SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Carbon and Organic Chemistry
These skills are usually tested on the SAT Subject Test in Chemistry. You should be able to …
• Describe the bonding patterns of carbon and its allotropic forms.
• Explain the structural pattern and naming of the alkanes, alkenes, and alkynes, and their isomers.
• Show graphically how hydrocarbons can be changed and the development of these functional groups, their structures, and their names: alcohols, aldehydes, ketones, esters, and amines.
This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter.
Carbon is unique. It forms inorganic substances such as carbon dioxide, graphite, and diamonds. It also forms organic substances without which life could not exist. It forms planar substances, tetrahedrons, and rings.
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 difficult to melt. Diamond is the hardest natural substance. At atmospheric pressure, it melts at 3,550°C.
Allotropic forms of carbon:
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.
Diamond uses the sp3 hybrid orbitals to explain its tetrahedron structure.
Graphite uses the trigonal sp2 hybrids
Some common amorphous forms of carbon are charcoal, coke, bone black, and lampblack.
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 41.
Figure 41. Carbon Dioxide Cycle
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()
Continued passing of CO2 into the solution will eliminate the cloudy condition because the insoluble CaCO3 becomes soluble calcium bicarbonate (Ca(HCO3)2):
CaCO3(s) + H2O() + 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 CO2 is the acid anhydride of carbonic acid, it forms the acid when reacted with soft drinks, thus making them “carbonated” beverages.
CO2(g) + H2O() → 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 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:
Know the photosynthesis process.