Chapter 9. Liquids and Solids.

Properties of liquids and solids play major roles in the environment. In this chapter we will look at two examples of applications of hydrogen bonding in the environment and one application of a phase diagram to explain the role of the heat of vaporization in the operation of a refrigerator.

Hydrogen Bonding.


A hydrogen bond is a moderately strong force of attraction between a hydrogen atom bonded to N, O, or F in one molecule, and a lone pair of electrons on a N, O, or F atom in an adjacent molecule. An individual water molecule has two hydrogen atoms directly bonded to an oxygen atom by strong covalent bonds. As water freezes, each oxygen becomes bonded to two additional hydrogen atoms by moderately strong hydrogen bonds. The OH4 unit will have tetrahedral geometry according to VSEPR making all of the H-O-H bond angles 109.5 deg. Below is a drawing showing the repeating unit in solid ice and a segment of solid ice.

Figure 3-13. (a) Tetrahedral cluster of four water molecules around a fully hydrogen-bonded water molecule in the center. The hydrogen bonds are indicated by dashed lines.

(b) Hydrogen bonding in the structure of ice. In liquid water the hydrogen bonding is not as extensive as it is in ice.

Joesten, M.D. et al, World of Chemistry Essentials, Saunders, Philadelphia, 1993, p. 67.

A snowflake reflects the hexagonal structure of the hydrogen bonded ice as shown below.

Figure 13.41. A snowflake. The six-sided or hexagonal geometry of the snowflake is a reflection of the six-sided rings formed when water molecules hydrogen bond to form ice.

Kotz, J.C., Purcell, K.F., Chemistry and Chemical Reactivity, 2 nd, Saunders, 1991, p. 544.

When many of the above repeating units join together in three-dimensions, they form a network type of structure consisting of parallel puckered hexagons as shown below.

Figure 13.40. Ice. Each water molecule in ice is hydrogen bonded to four others. Oxygen atoms of hydrogen bonded water molecules are at the corners of puckered six-membered rings.

Kotz, J.C., Purcell, K.F., Chemistry and Chemical Reactivity, 2 nd, Saunders, 1991, p. 544.

Notice that the network structure of ice has hexagonal channels that are parallel to each other. These channels in pure ice are empty space and their formation causes the density of ice to be less than that of liquid water. This means that when water freezes, it expands in volume. Nature uses that expansion caused by hydrogen bonding to weather rocks and produce vital components of soil. (See "physical weathering" below).

The hexagonal channels are also large enough to contain small molecules of other substances and function as two-dimensional molecular "cages". The early atmosphere of Earth contained methane and as the polar icecaps formed, the methane became trapped in the hexagonal channels to produce compounds which are called gas or methyl hydrates. (See "gas hydrates" below.)

Weathering

Thompson, G.R., Turk, J. Earth Science and the Environment , Saunders, Philadelphia, 1995, pp 246-248.

Weathering is the decomposition and disintegration of rocks and minerals at the Earth's surface by both mechanical and chemical processes. It converts solid rock to gravel, sand, clay, and soil. Weathering involves little or no movement of the decomposed rocks and minerals.

Erosion is the removal of weathered rocks and minerals from the place where they formed. Flowing water, wind, glaciers. and gravity erode weathered material from its place of origin. Then the material may be transported great distances by the same agents: water, wind, ice, and gravity. Finally, those agents of transport deposit the sand, clay, gravel, and other material in layers at the Earth's surface.

Weathering, erosion, transport, and deposition typically occur in an orderly sequence. For example, water freezes in a crack in granite, loosening a grain of quartz. A hard rain erodes the grain and washes it into a stream. The stream then transports the quartz to the seashore and deposits it as a grain of sand on a beach.

Surface processes create landforms by wearing away mountain ranges, sculpting coastlines, and carving canyons and valleys. They also build landforms by depositing sediment. Although the processes work slowly from the perspective of human life, in geologic time they can erode an entire mountain range to a flat plain or excavate a deep canyon.

The physical and chemical environment at the Earth's surface is corrosive to most materials. A pocketknife rusts when it is left out in the rain. For similar reasons rocks decompose naturally. Thus, over the centuries, stone cities have fallen into ruin.

If you visit the remains of ancient Greece or Rome, you can see two types of changes. First, large building stones have broken into smaller fragments. Mechanical weathering is the physical disintegration of rock into smaller pieces. For example, vegetation grows in cracks in building stones. Roots enlarge the cracks, pushing the stones aside and eventually toppling the wall. In cold climates, water expands as it freezes in cracks, fracturing rocks and reducing stone buildings to rubble. Such mechanical processes break rocks into smaller pieces, but they do not alter the chemical compositions of the rocks and minerals.

If you look closely at a building stone in an ancient city, you may see a second type of disintegration. Its face may be pitted and discolored, and once-sharp edges are rounded. In addition, the rock may be soft and earthy rather than hard and solid as it was when originally quarried. Chemical weathering occurs when air and water chemically attack rocks. The chemical changes are similar to rusting in that a chemically weathered rock contains different minerals, and has a different chemical composition, from those of the original rock. Certain kinds of air pollution accelerate chemical weathering. Thus, decomposed building stones can be seen in most modern industrial cities as well as in ancient cities.

Just as building stones in ancient cities decompose both mechanically and chemically, rocks in natural settings weather by both processes. Mechanical and chemical weathering reinforce one another. For example, chemical processes generally act on the surface of a solid object. Therefore, a chemical process will speed up if the surface area increases. Think of a burning log: the fire starts on the outside and works its way inward. If you want the log to burn faster, simply split it in half to increase its surface area. Mechanical weathering cracks rocks, exposing more surface area for chemical agents to work on.

Mechanical Weathering

Recall that mechanical weathering does not alter the chemical nature of rocks and minerals; it simply breaks them into smaller pieces. Think of breaking a rock with a hammer: the fragments are smaller than, but otherwise identical to, the original rock. Six processes mechanically weather rocks: frost wedging, salt cracking, abrasion, biological activity, pressure release fracturing, and thermal expansion and contraction.

Frost Wedging

Water collects in natural cracks and crevices in rocks. If the outside temperature drops below 0 deg.C, the water may freeze. Water expands when it freezes.(Due to hydrogen bonding!) Thus, water freezing in a crack pushes the rock apart in a process called frost wedging. The ice may cement the rock together, but when it melts, rocks may tumble from a steep outcrop. If you walk through the mountains during a season when water freezes at night and thaws during the day, be careful. Rocks tumble from cliffs when the rising sun melts the ice. Mountaineers commonly travel in the early morning before rocks begin to fall.

Anyone who has spent time in the mountains has noticed large piles of broken, angular rocks at the bases of cliffs. These piles are called talus slopes (Fig. 10-4). The rocks in talus slopes have broken from the cliffs, mainly by frost wedging.

Figure 10-4. Frost wedging dislodges rocks from cliffs to create talus slopes. Below - A talus slope along the San Juan River, Utah.

Thompson, G.R., Turk, J. Earth Science and the Environment , Saunders, Philadelphia, 1995, p. 248.

Gas Hydrates

Turk, J.T., Thompson, G.R., "Environmental Geoscience", Saunders, Philadelphia, 1995, pp. 389-391.

In the 1930s, when the first high-pressure natural gas pipelines were built, engineers were troubled by solids that formed in the pipe and blocked the flow of gas. They learned that natural gas (methane) reacts with water under the high pressure and low temperatures found in some pipelines to form an icelike solid called methyl hydrate (Figure 3.B.). The problem was solved by carefully drying the gas before it was fed into the pipe.

Figure 3.B. In the hydrate formed by methane, a CH4 molecule is trapped within a cage of water molecules bonded to one another by hydrogen bonds. (MastertonW.L., Hurley, C.N., Chemistry Principles and Reactions, 3rd, Saunders, Philadelphia, 1996, p.73.)

In the 1970s, oceanographers built specialized drilling ships to study the sediments of the continental shelves. When certain sediment cores were brought to the surface and stored on deck, sudden gas releases blew the sediment out of the drill pipes. Researchers learned that below a depth of about 500 meters, seawater temperature is low enough and the pressure is high enough to convert methane trapped in sea-floor mud to methyl hydrate. When brought to the surface, the hydrate decomposed to release methane gas. Scientists have found similar methyl hydrate deposits in Arctic permafrost. Many geologists contend that the sea-floor methane was originally produced by microbes, but not everyone agrees. Using both drill samples and studies of seismic waves, scientists have located sea-floor methyl hydrates world wide. Geochemist Keith Kvenvolden of the United States Geological Survey estimates that methyl hydrate deposits hold twice as much carbon as all conventional coal, oil, and gas reserves combined.

At present, commercial extraction of methyl hydrates is impractical. It is expensive to drill in deep water, and before such ventures are attempted, energy companies must develop more reliable estimates of the hydrate concentrations. A thin layer spread throughout the continental shelves would be prohibitively expensive to exploit, whereas concentrated reservoirs would be more attractive.

In addition to their possible commercial applications, methyl hydrates have considerable scientific interest. Tectonic activity at subduction zones or landslides on continental slopes could reduce pressure and release methane from hydrate deposits. In turn, increased atmospheric methane could trigger greenhouse warming. Ice core studies show that global atmospheric methane concentration has altered rapidly in the past, perhaps by sudden releases of oceanic methyl hydrates.

Refrigeration


Hinrichs, R.A., Energy its Use and the Environment, 2nd, Saunders, Philadelphia, 1996, p. 139.

"The basic components of a refrigerator are shown below in Figure 5-14.


Figure 5-14. Basic components of a refrigerator.

Hinrichs, R.A., Energy its Use and the Environment, 2nd, Saunders, Philadelphia, 1996, p. 140.

A working fluid such as Freon-12 is used as the refrigerant. In the evaporator (the coils inside the refrigerator), the liquid Freon absorbs heat from the warmer surrounding air and turns into a gas." This endothermic process removes the heat of vaporization of the liquid from the inside of the refrigerator and thus cools it. "The electrically driven compressor raises the pressure and temperature of the gas and forces it into the condensor coils outside the refrigerator. Bacause the temperature of gas is higher than that of the room, the gas condenses in an exothermic process that releases the heat to the room. The liquid then flows back through a pressure-reducing valve to the evaporator coils, and is cooled through expansion.

The path of the process is shown on the phase diagram below.