科普英文：化學第一輯

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The Importance of Oxygen

Oxygen has been present only for the last half of the earth's history-appearing about 2.5 billion years ago in what is called The Great Oxidation Event. Before this time the earth's atmosphere contained large quantities of methane, which reacted with oxygen and prevented its buildup. Now there is geological evidence that about 2.7 billion years ago the amount of dissolved nickel in the oceans began to decrease. This was important because the organisms that produced methane required nickel ions to exist. As the meth- ane concentrations in the atmosphere decreased, oxygen produced by chlorophyll-containing organ- isms began to build up. In contrast, oxygen was removed from the atmosphere by mountain build- ing and erosion as freshly exposed rocks combined with oxygen to form oxygen-containing minerals.

The concentration of oxygen in the atmosphere has varied greatly over the last 600 million years, as shown in the accompanying graph. Note that 300 million years ago (at the end of the Carbonif- erous period) the air consisted of about 35% oxy- gen. The fossil record indicates that during this time insects and other arthropods that absorb oxygen through holes in their exoskeletons were extraordinarily large. It is thought that mayflies as big as today's robins and dragonflies as big as modern hawks were commonplace in this period.

About 255 million years ago, the oxygen con- centration in the atmosphere was about 30%, but for some reason the oxygen content plunged to about 13% (about the concentration at an eleva- tion of 15,000 feet in today's world) in the rela- tively short geological time span of 10 million years. Die-offs during this period claimed as many as 95% of the species living in the ocean and about 70% of those living on land. The oxygen content then began to rebound (to about 16% )

200 million years ago, which led to a dramatic increase in biological innovation. For example, the first dinosaurs appeared only about 15 million years after the mass die-offs.

During the last 200 million years the oxygen content has increased rather steadily, making pos- sible the existence of fuel-intense species such as mammals. In fact about 25 million years ago when oxygen concentration maximized at 23%, many mammals had become gigantic. For example, the relatives of today's rhino stood almost 5 m tall and weighed 15 metric tons-the largest ever land mammals. After peaking at 23%, the oxygen levels dropped to today's level of 21% and the "mega- mammals" disappeared. If history is any indica- tion, the oxygen levels in the atmosphere will continue to change significantly over time, but this is obviously not a high-priority problem for us in the twenty-first century.

Reprinted with permission from Science News, the weekly news magazine of science. Copyright © 2005 by Science Service.

Chemical Analysis of Cockroaches

Cockroaches can be a big problem. Not only are these hardy pests unpleasant to live with, but they also consume significant quantities of the world's precious food and grain supplies. Because the many different species of cockroaches require dif- ferent control measures, determining which species is causing a particular problem is important. Careful examination of a cockroach can reveal its spe- cies, but this process is very time-consuming. However, a new method of cockroach identifica- tion, based on gas chromatography, has been developed at the U.S. Department of Agriculture by D. A. Carlson and R. J. Brenner. In gas chroma- tography, the compounds to be separated are dis- persed in a carrier gas that passes through a porous solid. Because different substances have differing tendencies to adhere to the solid, the components of the mixture travel at different rates through the system, causing them to spread out so that they can be separated and identified.
In the cockroach identification study, Carlson and Brenner found that the composition of the outer, waxy layer of a roach is distinct to the particular species. Thus, by dissolving this waxy coating and injecting it into the gas stream of a gas chromatograph, scientists can identify the cockroach unambiguously in less than half an hour. This technique is particularly useful for identifying hybrid Asian-German cockroaches, which have become a major problem for the food industry.
Although biologists might argue that the gas chromatographic method takes the fun and the challenge out of identifying cockroaches, this tech- nique should lead to significant advances in the control of these insects.

Fracking: What is It?

Hydraulic fracturing, universally called fracking, has revolutionized the drilling operation for petro- leum and natural gas. So far fracking has yielded more than seven billion barrels of oil and 600 tril- lion cubic feet of natural gas in the United States alone. By 2040, fracking is expected to account for 50% of the natural gas consumed in the United States compared to 35% today.

However, fracking is not without problems. The process consumes huge supplies of water and sand, and it uses chemicals that have the potential to pollute groundwater. Scientists and engineers are working hard to find ways to optimize the process and minimize accompanying pollution.

In the fracking process, a viscous fluid consisting of 90% water and about 10% sand and less than 1% chemical additives is injected down a predrilled well hole into deep shale deposits and subjected to high pressures. Although shale is porous, it is not permeable. Thus the fracturing generated by the fracking process frees trapped gas and oil, which can be pumped to the surface. The sand (called the proppant) remains in the hairline fractures to prop them open so that the gas and oil can continue to seep out, a process that may continue for decades.

The wells typically are a mile deep and can extend laterally for a mile. Each time the well is fracked, the lateral distance is extended by about 200 yards, and this process can be repeated as many as 20 times. 

One of the major problems associated with fracking is the amount of water required. To miti- gate this problem, a Canadian company named GasFrac is now using a gel made from propane as the main fracking fluid instead of water. Because the propane gel retains the proppant much better than water, only about 10% as much fluid is required for fracking, and the propane, being a hydrocarbon, becomes part of the oil or gas pro- duction stream. Although this new technology has been used successfully in thousands of fracking operations, at this time it represents only a small fraction of total fracking procedures.

Because fracking operations occur thousands of feet below the surface in solid rock, pollution of the groundwater used for drinking should be nonexistent. The main pollution threat occurs from how operators handle the fracking fluid and the waste flowback fluid above ground. Efforts are now underway to ensure safe fracking operations, but accidents do occur, and fracking remains controversial in some areas. The goals of fracking operations are to increase the efficiency of the process by experimenting with new fluids and proppants and to minimize the environmental effects.

Farming the Wind

In the Midwest the wind blows across fields of corn, soybeans, wheat, and wind turbines-wind turbines? It turns out that the wind that seems to blow almost continuously across the plains is now becoming the latest cash crop. One of these new-breed wind farmers is Daniel Juhl, who erected 17 wind turbines on six acres of land near Woodstock, Minnesota. These turbines can generate as much as 10 megawatts (MW) of electricity, which Juhl sells to the local electri- cal utility,

The largest wind farm in the world, located on the Oregon-Washington border, generates 300 MW. A controversial wind farm called Cape Wind, in the ocean five miles off the coast of Cape Cod, is planned to produce more than 400 MW of power.

There is plenty of untapped wind power in the United States. Wind mappers rate regions on a scale from 1 to 6 (with 6 being the best) to indicate the quality of the wind resource. Wind farms are now being developed in areas rated from 4 to 6. The farmers who own the land welcome the increased income derived from the wind blowing across their land. Economists esti mate that each acre devoted to wind turbines can pay royalties to the farmers of as much a $8000 per year, or many times the revenue from as growing corn on that same land. Daniel Juhl claims that farmers who construct the turbines themselves can realize as much as $20,000 p year per turbine. Globally, wind generation of per electricity has nearly quadrupled in the last five years and is expected to increase by about 60% per year in the United States. The economic feasi bility of wind-generated electricity has greatly improved in the last 30 years as wind turbines have become more efficient. Today's turbines can produce electricity that costs about the same as that from other sources. The most impressive thing about wind power is the magnitude of the supply. According to the American Wind Energy Association in Washington, D.C., the wind-power potential in the United States is comparable to or larger than the energy resources under the sands of Saudi Arabia.

The biggest hurdle that must be overcome before wind power can become a significant elec tricity producer in the United States is construction of the transmission infrastructure-the p lines needed to move the electricity from the rural areas to the cities where most of the power is used. For example, the hundreds of turbines planned in southwest Minnesota in a development called Buffalo Ridge could supply enough electricity to power 1 million homes if transmission prob lems can be solved. power

Another possible scenario for wind farms is to use the electrical power generated to decompose water to produce hydrogen gas that could be carried to cities by pipelines and used as a fuel. One real benefit of hydrogen is that it produces water as its only combustion product. Thus it is essen- tially pollution-free.

Within a few years wind power could be a major source of electricity. There could be a fresh wind blowing across the energy landscape of the United States in the near future.

The Chemistry of Air Bags

The inclusion of air bags in modern automobiles has led to a significant reduction in the number of injuries as a result of car crashes. Air bags are stored in the steering wheel and dashboard of all cars, and many autos now have additional air bags that protect the occupant's knees, head, and shoulders. In fact, some auto manufacturers now. include air bags in the seat belts. Also, because deployment of an air bag can severely injure a child, all cars now have "smart" air bags that deploy with an inflation force that is proportional to the seat occupant's weight.

The term "air bag" is really a misnomer because air is not involved in the inflation process. Rather, an air bag inflates rapidly (in about 30 ms) due to the explosive production of N gas. Originally, sodium azide, which decomposes to produce N2,

2NaN,(s)-2Na(s) + 3N,(g)

was used, but it has now been replaced by less toxic materials.

The sensing devices that trigger the air bags must react very rapidly. For example, consider a car hitting a concrete bridge abutment. When this happens, an internal accelerometer sends a message to the control module that a collision possibly is occurring. The microprocessor then analyzes the measured deceleration from several accelerometers and door pressure sensors and decides whether air bag deployment is appropriate. All this happens within 8 to 40 ms of the initial impact. Because an air bag must provide the appropri- ate cushioning effect, the bag begins to vent even as it is being filled. In fact, the maximum pressure in the bag is 5 pounds per square inch (psi), even in the middle of a collision event. Air bags represent a case where an explosive chemical reaction saves lives rather than the reverse.

Entropy: An Organizing Force?

In this text we have emphasized the meaning of the second law of thermodynamics-that the entropy of the universe is always increasing Although the results of all our experiments sup port this conclusion, this does not mean that order cannot appear spontaneously in a given part of the universe. The best example of this phenomenon involves the assembly of cells in living organisms. Of course, when a process that creates an ordered system is examined in detail, it is found that other parts of the process involve an increase in disorder such that the sum of all the entropy changes is positive. In fact, scientists are now finding that the search for maximum entropy in one part of a sys tem can be a powerful force for organization in another part of the system.
To understand how entropy can be an organiz- ing force, look at the accompanying figure. In a system containing large and small "balls" as shown in the figure, the small balls can "herd" the large balls into clumps in the corners and near the walls. This clears out the maximum space for the small balls so that they can move more freely, thus maximizing the entropy of the system, as demanded by the second law of thermodynamics.

In essence, the ability to maximize entropy by sorting different-sized objects creates a kind of attractive force, called a depletion, or excluded- volume, force. These "entropic forces" operate for objects in the size range of approximately 10 m to approximately 10 m. For entropy-induced ordering to occur, the particles must be constantly jostling each other and must be constantly agitated by solvent molecules, thus making gravity unimportant.

There is increasing evidence that entropic ordering is important in many biological systems. For example, this phenomenon seems to be responsible for the clumping of sickle-cell hemo- globin in the presence of much smaller proteins that act as the "smaller balls." Entropic forces also have been linked to the clustering of DNA in cells without nuclei. Allen Minton of the National Insti- tutes of Health in Bethesda, Maryland, is studying the role of entropic forces in the binding of pro- teins to cell membranes.

Entropic ordering also appears in nonbiological settings, especially in the ways polymer molecules clump together. For example, polymers added to paint to improve the flow characteristics of the paint actually caused it to coagulate because of to depletion forces,
Thus, as you probably have concluded already, entropy is a complex issue. As entropy drives the universe to its ultimate death of maximum chaos, it provides some order along the way.