Lecture Notes from CHM 1341
11 June 1996
The atmosphere is both finite and infinite in different senses. It trails off as altitude increases, getting thinner and thinner exponentially according to the following (low altitude) approximation:P = Psea level e -H / 6.5 km
But it is that rapid decline with height (H) that means that the integrated atmosphere contains only finite molecules. How high would the atmosphere be if it didn't fall off in pressure at all? Sounds like a silly question at first because it is Earth's gravity which causes atmospheric pressure in the first place. And the atmosphere's response to gravity is to align itself in this exponentially thinning way. So we'd have to turn OFF gravity in order to do our shrinkwrap. (Since the atmosphere would fly off as gravity quit, put that Saran Wrap in place first!)
OK, with gravity turned off (mentally), we pull the shrinkwrap tighter and tighter exactly like a constricting balloon until the rising pressure (as the atmosphere tries more desparately to escape the decreasing volume) has come up to Sea Level Pressure (defined as ONE ATMOSPHERE). Now we ask how high off the surface of the Earth is the shrinkwrap with all the atmosphere trapped beneath it? About 4 miles. We've got several mountains that would be sticking up through the wrap! You've flown at altitudes higher than that!
This "Gedanken" (thought) Experiment means to impress you that although a common dictum of Society is "the solution to pollution is dilution," there's really an end to what the atmosphere can blow away for us...because it is finite.
Nowhere is that more of a problem than up above the clouds. Interesting question: if the atmosphere extends forever ('til it runs into the Solar Wind), why don't the clouds also climb arbitrarily high? We've all seen thunderstorms in which the tops of the highest flatten out into "anvil" thunderheads; what kind of barrier would be so powerful as to thwart the will of those enormous updrafts? You know they're enormous because of the size of hail which can fall as big as grapefruit! Hail gets that large by being lifted again and again and again up into the freezing regions of chilling alititudes. How fast must updrafts be going to lift grapefruits 10 miles into the sky, eh? And what's to stop it?
You don't believe that, do you? Ozone isn't a solid wall, it's a gas too. But it's a one which falls apart under ultraviolet light as:
O3 -----> 02 + O
And when the energy of the original UV photon gets returned as that terribly-reactive oxygen atom recaptures an oxygen molecule
O + O2 -----> O3 + heat
where M is any partner to the collision, perhaps diatomic nitrogen, which can carry away that heat. So what? It means that wherever in the atmosphere this happens, it's warmer than it should be, and RISING AIR CURRENTS ARE NO LONGER BUOYANT. Why? Because hot-air balloons only rise in a colder atmosphere where they are comparatively less dense. Those thunderhead updrafts cool as they expand upward into lower pressures (why the hail freezes, no?) but meet their match when the atmosphere itself ceases to cool with altitude and begins to warm instead DUE TO OZONE RECOMBINATION.
So that ozone saves us from even larger hail (and more frequent and damaging tornadoes here in Middle America), but that's not all. The very ultraviolet light it absorbs is thus prevented (well, discouraged) from penetrating to the Earth's surface to give us all skin cancer and fry our crops and livestock.
And there's the rub. If the weather stops at the point ("tropopause") where the turbulent, cooling-with-altitude lower atmosphere ("troposphere") collides with the calm, warming-with-altitude, STRATified, next-higher layer ("Stratosphere"), there's no cleansing, weather wash-out effects up where the ozone is doing its critically good work. Any pollutant up there, we're stuck with for many, many years. And if that pollutant works against ozone, as do the chlorofluorocarbons (CFC), we're in for perilous times ahead. The "Ozone Hole" now is found above BOTH poles and is expanding. More on this cliff-hanger in a later chapter.
To complete the story, rising in the stratosphere, the air thins to the point that it cannot effectively capture that UV light, so we arrive at the "mesosphere" (middle) where once again the air cools with altitude...but it's so tenuous out there that no weather is supported anymore. Higher still, cosmic rays and solar particles bombard what little remains of the air, ionizing (electrically charging) it, producing the aurora, and raising the temperatures of the gas at virtually no pressure to very high values in the "ionisphere." After that, it's interplanetary space...space...space.
Meanwhile, back on Planet Earth...
Our "dry" atmosphere is 78% diatomic nitrogen and 21% diatomic oxygen with the remaining 1% composed of mostly Argon (A, ~1%) but odds and ends of important stuff like carbon dioxide (0.03+%), sulfur and nitrogen oxides (0.0002-%), but the most important minor component of the atmosphere isn't found in a dry atmosphere at all. It's water vapor, without which we'd have little weather and no life to appreciate it. Humid atmospheres can carry up to 5% (by volume) of water vapor; that, of course, reduces all the other components to 95% of their "dry" values. So on humid days, we have only 20% oxygen; no wonder we say we're stifling! (Just kidding; that 1% loss is completely ignorable.)
Of course, it wasn't always so. We've inherited most of our atmosphere from the "outgassing" of the Earth (vulcanism), but that would've given us an oxygen-free environment. So Earth started life with no oxygen.
..well...almost no oxygen; when water is zapped by UV light, it can decompose. The hydrogen escapes the atmosphere since, being the lightest, it's the most buoyant gas. The oxygen from that water decomposition would hang around only briefly, however, geologically-speaking, since it would rust out surface iron deposits and get buried as iron oxide.
That's a blessing because, being terribly reactive (causes fires, you know), oxygen is a deadly poison to life. I must be joking, right? Without oxygen, we'd perish, being as the biologist put it "obligate aerobes." But we've EVOLVED to our oxygen tolerance with the help of parasite, mitochondria. Stranger and stranger, I know, but bear with me.
Early life would have been destroyed by oxygen, but it was a waste product of photosynthesis! So Early Life had an oxygen pollution problem. It prospered, creating oxygen faster than the iron deposits could rust it away! And the more it created, the less ultraviolet light bombarded the Earth, making the surfaces of the oceans more hospitable to life. The excellent mixing of atmospheric gases (e.g., carbon dioxide, a reactant in photosynthesis) made the surface THE most desirable place to be, if you were a growing anaerobe...but it was becoming the least desirable because of the upswing in your detritus poison, oxygen!
Enter the ancestors of mitochondria, creatures who developed the trick of surviving oxygen by moderating its reactions with catalysts (called enzymes when Life produces them). So it was safe...from oxygen...not from phagocytes, our rapacious ancestors, one of whom ate a proto-mitochondria who refused to be digested, clever cell. Indeed, it set up housekeeping INSIDE the phagocyte where it got its nutrition and protection and where it resides in every one of our cells to this day. Now before you cringe at this thought, although the our mitochondria, critical to our aerobic respiration (metabolism of our food), even have their own cell nuclei, they are "us" just as surely as everything else in our cells! We can't get along without one another and so prosper together.
But just as Early Life had an atmospheric pollution problem in oxygen which it solved be evolving into aerobes, we have several such problems with Acid Rain, CFCs, and (less obviously) the Greenhouse Effect. Only the first of these has a quick fix; sulfur and nitrogen oxides, which become acids when combined with water, get rained out thereby...in a matter of hours. So if we stopped sulfur and nitrogen pollution today, tomorrow there'd be no acid rain...acid lakes left over from our previous thoughtlessness, of course, but no acid rain. We've already pointed out that CFCs kill ozone in a stratosphere which doesn't wash anything out; we're stuck with ozone holes for perhaps a century after we cease CFC pollution! Greenhouse gases, like carbon dioxide, which trap sunlight are another story. Stay tuned for a later chapter.
So nitrogen and sulfur oxides are acidic. The Germans called oxygen "sauerstoff," literally "sour stuff," because of the tendency of oxides (of non-metals) to be "sour" or "acidic." They also call nitrogen "stickstoff," which means "choking stuff," because, being obligate aerobes, we are asphixiated in an oxygen-free atmosphere, which is what pure nitrogen would provide! And German hydrogen is "wasserstoff," or "the stuff which comes from water" by electrolysis, say, or mesospheric heavy ultraviolet radiation. This makes reading a German Periodic Table an interesting exercise.
Of course, oxygen can "oxidize" (make an oxide of) metals as well as non-metals. When it does, a basic (alkaline) compound usually results. When your stomach is over-acidified, you neutralize it with some elementary base like "milk of magnesia" because
2 Mg(s) + O2(g) -----> 2 MgO(s)
and MgO(s) + H2O(l) -----> Mg(OH)2(s), magnesia
The OH portion of magnesia is the hallmark of compounds with the possibility of being basic in the aqueous solution of your stomach. Same thing holds true of, say, calcium "hydroxide," which is probably better for you since the hydroxide (OH) neutralizes your TUMmy (Legal Notice: Neither U.T. Dallas nor the State of Texas endorses any particular antacid) while the calcium protects your bones.
While oxidation is the addition of oxygen to elements to create "oxide" compounds like:
SO2(g), CO2(g), Fe2O3(s), NO(g), NO2(g), CaO(s)
the taking away of oxygen from an oxide is an example of "reduction," where the word comes from the prehistoric art of "reducing ores to their metals." That art heralded the Bronze and Iron Ages of (wo)Man's prehistory with the ancient chemistry of:
2 CuO(s) + C (charcoal) -----> 2 Cu(s) + CO2(g)
Fe2O3(s) + 3 CO ------> 2 Fe(s) + 3 CO2(g)
So oxygen plays many, many roles in our lives. But so does the almost inert nitrogen! It is rather had to "fix" nitrogen into biological compounds; only a few bacteria know the trick. They can be found in abundance in legumes and in other plants where they turn atmospheric nitrogen into ammonia, critial plant food since nitrogen gets from there into amino acids ("amino" from "ammonia"), the building blocks of Life via proteins and DNA! We (us animals) can't do this trick, so we must steal our nitrogen-containing amino acids from plants and animals which have themselves feed on plants. But don't feel bad about that...you can't photosynthesize either! No human chloroplasts...pity...but it wouldn't be easy being green.
We humans turn to chemical engineering to mass produce ammonia for artifical fertilizer to fuel our Green Revolution. Unfortunately, the best reaction so far we've come up with is woefully inefficient...the high pressure, high temperature, metal-catalyzed, DIRECT hydrogenation of nitrogen via the Haber Process:
N2(g) + 3 H2(g) -----> 2 NH3(g)
Those 3 molecules differ in interesting ways in their boiling points. Clearly, since I've labelled them with (g), they're all gases at room temperature. Ammonia condenses at -33°C making it ideal as a working fluid for commercial refrigerators (and early home units). Nowadays, that role has gone to the less toxic CFCs with the unfortunate effect on stratospheric ozone. Ammonia is toxic, by the way, because it is SUCH a small molecule that the blood-brain barrier to poisons is ineffective against it; so your brain has evolved to WAKE UP whenever it smells ammoniacle "smelling salts" or perish!
Dropping in temperature to -196°C, we liquify nitrogen; oxygen condenses just a little above that. Liquification of air is now a trivial trick; witness the one story liquid nitrogen tank just outside the Chemistry building, for example. It's so cheap that telephone companies use it to pressurize underground phone lines to prevent water leaking in. But liquid nitrogen may soon have a fascinating new use because of a trick that liquid hydrogen already knows.
When we get down to -253°C, near absolute zero (-273°C), hydrogen liquifies and from there down to liquid helium temperatures (-269°C), many substances, especially metals, lose all resistance to electric current! These "superconductors" do NOT fritter their currents away into resistive heat, so there are no power conduction losses and currents started in rings of supercold metal run FOREVER! Currently, many laboratories all over the world are finding new materials, mostly ceramics, that can do that same trick at higher temperatures...now exceeding that of liquid nitrogen! Since liquid nitrogen is such a cheap commodity, as soon as one of these compounds can be drawn into wire or otherwise machined conveniently, we'll be able to replace copper transmission lines with LOSS-FREE superconductors!
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Chris ParrUniversity of Texas at DallasPrograms in Chemistry, Room BE3.506P.O. Box 830688 M/S BE2.6 (for snailmail)Richardson, TX 75083-0688
Voice: (214) 883-2485 Fax: (214) 883-2925 BBS: (214) 883-2168 (HST) or -2932 (V.32bis)Internet: [email protected] (sends Chris e-mail.)
Modified 2 September 1996.