SAT Chemistry Carbon and Organic Chemistry - Carbon

SAT Chemistry Carbon and Organic Chemistry - Carbon

Forms of Carbon
The element carbon occurs mainly in three allotropic forms: diamond, graphite, and amorphous (although some evidence shows the amorphous forms have some crystalline structure). In the mid-1980s, fullerenes were identified as a new allotropic form of carbon. They are found in soot that forms when carbon-containing materials are burned with limited oxygen. Their structure consists of near-spherical cages of carbon atoms resembling geodesic domes.

The diamond form has a close-packed crystal structure that gives it its property of extreme hardness. In it each carbon is bonded to four other carbons in a tetrahedron arrangement like this: These covalent solids form crystals that can be viewed as a single giant molecule made up of an almost endless number of covalent bonds. Because all of the bonds in this structure are equally strong, covalent solids are often very hard, and they are notoriously dif¬ficult to melt. Diamond is the hardest natural substance. At atmospheric pressure, it melts at 3,550°C.


It has been possible to make synthetic diamonds in machines that subject carbon to extremely high pressures and temperatures. Most of these diamonds are used for industrial purposes, such as dies.
The graphite form is made up of planes of hexagonal structures that are weakly bonded to the planes above and below. This explains graphite’s slippery feeling and makes it useful as a dry lubricant. Graphite is also mixed with clay to make “lead” for lead pencils. Its structure can be seen below. Graphite also has the property of being an electrical conductor.


Some common amorphous forms of carbon are charcoal, coke, bone black, and lampblack.

Carbon Dioxide
Carbon dioxide (CO₂) is a widely distributed gas that makes up 0.04 percent of the air. There is a cycle that keeps this figure relatively stable. It is shown in Figure 37.


LABORATORY PREPARATION OF CO₂
The usual laboratory preparation consists of reacting calcium carbonate (marble chips) with hydrochloric acid, although any carbonate or bicarbonate and any common acid could be used. The gas is collected by water displacement or air displacement.

The test for carbon dioxide consists of passing it through limewater, (Ca(OH)₂). If CO₂ is present the limewater turns cloudy because of the formation of a white precipitate of finely divided CaCO₃:
Ca(OH)₂(aq) + CO₂(g)  → CaCO₃(s) + H₂O(ℓ)
Continued passing of C0₂ into the solution will eliminate the cloudy condition because the insoluble CaCO₃ becomes soluble calcium bicarbonate (Ca(HCO₃)₂):
CaCO₃(s) + H₂O(ℓ ) + CO₂(g) → Ca²⁺(HCO₃ )²(aq)

This reaction can easily be reversed with increased temperature or decreased pressure. This is the way stalagmites and stalactites form on the floors and roofs of caves, respectively. The ground water containing calcium bicarbonate is deposited on the roof and floor of the cave and decomposes into solid calcium
carbonate formations.

IMPORTANT USES OF CO₂

1. Because C0₂ is the acid anhydride of carbonic acid, it forms the acid when reacted with soft drinks, thus making them “carbonated” beverages.

CO₂(g) + H₂O(ℓ ) → H₂CO₃(aq)

2. Solid carbon dioxide (-78°C), or “dry ice,” is used as a refrigerant because it has the advantage of not melting into a liquid; instead, it sublimes and in the process absorbs 3 times as much heat per gram as ice.
3. Fire extinguishers make use of CO₂ because of its properties of being 1-t times heavier than air and not supporting ordinary combustion. It is used in the form of CO₂ extin­guishers, which release CO₂ from a steel cylinder in the form of a gas to smother the fire.
4. Plants consume CO₂ in the photosynthesis process, in which chlorophyll (the catalyst) and sunlight (the energy source) must be present. The reactants and products of this reaction are:

6CO₂(g) + 6H₂O(f) → C₆H₁₂O₆(s) + 6O₂(g)
cellulose

ORGANIC CHEMISTRY
Organic chemistry may be defined simply as the chemistry of the compounds of carbon. Since Friedrich Wohler synthesized urea in 1828, chemists have synthesized thousands of carbon compounds in areas of dyes, plastic, textile fibers, medicines, and drugs. The number of organic compounds has been estimated to be in the neighborhood of a million and con­stantly increasing.
The carbon atom (atomic number 6) has four electrons in its outermost energy level, which show a tendency to be shared (electronegativity of 2.5) in covalent bonds. By this means, carbon bonds to other carbons, hydrogens, halogens, oxygen, and other elements to form the many compounds of organic chemistry.