heat that will boil water and make steam, hot steam to push a piston (mechanical energy) or rotate a turbine that makes electricity that in turn can be changed to light in a light bulb or to sound in an audio speaker system, and so forth. That's it. Important but dull.
Q: Sounds fair. I'm listening.
A: Look at the direction that energy flows in any happening or process or event. That is the first step to understanding what the second law of thermodynamics is and what it applies to.
(Later we'll come back to those two tricky words "spontaneously" and "tends".) That's it, the big idea. The perfect illustration is: A hot frying pan cools down when it is taken off the kitchen stove. Its thermal energy ("heat") flows out to the cooler room air. The opposite never happens.Energy spontaneously tends to flow only from being concentrated in one place
to becoming diffused or dispersed and spread out.
Q: Come on. All this build up for that dumb example?
A: Don't put me down. I could have snowed you with differential equations and diagrams instead of what you see everyday. We're being practical and visual rather than going the math route, essential as that is in chemistry.
The big deal is that all types of energy spread out like the energy in that hot pan does (unless somehow they're hindered from doing so) They don't tend to stay concentrated in a small space; they flow toward becoming dispersed if they can -- like electricity in a battery or a power line or lightning, wind from a high pressure weather system or air compressed in a tire, all heated objects, loud sounds, water or boulders that are high up on a mountain, your car's kinetic energy when you take your foot off the gas. All these different kinds of energy spread out if there's a way they can do so.
Get the picture? The second law of thermodynamics summarizes that totally different events involving all kinds of energy have a common cause. A blowout in a tire or a car battery shorting out or slowly running down -- what could seem to be more unlike than those! Yet the reason for their occurring is the same, the tendency for concentrated energy not to stay localized, to disperse if it has a chance and isn't hindered somehow.
A major goal in life is to find true BIG ideas that describe how the world works, to understand why and how things happen around us in terms of a small number of basic principles. Principles that can be tested and checked. You can't do better than the second law of thermodynamics. And the direction of energy flow is just a tip of the iceberg of that law.
Q : Iceberg? Titanic iceberg?
A: Come on now. You know that's just a figure of speech to give a feeling for the size of this principle. But... OK, let's get literal: Run that Titanic movie as the ship hits the iceberg. See those steel plates ripped open and the ship begin to sink. Realistic, right? Can you imagine a real happening in which the reverse occurs? A sinking ship whose steel side heals up as it comes up out of the water and floats? Ridiculous. Too stupid to think about. But why is it stupid? Because it is so improbable from your and my experience. Only a movie run backward would show that kind of unrealistic fantasy. The second law isn't some weird scientific idea. It fits with everything common happening that we know.
Sinking ships are like rocks rolling down a mountain -- as they sink, their potential energy due to being high above sea-bottom is diffused, spread out to the water that they push aside (or, in the case of mountain rocks, diffused as they roll down to the valley and hit other rocks, give those a bit of kinetic energy, and warm them slightly by friction.)Our psychological sense of time is based on the second law.
It summarizes what we have seen, what we have experienced, what we think will happen.
In a video that is run backward, you may have laughed at some diver who zooms up from the water to a ten-meter diving board, but you're never fooled that the video is going forward, i.e., that you are seeing an event as it actually happened in real time. Unconsciously, you are mentally comparing what you see now with your past practical experience -- and that has all followed the second law. Even though you may never have heard of the law before, in the years of your everyday experience you have seen thousands and thousands of examples of energy flowing from being concentrated to becoming diffused.
A swimmer doesn't come shooting up out of the water to the diving board, rocks in a valley don't suddenly roll up a mountain, outside air doesn't rush into a flat tire, batteries don't get charged by sitting around. Those events all would have energy spontaneously become more concentrated, the opposite of energy spreading out. We sense that videos or movies are shown in the right direction of showing time going forward only if the events in them agree with our lifetime experience about the direction of energy flow: concentrated to dispersed and spread out. The second law points the direction of how we feel time goes.
Q: So that's why people call the second law "time's arrow"?
A: You got it. Now let's clear up those tricky words, "spontaneously" and "tends" in the statement of the second law, "Energy spontaneously tends to flow only from being concentrated in one place to becoming diffused and spread out."
Q: Why bother? "Spontaneously" means fast, quick, ad lib .......
A: Hold it! That may apply to people, but we're talking about how things and chemicals behave. In the second law "spontaneously" means only that any energy which is available in the object or substance for diffusing will spread out from it -- if given a chance. It doesn't have anything to do with how fast or slow that occurs after the dispersal of energy starts, or even when it might start. That's why "tends" is so important to understand as part of the second law.
The energy available in a hot frying pan or in a loud BOOM from a drum immediately and rapidly begins to spread out to their environments. Nothing hinders that from happening. Lots of ordinary and also unhappy events are like that. But there are an enormous number of "energy diffusing" second-law happenings that are hindered so they don't occur right away. Here's a simple illustration: If I hold a half-pound rock in my fingers so it is ready to fall, it has potential energy concentrated in it because it is up above the ground. If the second law is so great and powerful, why doesn't the energy that has been concentrated in the rock spread out? Obviously, it can't do that because my fingers are "bonding" to it, keeping it from falling. The second law isn't violated. That rock tends to fall and diffuse its energy to the air and to the ground as it hits -- and it will do so spontaneously by itself, without any help -- the second I open my fingers and "unbond" the rock.
Q: Is that really so important?
A: Yes, it is. Many philosophers and novelists learned about the second law only from physicists.
Unfortunately, physics emphasizes what happens in a closed or isolated system of tiny particles rather than in our real open-flow world of trees, shiny steel, sunshine, rocks and people, the world of sun energy and things made out of chemicals. Thus, many readers of popular philosophy articles and recent novels have been misled and frightened by talk about the second law as a fast-approaching doomsday. The writers pass too quickly over the fact that it is a tendency rather than a prediction of what will happen right away.
In many real-world chemicals and things the second law can be obstructed or hindered for millions of years. Certainly, the mountains of the world haven't all slid down to sea level in the last several hundred centuries! Similar to my fingers holding the small rock (but millions of times more tightly), even overhanging stone in cliffs or mountains is bonded, chemically bonded, to adjacent atoms in the stone and so the stone can't obey the second law tendency for it to fall to a lower level. Here, as in countless other examples, the second law is blocked by the strength of chemical bonds. It takes a huge number of repetitions of outside energy input like freezing and thawing and earthquakes and windy rainstorms to break the bonds along even a weak bond-line, make a crack, and free particles or pebbles or rocks so they can follow the second law by falling to a lower level. (But even then, they may just fall into a mile-high valley and be kept from dropping any closer to sea level; so here in a different way the second law is further hindered.)
Blockage of the second law is absolutely necessary for us to be alive and happy. Not one of the complex chemical substances in our body and few in the things we enjoy would exist for a microsecond if the second law wasn't obstructed. Its tendency is never eliminated but, fortunately for us, there are a huge number of compounds in which it is blocked for our lifetimes and even far longer.
Q: About time we got to something human. Are we through with rocks?
A: Yes, but don't forget what they have illustrated.
I think it is helpful to see some of the ways the second law works in the ordinary dusty world of actual objects before looking at its relation to pure substances, the chemicals that make up those objects. Chem profs approach the second law the other way around, starting with atoms and molecules first. And that's certainly OK. Professors rightfully avoid much talk about the behavior of big visible things at all. In the limited time of a chemistry course they can only develop the nature of atoms and molecules and of chemical substances. Objects made from chemicals like a gear or a bridge or a wooden house or a book or a bone just have to be assumed to behave like their constituent substances. What they do because they are big things composed of tiny molecules just has to be left for other courses or our future experience.
We'll go further to find how the second law affects common objects and how it is really the mother of all serious Murphy's Laws that apply to things. (We'll omit the zillions of humorous or stupid variations about the way fallible humans behave, as well as all the problems surrounding computers and programs!)
Q: The second law and Murphy's Law -- they're related?
A: Yes, but I shouldn't have mentioned it. First, we need to know more about bad things that can happen to us because of the behavior of chemicals in the objects around us. For a grim example, wooden houses burn down with disastrous financial loss even when people aren't killed. What's going on here in terms of energy flow?
Wood and paper are both primarily cellulose. Paper is easier to experiment with so let's think about its burning in air. When paper catches fire and burns, there's a lot of energy given out as heat and some as yellow light. It's well known now that the products of the combustion of cellulose with the oxygen of the air are carbon dioxide and water. (The slight amount of black ash is due to the clay that was on the paper adsorbing a small amount of carbon.) Once started, the burning is spontaneous --i.e., the process goes on by itself without any further help after a match starts it -- and also the burning is really fast. Now, if all that energy is flowing out in this reaction of paper with oxygen, the paper and oxygen must have had a lot more energy inside them before the reaction than do the carbon dioxide and water after the reaction -- as shown in the diagram above.
What's happening here is a beautiful illustration of the predictions of the second law. Systems (groups) of chemicals -- or objects made from them (like sheets of paper or houses) -- tend to react if they have more energy bound inside their molecules than do the reaction products that they can form. [[Note for Advanced students: "Energy change here means
G, not just H".]] Thus, when systems react spontaneously, their reactant molecules are spreading out their internal energy in two ways: 1) part to the bonds in each molecule of the products because each of those has less energy concentrated in it than was in the molecules of the starting materials, and 2) a part to making all those product molecules move much faster (i.e., they have more kinetic energy than the original cellulose and oxygen). These faster molecules show a high temperature on a thermometer. We say they are hot, not because heat is a "something" but because heat is the process of energy transfer from one kind of matter to another -- from fast molecules of gas to the thermometer bulb or to one's hand if you get it near the flame. Q: Wait a minute! Wait a minute!! You had to put a match to that paper to start it burning! What's spontaneous or second law about that??
A: Wait a minute yourself. Have you already forgotten that essential word "tends" in the second law?
All the paper and wood and things made from them in the entire world tend right now to react with the oxygen in the air and form one gigantic fireball. Why don't they? Well, why don't all the mountains on earth spread out the potential energy in their high stone cliffs this second and collapse into spread out much-lower mounds of sandy particles? It's the strength of the chemical bonds (between silicon, oxygen, potassium, aluminum and other atoms and ions) that holds stone together and acts as an obstacle to the second law's immediate execution. The potential energy of high rocks/mountains is hindered from spreading out instantly.
Just so, the strength of the chemical bonds (between carbon, hydrogen and oxygen) in cellulose holds it together and obstructs the instant spreading out of the energy inside the cellulose by reacting with the oxygen of the air. This strength prevents oxygen from instantly breaking into the cellulose molecules to form the even stronger bonds (of carbon dioxide and water) and to release large amounts of energy. However, it takes just a little extra push of energy from the hot match flame to begin to break a few quadrillion bonds in the cellulose of paper or wood so that oxygen molecules also striking those 'breaking bonds' can complete the breaks and make the first 'gazillion' CO2 and H2O molecules.
As these first "heated up" bonds are breaking, the oxygen from the air begins to grab carbon and hydrogen atoms to form carbon dioxide and water molecules. But the formation of new strong bonds in the CO2 and water gives out a lot of energy -- enough to start to break many many more quadrillions of bonds of cellulose (no bond being totally broken before oxygen has simultaneously begun to form a new CO2 and water molecule from the developing fragments). These new molecules of CO2 and water also absorb some of the energy from the new bonds as they are formed and many move faster than twice the speed of sound. We sense those fast moving molecules as hot gas and we call it "heat".The initial energy push (usually from heat), shown by the small energy "hill" in the diagram below, is the activation energy, Ea, that is necessary to overcome the bond-strength obstacle to the second law in most chemical reactions. Thus, this requirement for input of an initial energy, the energy of activation, hinders both desirable and undesirable reactions from occurring.
An important idea is "Activation energies protect substances from change."
Q: Aha. I remember that in the Malibu fires a few years ago some houses started to blaze from the inside because heat from the nearby burning trees and brush ignited the cloth drapes inside the picture windows. Then there were others with big windows that didn't catch fire because they had aluminum blinds which were closed. That involved activation energy, right? Cotton cloth is cellulose, isn't it?
A: Yes to both questions. First of all, the glass of the windows probably got extremely hot, both from the heated air of the fire and the fire's infrared radiation. In addition, as you suggest, the intense IR radiation went right through the windows and heated the fabric drapes even more -- enough to exceed their activation energy that normally hinders their oxidation in air. They began to burn and this gave out enough energy to ignite the whole interior -- by exceeding the activation energy of oxidation of all the other flammable materials in the house.
Just as does every idea that we've been talking about, the concept of activation energies gives us tremendous power in understanding how the world works, even in unusual events.
For instance, you've heard about the dangers of nitroglycerin, a liquid that explodes violently just from being shaken hard or jarred sharply. Do you think that its energy diagram would look like the one for cellulose above? Of course not. It must have a very low activation energy, Ea. That leads to an extremely fast formation of hot gaseous products, an explosion (despite the relatively smaller difference in energy between "nitro" and the products). Explosives form hot gases so rapidly because they all have oxygen atoms as part of their molecules. Thus, those molecules don't need to wait until some molecules of atmospheric oxygen happen to hit them -- the way most substances have to do. Alfred Nobel was driven to invent a safer explosive when four workers and his brother were killed in the family nitroglycerin plant. He made what he called "dynamite" when he mixed oily nitroglycerin with some powdery silica material to form a seemingly dry solid that could be pressed into stick shape. They didn't detonate just from being hit or dropped. Obviously, therefore, an energy diagram for dynamite must look like the dotted line, a considerably higher Ea indicating that more energy must be put in, e.g., by a blasting cap, to initiate the spontaneous decomposition of the nitroglycerin. (TNT, used in armor piercing shells, is about six times more resistant to shock than nitroglycerin. Thus, you can guess at TNT's activation energy for reaction.) Dynamite has been mainly replaced by other explosives for excavation, etc., today. There. We've seen some substances with low activation energies but we don't often handle nitro or TNT! How about a more important problem to many of us, rusted iron, the result of iron reacting with oxygen to form iron oxide. Of course, I'm running the risk here of opening the whole can of worms about human activity and the second law.
Q: What do you mean, "can of worms about human activity and the second law"? What has that got to do with simple old rusting iron?
A: Mainly that we usually don't want it -- except for iron ore (which is a mixture of dirt or rock that has a lot of iron rust , i.e., iron oxide, in it). We really like millions and millions of tons of that because it's worth millions and millions of dollars! But before we start digging in an iron mine, let's look at how we humans use the second law for our purposes. Whenever we run a truck or any kind of engine, we're using the second law for our benefit:
[Note! From here on when I write "the second law", it is using those words as a code phrase or shorthand for "what the second law describes", namely: "some process in which energy disperses or spreads out"]
| taking energy inside of substances that tend to spread out, but can't because of Ea, |
| (i.e, a high-energy mixture like oxygen (of the air) plus liquid gasoline (or solid coal), |
| giving it the necessary activation energy (a spark or a flame or heating at high pressure), |
| having the diffusing energy (in the form of hot expanding gases of CO2 and H2O) push a piston that turns |
| crankshafts, gears and wheels (with the exhaust gases, still fairly hot, but no longer |
| available for any more piston-pushing in this engine going out the tailpipe). |
So in this example, it looks like the second law is a good deal for us. It is, whether in engines or in our biochemistry (where oxygen plus food is the concentrated energy source -- but with totally different "sparkplugs" in our bodies to start our oxidation reactions that are far more controlled and in very tiny quantities compared to the violent explosions in a car engine!) Nature's second law predicts that the energy concentrated inside a chemical mixture like oxygen with oil or coal (or food) tends to spread out. It will do so, if that necessary little energy push to overcome an activation energy barrier is applied to that kind of high-energy mixture compared to the lower energy products of CO2 and H2O.
We make our whole technological world run by grabbing as much as we can of the energy flow available from concentrated energy mixtures like oxygen and fuels to run an infinite variety of machines, electrical generators and vehicles. (Our bodies, as we have said, use second-law energy flow from the oxidation of food for the synthesis of essential compounds and for all activity, from biochemical to muscular to mental.) However, when we change energy from one form to another, from energy in a fuel plus O2 to pushing a piston or even water running down from Hoover Dam to the dynamos below, it is impossible for us to get to use all of the energy in the concentrated energy source for the jobs we want it to do. Some always is diverted as the unusable energy due to faster moving molecules (i.e., "heat") to the environment. (That's where our body gets heat to maintain our 98.6º F/37º C.)Q: OK, I get it. Every time I start our car, I'll think of the energy the gasoline's giving out by reacting with oxygen, making hot gases, pushing those pistons and turning that crankshaft….
A: And every time you breathe, don't forget the oxygen going all over your body and …well, let me do some more summarizing before talking about that biochemistry angle a little more:
This minute all around the world there are tens of thousands of people who are "using" (transforming to mechanical work, losing some to waste heat spread out to the environment) the concentrated energy of a mixture of oxygen with coal, oil or gas to dig up the iron ore with giant scoops and transport it via trucks, trains, and ships from different mines to steel mills. Then, more energy is used by more thousands of people to change it into iron and finally to shiny steel...What a long parade of actions based on using the second law to get what we want!
Every step from the original rusty dirt in the ground requires transformation of concentrated energy (of oxygen plus coal, oil, gas) to do a lot of mechanical work (along with that dispersing of less concentrated energy in the hot exhaust gases of CO2 and water). Then bringing together thousands and thousands of tons of ore, coal and limestone to one place, the steel mill, is another enormous expenditure of concentrated energy in fuels (not counting the human effort in muscle and brain). Next, a totally different variety of energy transformation is done, changing the iron (oxide) ore to almost pure iron metal that has a larger internal energy content in its bonds than does the iron oxide. Wait a minute! Doesn't it seem against the second law to force a dispersed-energy chemical like iron oxide to change into a concentrated-energy chemical like nearly pure iron? Sure it is, but there's no problem. Just as in running all those truck, train and ship engines, we can take energy flow from a spontaneous process (here in this case, from two chemical processes):
The spontaneous reaction of carbon from coal with a little oxygen to form CO whose molecules are moving very fast (i.e., are very hot), followed by
the spontaneous reaction of CO with iron oxide to form fast moving CO2 molecules plus pure iron and cause the nonspontaneous change of iron oxide to iron. Of course, in doing that we will lose a large flow of energy as waste heat. To give an idea of the size of it in iron making, a ton of near-pure carbon (coke from coal) reacts with four tons of air at around 1000 C in a blast furnace to form a ton of pig iron from two tons of ore. The energy price is six tons of hot flue gas that the process spews out, some of which isn't available for more changing of iron oxide to iron. Pretty big operation.
Did we beat the second law? No way. But by using the second law (taking the energy from two spontaneous "downhill" reactions and transferring much of it to force a nonspontaneous process to go "uphill" energy-wise and make something), just as we take gasoline energy and change some of its energy into mechanical energy (to make pistons, crankshafts, and driveshafts turn the car wheels), we got what wanted: iron from which we can produce steel, the structural material for a near-infinite number of useful objects. Better than rusty dirt, right?
Q: Are you trying to make an iron man out of me?
A: No, no. Stick to the triathlon for that. The reason I went so long on that kick of ore to iron is that it's a perfect summary of the tremendous variety of what humans can do with the aid of the second law.
We gather objects and mixed-up raw materials from all over the world. Just bringing stuff of all sorts from so many widely separated places to one spot as in iron and steel making is certainly not a probable occurrence in inanimate nature! It's a human act, especially when you consider the further elaborate arrangements that we make with all varieties of matter, from lining up botanicals in a National Arboretum in Washington, DC to joining metal and many other kinds of materials into building a skyscraper in Chicago or a Getty Center in LA: Those are big things.
Equally as spectacular are the human actions in smaller things, bringing together the materials and fabricating a Boeing 747 or a jet engine with so much power that a couple of them could move a Titanic. Gathering, arranging, building, fabricating -- in all of these we use (what we can of) the directional energy flow from spontaneous chemical reactions such as the oxidation of petroleum and coal.
Let’s finish this recap of human use of the second-law energy flow: Besides making concentrated-energy chemicals like pure metals -- iron, copper, chromium and silver -- from their diffused-energy ores (and innumerable objects from them), we make thousands of other high energy substances for our pleasure or our needs. Minor things like flavors for foods. Important pharmaceuticals that save millions of lives. It may take dozens of reactions (milder than that violent one for iron from iron oxide!) to change starting materials stepwise to the final chemical product, but the overall process involves diverting energy from spontaneous 'downhill' reactions to make the 'uphill', more concentrated-energy substance that we want.
Of course, this is the kind of coupled process (i.e., a spontaneous + a non-spontaneous) that nature uses – taking a tiny bit of sunlight energy and, with the aid of extremely complex processes in organisms like plants, changing lower-energy carbon dioxide and water and traces of minerals into thousands of higher-energy substances. But don’t think that "natural" or "from natural materials" has something to do with good or harmless! There are hundreds of harmful or even poisonous chemicals in nature – from strychnine to the extremely deadly compound in simple castor beans. (Also usually omitted when someone extols the beneficial qualities of everything "natural" is the fact that all terribly toxic viruses and bacteria are totally natural!)
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