Volume 3

Natural Occurrence • Allotropes of Carbon. Diamond, Graphite, and Fullerenes. Coal and Coke. Activated Carbon. Adsorption and Absorption • Organic Chemistry: A Separate Branch of Chemical Sciences • Carbon Oxides and Carbonic Acid • Silicon, Silicon Oxide, and Silicic Acid. Cement and Glass. Silica Gel • Exercises
3.6.1. Natural Occurrence. Carbon (C) and silicon (Si) of Group 14 of the periodic table are ubiquitous in nature. Naturally occurring graphite and diamond are the two most common allotropes of carbon. Coal is largely carbon. Natural gas and oil are mixtures of carbon compounds. Finally, flora and fauna (ourselves included) are made up of organic molecules, compounds of carbon. Silicon is the second most abundant element in the Earth's crust (> 25% by mass), next only to oxygen. Over 90% of the Earth's crust is composed of SiO2 (sand, quartz) and various silicates, derivatives of oxoacids of silicon. Unlike carbon, however, elemental silicon is not found in nature.

3.6.2. Allotropes of Carbon. Diamond, Graphite, and Fullerenes. Coal and Coke. Activated Carbon. Adsorption and Absorption. There are many allotropic forms of carbon. Figure 3-71 displays structures of three carbon allotropes: diamond, graphite, and buckminsterfullerene.
Figure 3-71. Structures of diamond (left; source), graphite (center; source), and buckminsterfullerene (right; source).

While natural graphite and diamond have been known for a long time, the amazing fullerenes were discovered only in 1985. Besides being expensive, diamonds are remarkable in two more respects. First, diamond is the hardest material known. Second, as follows from the structure of diamond (Figure 3-71, left), each diamond is one single big molecule. Graphite is composed of sheets of hexagonal cells formed by carbon atoms, resembling a honeycomb (Figure 3-71, center). However, unlike honeycombs that are three-dimensional, a flat layer of graphite is two-dimensional. Interactions between the layers in graphite are weak, which makes graphite an ideal material for making pencils. As we write with a pencil, the graphite layers of the pencil core rub off and stick to the paper. Graphite is an excellent conductor of electricity.

Coal, a combustible organic mineral, is mostly carbon. Yet coal also contains much smaller, variable quantities of oxygen, hydrogen, nitrogen, and sulfur. Certain types of coal, such as anthracite, are almost pure carbon, up to 95%. Heating coal in the absence of air produces coke, a higher carbon content fuel. Another widely known and broadly used form of carbon is charcoal, which is made by the heating of wood in the absence of oxygen. A very important grade of charcoal is activated carbon, also known as activated charcoal. The structure of charcoal is similar to that of graphite (Figure 3-71), except the hexagonal sheets are smaller in size and disordered.

The most remarkable feature of activated carbon is its porosity. Porosity is the amount of "void" in a material. For example, the porosity of lacy Swiss cheese is much higher than that of cheddar cheese (Figure 3-72). We can also say that lacy cheese is more porous than cheddar cheese.
Figure 3-72. Lacy cheese (left; source) is more porous than cheddar cheese (right; source).

Activated carbon is extremely porous. It is hard to believe, but true, that the surface area of just 1 g of activated carbon is approximately 3,000 m2 (32,000 sq ft), exceeding the square footage of most mega mansions, like the one shown in Figure 3-73.
Figure 3-73. The surface area of activated carbon in one 1-g capsule (left; source) is larger than the square footage (30,000 sq ft) of the Brookville mega mansion (right) featuring 9 bedrooms, 16 bathrooms, 2-story foyer with staircase, grand formal living and dining rooms, den, library, gourmet kitchen, breakfast room, family room, 4-car garage, and more.

The enormously large surface area of activated carbon makes it a unique material for many important applications. Unlike atoms and molecules underneath the surface of a solid, those on the surface are partially exposed and, consequently, available for interaction with molecules on the outside. Such interactions can be very strong, resulting in a chemical reaction, such as spontaneous air-oxidation of alkali metal atoms on the surface of the bulk metal. In many cases, however, external molecules do not react with the surface of a solid but are only attracted to it by van der Waals forces. Without going into details, van der Waals forces are rather weak, attracting molecules due to Coulomb-type interactions of permanent or transient partial electric charges and electron motion within the molecules (London dispersion forces).

The phenomenon of attraction of molecules to the surface of a material is called adsorption. Adsorption should not be mixed up with absorption, a different phenomenon where external molecules permeate the entire volume of a solid. Figure 3-74 illustrates the difference between the two.
Figure 3-74. Adsorption (attraction to surface) vs. absorption (permeation).

It is clear that the larger the surface area of a particular solid, the more molecules of a given type that solid can adsorb (accommodate for adsorption). The huge surface area of activated carbon makes it a superb adsorbent, a material used to adsorb gases or dissolved substances. One of the most widely known applications of activated carbon is in gas masks. A gas mask is equipped with a filter cartridge filled with activated carbon that adsorbs molecules of toxic substances in the air, thereby making it safe to inhale. Another application of activated carbon is in the treatment of bloating, an uncomfortable pressure in the abdomen caused by excess gas. If taken orally, activated carbon efficiently adsorbs abdominal gas, thereby alleviating the bloating and discomfort. Activated charcoal is also used in water purification systems, in odor-destroying insoles, and to treat overdoses.

3.6.3. Organic Chemistry: A Separate Branch of Chemical Sciences. The chemistry of carbon is uniquely rich. No other chemical element can form as many compounds as carbon does, not even close. The exceptional number and diversity of carbon compounds stem from the ability of carbon atoms to form stable bonds to one another as well as to atoms of hydrogen, oxygen, nitrogen, and other elements. With a small number of exceptions (see below), compounds of carbon belong to the class of organic compounds and are studied in a separate branch of chemical sciences called organic chemistry. Selected examples of organic compounds commonly found in households are presented in Figure 3-75. We will be familiarizing ourselves with the basics of organic chemistry in the next course module, Volume 4. Meanwhile, we will learn about a small number of substances that are classified as inorganic compounds of carbon.
Figure 3-75. Selected household organic compounds.

3.6.4. Carbon Oxides and Carbonic Acid. Carbon in any allotropic form burns in the presence of oxygen. Many of us have seen charcoal burn on a grill. Do diamonds burn? Against the famous De Beers slogan "A Diamond is Forever", they do. Watch this video demonstration of diamonds burning in oxygen. A brilliant hoax video showing the burning of graphite and a diamond features the late Harold Kroto, a Nobel Prize winner, his wife, and her diamond. By the way, Dr. Kroto received the Nobel Prize for the discovery of fullerenes, the previously unknown allotrope of carbon (Figure 3-71, right).

When enough O2 is provided for the burning of carbon, carbon dioxide (CO2) is produced. However, if the system starves for oxygen, the reaction gives rise to carbon monoxide, CO (Figure 3-76).
Figure 3-76. Depending on the amount of O2 used, carbon is oxidized to CO2 and/or CO.

Carbon monoxide is a highly poisonous, colorless, and odorless gas. The toxicity of CO stems from its ability to strongly bind to the iron atom of hemoglobin, the oxygen transporter in the red blood cells. With a molecule of CO bonded to it, hemoglobin is not capable of performing its vital function of picking up the oxygen in the lung and delivering it to the tissues. Carbon monoxide poisoning usually occurs accidentally, due to incomplete combustion of carbon and carbon-based fuels in heaters, cooking ovens, and engines. By the way, against the common belief, diesel engines produce lower levels of CO emission than internal combustion gasoline engines (read more here).

The oxidation state of carbon in CO2 is +4, the most common and stable oxidation state for this element. In CO, however, the carbon atom has the oxidation state of +2, which suggests that CO could be a reductant. This is indeed the case. Carbon monoxide burns in air (2 CO + O2 = 2 CO2), as demonstrated in this video, and reduces some metal oxides to metals (Figure 3-77). As we will learn soon, it is CO that reduces iron ores to iron metal in the production of iron and steel.
Figure 3-77. Reduction of CuO with carbon monoxide.

A very important redox reaction of CO is the water-gas shift reaction, in which CO is oxidized by the H atoms of the water in the presence of a catalyst to give carbon dioxide and hydrogen (Figure 3-78). This reaction is an important industrial method to make hydrogen gas.
Figure 3-78. The water-gas shift reaction (requires a catalyst).

We have previously learned that oxides can be basic, acidic, or amphoteric. There is also a small group of oxides, sometimes called neutral oxides. Neutral oxides do not fall into any of the three main categories of oxides because they do not react with either acids or bases to give a salt and water. Carbon monoxide is one such oxide. Another example is nitric oxide (NO). Carbon dioxide, CO2, is an acidic oxide.

Carbon dioxide is moderately soluble in water, approximately 1.5 g/L at 25 oC and 1 atm. As for any gas, the solubility of CO2 increases at higher pressures and lower temperatures. Carbonated water, also known as sparkling water and seltzer, is a solution of CO2 in water sealed under pressure. When we open a bottle of carbonated water, the pressure inside drops to atmospheric, the solubility of CO2 lowers, and the gas starts bubbling off. There is less bubbling if the bottle has been chilled before opening. These simple observations point to the dependence of gas solubility on temperature and pressure.

Importantly, not only does CO2 dissolve in water, but it also reacts with water to give carbonic acid, H2CO3. This reaction is reversible (Figure 3-79). Carbonic acid exists only in solution and cannot be isolated pure.
Figure 79. Reversible formation of carbonic acid from CO2 and water.

Carbonic acid is a weak acid, even weaker than acetic acid, which itself is a weak acid. Nevertheless, H2CO3 readily reacts with strong bases. Being dibasic, H2CO3 forms two types of salts, carbonates and bicarbonates, also called hydrogen carbonates (Figure 3-80). As we already know, a solution of Ca(OH)2 (limewater) is a well-known reagent for CO2 detection. In the presence of CO2 limewater turns cloudy due to the formation and preciptation of insoluble CaCO3.
Figure 3-80. The formation of sodium carbonate (soda ash, washing soda) and sodium bicarbonate (baking soda) from H2CO3 and NaOH.

Acids that are stronger than H2CO3 decompose carbonates and hydrogen carbonates to CO2, which bubbles off as the reaction occurs (Figure 3-81). Even vinegar (aqueous acetic acid) frees up CO2 from carbonic acid salts. Pour vinegar onto baking soda (NaHCO3) or a piece of chalk (CaCO3) placed in a glass to see the formation of CO2 bubbles. You may also watch video demonstrations of these experiments with chalk and baking soda.
Figure 3-81. Salts of carbonic acid are decomposed to CO2 by stronger acids.

3.6.5. Silicon, Silicon Oxide, and Silicic Acid. Cement and Glass. Silica Gel. Silicon is made by the high-temperature reduction of naturally abundant SiO2 (sand, quartzite) with coke (Figure 3-82).
Figure 3-82. Reduction of SiO2 with carbon to make silicon.

Although crystalline silicon has a metal luster, it is brittle and is a semiconductor. In air or oxygen, silicon burns to give silicon dioxide, SiO2, also known as silica. Silicon dioxide is an acidic oxide that reacts with bases and basic oxides. These reactions produce silicates, salts of silicic acid (Figure 3-83).
Figure 3-83. Reactions of silicon dioxide with a base (top) and a basic oxide (bottom).

Natural silica is found in the form of sand and quartz (Figure 3-84), the second most abundant mineral on Earth (after feldspar, a silicon-aluminum rock). There are many quartz gemstones, including amethyst (Figure 3-84) as well as citrine, onyx, agate, and tiger's eye.
Figure 3-84. Natural SiO2: quartz (Tibet; source), left, and amethyst (South Africa; source), right.

Silicon dioxide (silica) and calcium oxide (quicklime) are the two key ingredients of Portland cement, the most important and broadly used type of cement. The chemistry of cement is complex, but the formation of calcium silicate from the CaO and SiO2 components (Figure 3-83, bottom) is one of the key transformations in the cement hardening process. (Actually, it is not the CaO that reacts directly with the SiO2 upon addition of water to cement, but Ca(OH)2, which is formed in the hydration reaction of calcium oxide.)

Another important application of SiO2 is in the production of glass. Many books still state that glass is a very viscous (thick) liquid. This is not the case. Glass is neither a liquid nor a solid, but something in between. Glass is an amorphous solid, a solid that is devoid of ordered structure. Glass is made by co-melting SiO2 (melting point 1,710 oC) with Na2CO3 and CaCO3 in certain proportions. At the high temperature used in the glass making, both Na2CO3 and CaCO3 decompose to the corresponding basic oxides, which then react with SiO2 (Figure 3-85).
Figure 3-85. Chemical transformations involved in the production of glass.

The CaSiO3 and Na2SiO3 formed dissolve in the excess molten SiO2 to form glass. The molar ratio of CaSiO3 to Na2SiO3 to SiO2 in the standard clear window glass is approximately 1:1:4.

Glass has been around for thousands of years. Cement was used by the ancient Greeks and Romans. The currently most widely used Portland cement was patented "only" about 200 years ago. (Portland cement received its name from its similarity in appearance to Portland stone, the then widely used construction stone quarried on the Isle of Portland in England.) The most important modern application of silicon – as a simple substance – is in the production of semiconductors to make microchips, the integrated circuits that serve as the "brain" of computers and smartphones. The semiconductor purity requirement for silicon to fabricate microchips is very tough, typically 99.9999999%, often called "nine-9".

Silicic acid is a weak acid. On addition of a stronger acid such as HCl or H2SO4 to a solution of Na2SiO3 or K2SiO3 silicic acid is produced (Figure 3-86) in the form of a gel. I was able to find only one reasonably good video demonstration of this reaction, and it is in Russian. In this experiment, aqueous HCl is slowly (way too slowly, imho) added to a solution of sodium silicate to give rise to a gel of silicic acid. As a matter of fact, the silicic acid produced in this way is a polymer whose structure and formula differ from those of H2SiO3. It is not a big deviation from the truth, however, to use the simplified representation of the gel formed in the reaction as H2SiO3.
Figure 3-86. Silicic acid is formed on addition of an acid to sodium silicate.

A gel of silicic acid loses water on heating (H2SiO3 = SiO2 + H2O) to give silica gel, amorphous SiO2. Silica gel is a desiccant. If you have ever been curious enough to open one of those small packets saying "silica gel" and "desiccant", you know what silica gel granules look like (Figure 3-87).
Figure 3-87. Silica gel desiccant pockets and beads (source).

In the form of a fine powder, silica gel is used as a food additive. Being non-toxic, silica gel is permitted to be added to food in quantities of up to 2% in the U.S.A. and up to 5% in the E.U. countries. A very important application of silica gel is in chromatography, a powerful method for separation and purification of chemical substances.

To conclude this section, I invite you to watch a stunning video demonstration of the silicate garden experiment. If you want to know why the garden grows and how it works, read this article.

3.6.6. Exercises.

1. Explain why carbon and silicon form covalent rather than ionic compounds. Answer

2. Name two most common naturally occurring allotropes of carbon. Answer

3. Explain how graphite pencils work. [Answer: See section 3.6.2]

4. What is adsorption and how does it differ from absorption? What is activated carbon (activated charcoal) and why is it used in gas masks? [Answer: See section 3.6.2]

5. Carbon monoxide is a colorless, odorless, and highly toxic gas. True or false? Why is CO poisonous? Answer

6. Carbon dioxide is widely used in fire extinguishers because CO2 is very good at putting out fires from burning wood, gasoline, cardboard, etc. Being more dense than air, CO2 displaces the air in the fire area and floods the fire site on the ground, thereby blocking access of O2 to the burning material. In the absence of O2, the burning stops quickly. In sharp contrast, CO2 cannot and must not be used for putting out burning active metals, such as Na, K, and Mg. Why? Answer

7. Both CO2 and CO react with water to give acids, H2CO3 and H2CO2, respectively. True or false? Answer

8. Write a chemical equation for the water-gas shift reaction. What makes this reaction highly important? Answer

9. Carbonic acid, H2CO3, is (a) a stable weak acid that can be isolated pure; (b) a strong acid that cannot be isolated pure but exists in low concentrations in aqueous solutions; (c) a weak acid that cannot be isolated pure but exists in low concentrations in aqueous solutions. Answer

10. How would you make pure baking soda (NaHCO3) from pure soda ash (Na2CO3) and white vinegar (aqueous solution of acetic acid)? Answer

11. How would you make a reagent for detecting CO2 from toothpaste? Answer

12. Silicon is the most abundant element in the Earth's crust. True or false? Answer

13. Unlike carbon, silicon in the form of a simple substance does not occur in nature. True or false? Answer

14. How is elemental silicon (silicon as a simple substance) made? [Answer: See Figure 3-82]

15. What are the main ingredients for making (a) cement and (b) glass? Answer

16. Write balanced chemical equations for the reactions involved in the making of glass from SiO2, Na2CO3, and CaCO3. [Answer: See subsection 3.6.5 and Figure 3-85]

17. What is silica gel and how is it made and used? [Answer: See section 3.6.5]