edited by HENRYA. BENT
bench remark/
North Camiina State Universiw Raleigh. NC 27695
Carbon Dioxide Its Principal Properties Displayed and Discussed Henry A. Bent North Carolina State University, Raleigh. NC 27695 One of the busiest chemicals on the planet is carbon dioxide. Tonnagewise, it's the chief input to green plants and the chief output of animals. No other chemical on the planet is turned over chemically in such massive amounts for such noble purposes: plant synthesis and animal maintenance. Yet we are largely unaware of its existence. For ordinarily, a t room temperatures and atmospheric pressure, carbon dioxide is an invisible, odorless gas. Dry Ice Exhlblted
Solid carbon dioxide is visible-all solids are, in air. Solid carbon dioxide's most remarkable property, however, is its temperature. It's very cold: -78 "C. It's probably the coldest substance you'll ever have a chance to touch: But CAUTION: Handle with care! Solid carbon dioxide causes "frost bite". It can quickly freeze the water in skin cells, causing them to rupture-like frozen water pipes. Like ordinary, cold ice, solid carbon dioxide can't exist warm. Unlike ordinary ice, however, solid carbon dioAide doesn't melt (at atmospheric pressure). It doesn't form puddles. You'll probably never see another substance like that: solid, cold, and nonmelting. For those reasons-its solidity, coldness, and lack of puddle-making-solid carbon dioxide is called "dry ice". When it absorbs heat, it doesn't leave a wet mess. Dry Ice In Acetone
We easily sense by feel that dry ice is cold. We can also see, in a sense, that dry ice is cold. Put some in a beaker about half full of a not-easily-frozen liquid, say acetone, freezing t h e d r y ice and initially ooint -95 OC. CAUTION:K e e ~ warm acetonc away from open flames! \%'henthe r w are ~ first mixed, there may he \ , i a o r ~ ~iizzing, ~ l s u,ith ewlution of flammable acetone spray. -
The acetone serves as a heat transfer medium. I t transfers heat from its warm surroundings to the colder dry ice. It's like the water in a car's radiator, or the molten sodium in an atomic pile. The water in a car transfers heat from a hot engine block to the colder radiator. The molten sodium in an atomic pile transfers heat from the hot pile to water in the boilers of steam turbines. In all cases, the liquids transfer heat from hot to colder things. In our case, the dry ice in the beaker cools the acetone next to it. Next the cold acetone cools the beaker. Then the cold heaker cools the surrounding air. See? The beaker is turning white, up to about the level of the acetone. It's frosting up, like windows of a house on a very cold day. The cold acetone inside the beaker corresponds to the cold air outside a house. The moisture-containing room air outside our beaker corresponds to the moisture-containing air inside a house. In both cases the air is demoisturized.
Lawtul Nature Cold is cold and humid is humid, and always their union is condensation, wherever they meet: on the exterior of a cold beaker or the interior of cold windows. Nature is dependable. Nature is lawful. Systems prepared alike behave alike. That's the philosophical basis on which student laboratory work is judged. If observed behavior is unexpected, system preparation must have been unusual. It's the basis of "scientific knowledge". Scientific knowledge is description of system-preparation and observed behavior in such ways that one may say "Nature is lawful." Three cheers for descriptive chemistry! Creation of lawcreating descriptions of Nature's chemical nature is the essence of the science of chemistry.
Volume 64
Number 2
February 1987
167
Weldlng wlth Water By the end of the period we may see a beautiful buildup of ice crystals on the surface of our beaker. And look! Already the beaker is frozen-welded if you like-to the pre-moistened block of wood i t is sitting on. Welding is joining two objects-usually metals-by beat, often with an intermediate solid that has a melting point that is high compared to the normal temperatures of the welded objects. In our case, the melting point of water is high compared to the temperature of a dry ice-cooled beaker and block. Water welds cold things. CAUTION:B e careful about placing moist skin-a tongue, e.g., o r wet hands-in contact with cold metal surfaces.The skin may stick! The cautionary tale is told of the attempt of a truck driver to fix a flat tire one winter in northern Siberia. He was found frozen solid, both hands in the icy grip of the rim of the truck's wheel. Subllrnatlon of Dry Ice How does dry ice stay so cold in a warm room? Dry ice stays colder than its thermal surroundings in the same way that watrr in a van on a red hot heating element of a stoveor water in a beaker over the hot flame of a Bunsen burnerstays relatively cool, a t about 100 OC. I t evaporates. Evanoration cools. That's how animals stav cool in hot days: by evaporation of water, chiefly. That's how a factory supervisor tried to cool a too-hot tank of methyl isocyanate in Bhopal, India: by wetting it down with water. And that's whv rubbina alcohol has been rubbed on feverish patients. TI; acetonlor ether on your hand. Now wave it in the air. Chilly, isn't it? You can deaden skin toward pain by evaporating from it a low-boiling liquid. A thermometer hulb wrapped with a bit of towel dipped in ether and waved vigorously through the air to promote evaporation gives a reading of about -5 "C. A chunk of dry ice sitting on a table shows evidence of sublimation in several ways. T o the touch it's cold. True, to the eye it may look-at first-steaming hot. The "steam" tends tosink, however, rather than rise. The vaporous, white wisps about dry ice in a humid room are droplets of water vapor condensed by cold, gasified carbon dioxide. EventualIv all the drv ice will sublime. I t will disappear. - Dry ice dLsappears fastest from points where heat enters it fastest. Where is that? Where it is in contact with the best conductor of heat, i.e., where our chunk on the table touches the table. For solids conduct heat much better than gases. That's whv thermal insulation is alwavs as much aas and as little soliddas possible. Look and feel (carefully) t6e bottom of our chunk of drv ice. It's flat and smooth. Barelv tap it on a flat, dry, and ciean table top and it glides along &nost frictionlessly, like an air puck. It's a wonderful demonstration on a long, clean, dry, smooth, almost horizontal chalk tray. It's easy to show the sublimation of dry ice in another way. Place some of it in a flask and attach a balloon. The balloon expands, impressively. A little solid produces a lot of gas. Gases are mostly empty space. That's why they are poor conductors of heat. A vacuum doesn't conduct heat a t all (although heat passes through it, by radiation). Most of the extraordinary physical properties of gases-particularly their densities and compressibilitiekstem from the fact that most of the space gases occupy is empty. Gas Densities Gas densities are extraordinarilv low. The least dense lia(lid at room temperature and ;itmospheric pressure is liquid ammmiusatursted with lithium. Its density is 0.5 r mT.. I.'c~r comparison, consider the density of carbon dioxide gas. T o determine density, we need to determine the volume of a given mass. That's not difficult to do for carbon dioxide gas if we have dry ice, a flask, a balance to weigh the dry ice, and 168
Journal of Chemical Education
a balloon (and string and a ruler) t o determine the volume of the gasified dry ice. We find: 22 g of freshly crushed dry ice yields on sublimation a balloon with a circumference, C, of 87 cm. The balloon's radius. R (= C/2r). is 13.8 cm. and the balloon's volume, V [='(4/3jr~3], is11.000 mL. ~ k d e n t l the y density of carbon dioxide eas is about 22 -e/11.000 mL = 0.00020 -a/ . mL, at room temperature. Correspondingly, the mass of 22.4 L of carbon dioxide gas a t 0 OC (1 mol. at standard conditions) is calculated to be about 48g. ~ h value k obtained from the formula Cop is 44 g-not bad agreement for such a quick, approximate experiment. For note that our inflated balloon isn't a perfect sphere (as we've assumed), and a t the outset, prior to weighing, moisture was condensing on the cold parts of our apparatus. Also, some carbon dioxide sublimed hetween the times we weighed it and attached the balloon. But now, something interesting. and perhaps unexpected. haonened as thedrv icesublimed in the tlask on the balance. ~ s ~ i attached he bailoou expanded, the balance became unbalanced. As the balloon became larger, what was on the balance pan appeared to become lighter! That may seem counterintuitive. But remember that we're doing our experiment a t the bottom of a sea of air. The balloon is buoved UD bv the mass of the displaced air. We hawn't exactly raised i h e ~ i t a n i cfrom the botiom of the ocean with bags of air, but we have lightened the load on the balance. Aiter wiping off the moisture that condensed on the tlask and balance pan due to the cooling effect of the dry ice, we see that the load on the balance has been liehtened bv 15 g. Evidently, 11,000 mL of air (thevolume of a; displaced by the gaseous carbon dioxide) weigh about 15 g. Correspondingly, the mass of 22.4 L of air a t 0 OC is calculated to be about 33 g. The value obtained from the formulas Np and Op and the composition of air (about 80% nitrogen and 20% oxygen) is 28.8 g. Dry ice is alecture demonstrator's friend. With it alecturer can demonstrate simply, quickly, safely, inexpensively, and strikingly the endothermicity of evaporation; frost formation; the relative conductivities for heat of solids and gases; and, with unparalleled ease and directness, the enormous free volume in gases-from which all the characteristic properties of gases, and the gas laws, stem. Also, one obtains the densities of carbon dioxide gas and air. Avogadro's Law With dry ice, a balance, and a balloon, we found that 11,000 mL of carbon dioxide gas a t room temperature weighs about 22 g, the same volume of air about 15 g. If we know the chemical formulas for molecules of carbon dioxide and air, we can say: the heavier a gas's molecules, the denser the gas. 'I'hat'5eass toshow. As vou ~robahlvnu tired.^ l~alloolifilled to s i i k in air. And watch what with carbon dioxide happens to a beaker balanced on a balance as one pours into it carbon dioxide from another beaker. I t sinks. The sinking of carbon dioxide gas in air illustrates one of the central laws of chemistry. The pioneer German physical chemist Nernst based his classic textbook on theoretical chemistry on that law-Avogadro's law. According to Avogadro's law, densities of gases are directly proportional to the masses of their molecules. That's an exceedingly useful fact for chemists. It's amazing that from something as prosaic as gas densities we obtain directly relative masses of molecules! T o put it another way: the volume per molecule is the same for all gases. Gases a t the same T and P have the same population densitv. How big the individual molecules are doesn't much atlect how much spaceisassoc~atedwith them in the gnseous .tare, for cases are mostly empty space. Thus. as mentioned, gases have low densities, andthey are extraordinarily compressible.
of a gas in a liquid, like liquefaction of a gas in its own liquid, is an exothermic transformation: C02(g) = CO,(in acetone) +Heat
According to Le Chatelier's principle, raising the temperature shifts the equilibrium to the left. The warmed, COzsaturated acetone s ~ a r k l e sas i t deeasses. Had there been exotic life living in the acetone consuming dissolved carhon dioxide. like rdd-blooded fish in cold water consuming disadved oxygen, the rise in temperature and decrease in solubility of the dissolved -pas mirht - have been life-threatening" to the species. To really zap the cold, carbonated acetone thermally try this: squirt in a little water from a squeeze bottle. Wow! I t sounds like dousinga red hot poker in water. The water with its high capacity for heat and its large heat of fusion liberates a lot of heat to the acetone as it cools down and freezes. Chemical Com~slllonof Dw Ice Dry ice, we've been saying, is solid carhon dioxide. But what evidence have we that the white solid that sublimes to a colorless, fire-extinguishing gas contains solid, black, combustile carhon and gaseous, combustion-supporting oxygen? In fact, chemists don't mean by the name "carbon dioxide"
that the substance so named contains carbon and oxygen as such. The name "carhon dioxide" means that carbon and oxygen can he ohtained from carhon dioxide. There's a splendid, short, simple, safe, striking, and inexpensive way to show that carbon and oxygen can be obtained from "carhon dioxide". Burn magnesium in it. First, carefully observe a strip of magnesium burning in air. CAUTION:You may not wish t o look at t h a t flame directly. It's one of the brightest flames in chemistry. Magnesium has an enormous affinity for oxygen. The white powder produced is magnesium oxide. Next check the sublimate above dry ice in a beaker t o see that i t extinguishes a burning splint. Then lower burning magnesium into it. The magnesium continues to burn! Again, a white powder is formed. I t looks like magnesium oxide. I t is magnesium oxide. The oxygen of the magnesium oxide must have come from the "carbon dioxide". That was the only substance in the beaker, besides magnesium. And those black specs? They look like carbon. They are carbon. With the aid of magnesium, we (the magnesium and ourselves) have ohtained carbon and oxygen-and, so far as we can tell, only those two suhstances-from sublimed dry ice. Those facts, and the density of the sublimate, fit only one formula: COz.
Volume 64
Number 2
February 1987
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