Carbon Dioxide

CHEMICAL NAME = carbon dioxide CAS NUMBER = 124–38–9 MOLECULAR FORMULA = CO₂ MOLAR MASS = 44.01 g/mol

COMPOSITION = C(27.3%) O(72.7%) MELTING POINT = −56.6°C

BOILING POINT = −78.5°C (sublimation point) DENSITY = 2.0 g/L (vapor density = 1.53, air = 1)

Carbon dioxide is a colorless, odorless gas present throughout the atmosphere and is an essential compound for life on Earth. It is found on other planets in the solar system. Mars’s icecaps are primarily frozen carbon dioxide and Venus’s atmosphere is mostly carbon dioxide.

Th e discovery of carbon dioxide, credited to Joseph Black (1728–1799), played a critical role in supplanting the phlogiston theory and advancing the development of modern chemistry.

Black, in his medical studies, was searching for a substance to dissolve kidney stones, but he switched his subject to a study of stomach acidity. Black was working with the carbonates magnesia alba (magnesium carbonate) and calcium carbonate (limestone) and observed that when magnesia alba was heated or reacted with acids, it produced a gas and a salt. Black, who published his work in 1756, called the gas “fi xed air” and noted that it had properties similar to those described by Jan Baptista van Helmont (1577–1644) for spiritus sylvestrius. Spiritus sylvestrius was the gas produced during combustion processes, and van Helmont realized that this was the same gas produced during fermentation and when acids reacted with seashells.

Carbon dioxide accounts for 0.037% by volume of the atmosphere. Its low concentra-tion means that most commercial supplies of carbon dioxide are acquired as by-products of industrial chemical reactions. Several methods can be used to produce large volumes of CO₂. Th e combustion of coke or other carbonaceous substances produces results in CO₂: C_(coke) + O₂ → CO_2(g). In combustion processes, CO₂ is concentrated by separating it from other gases using scrubbing and absorption techniques. Another source of CO₂ involves the calcination (slow heating) of carbonates such as limestone, CaCO₃: CaCO_3(s) CaO + CO_2(g).

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Th is process takes place in a lime kiln in the production of precipitated calcium carbonate at temperatures of from 500°C to 900°C. Carbon dioxide is also produced as a by-product in fermentation reactions to produce alcohols. An example is the fermentation of glucose, C₆H₁₂O₆ to ethanol (C₂H₅OH): C₆H₁₂O_6(aq) 2C₂H₅OH _(aq) + 2CO_2(g). Carbon dioxide is produced as a by-product in a number of syntheses, such as the Haber process, to produce ammonia.

Carbon dioxide has several major uses. Solid carbon dioxide, dry ice, is used as a refrigerant.

Dry ice was fi rst prepared in France by Charles Th ilorier (1797–1852) in 1834. He observed dry ice when carbon dioxide expanded from pressurized containers. Th ilorier reported his fi ndings in 1835, but dry ice was not used commercially until the 1920s. Th e term dry ice was the original trademark of Prest Air Devices of Long Island in 1924. Th e company built a com-mercial plant to produce dry ice and changed its name to DryIce. Dry ice has been adopted generically as the name for solid CO₂. Dry ice is an appropriate name because at atmospheric pressure, carbon dioxide exists as a solid or gas depending on the temperature. Th e phase change from solid to gas without passing through the liquid stage is called sublimation. Th e reverse process from gas to solid is called deposition. Sublimation and deposition of carbon dioxide occur at −78.5°C.

Another major use of carbon dioxide is in the soda industry. Soda is sodium carbonate monohydrate (Na₂CO₃• H₂O). Other forms of soda include washing soda, which is sodium carbonate decahydrate (Na₂CO₃• 10H₂O), and baking soda, which is sodium bicarbonate (NaHCO₃). Sodium carbonate is also referred to as soda ash because it was prepared by leaching the ashes of burnt wood. Th e Solvay process for producing soda ash is a series of reac-tions that uses ammonia, carbon dioxide, and water. Baking soda is produced from sodium carbonate and carbon dioxide according to the reaction: Na₂CO_3(s) + H₂O_(l) + CO_2(g) → 2NaHCO_3(s).

Carbon dioxide is used to produce carbonated beverages by bubbling CO₂ into them. Th e carbonation of beverages was discovered by the English chemist and Presbyterian minister Joseph Priestley (1733–1804). Priestley’s fi rst investigations in chemistry dealt with carbon dioxide produced in a brewery near his parish in Leeds. Priestley discovered that he could dis-solve the gas in water to produce a pleasant-tasting beverage. Priestley had actually produced soda water, the equivalent of sparkling water. Priestley’s process for producing carbonated water was made commercially feasible in the 1790s by Jacob Schweppe (1740–1821), the founder of the company that still bears his name.

Carbon dioxide is used as a gas in fi re extinguishers, as an infl ation gas for fl otation devices, and as a propellant (for example in air guns). In recent years, the use of carbon dioxide as a supercritical fl uid in green chemistry applications has increased. A supercritical fl uid is a fl uid with a temperature and pressure above its critical point. For CO₂, the critical temperature is 31.1°C and the critical pressure is 73 atmospheres. Beyond the critical point, the properties of liquid and gas merge. Th e phase diagram for CO₂ shown in Figure 22.1 illustrates subli-mation. Below 5.1 atmospheres, which defi nes the pressure at the triple point where all three phases exist, CO₂ passes directly from solid to gas. Supercritical CO₂ has the penetrating power of a gas but the solubility properties of a liquid. Supercritical CO₂, abbreviated scCO_2, has a number of advantages when used to replace traditional organic solvents. Th ese include its relative inertness, nonfl ammability, low toxicity, abundance, and low expense. Supercritical CO₂ has replaced the Freon substance methylene chloride, CH₂Cl₂, to extract caff eine from

70 ^| Th e 100 Most Important Chemical Compounds

coff ee and tea to produce decaff einated products. Supercritical CO₂ is also used in place of other organic solvents to extract compounds used for pharmaceuticals, spices, fl avorings, and industry. It can be used as a cleaner and degreaser. It is also fi nding increasing use in the dry cleaning industry, where it has started to replace perchloroethylene, C₂Cl₄.

Figure 22.1 Phase diagram for carbon dioxide.

Plants require carbon dioxide for photosynthesis, and during respiration carbon dioxide is produced. Photosynthesis involves a series of biochemical reactions in which plants (and some bacteria) take inorganic carbon dioxide and water and use energy supplied from the sun to build carbohydrates. Th e carbohydrates are oxidized for energy during cellular respiration and are also used to build other compounds such as lipids, proteins, and nucleic acids. Th e general reaction for photosynthesis can be represented as: nCO₂ + nH₂O (CH₂O)_n + nO₂. Th e production of the carbohydrate glucose, C₆H₁₂O₆, is therefore represented by the equation:

6CO₂ + 6H₂O C₆H₁₂O₆ + 6O₂

Th is reaction representing photosynthesis is highly simplifi ed; the actual process involves numerous biochemical reactions, which take place in two sets called the light-dependent and light-independent (sometimes called light and dark) reactions. Oxygen is produced in the light-dependent reaction when water is split, and carbon dioxide is converted to carbohydrates in the light-independent reactions. Th e conversion of carbon dioxide into carbohydrates is termed carbon fi xation.

Carbon dioxide readily dissolves in water to produce the weak acid carbonic acid, H₂CO₃. Th e presence of CO₂ in aqueous solution results in equilibria involving carbon dioxide, carbonic acid, bicarbonate, and carbonate: CO_2(aq) + H₂O_(l) H₂CO_3(aq)

H⁺ + HCO_3- H⁺ + CO₃^2-. Th e dissolution of CO₂ in water explains why rainwater is naturally acidic. Pure rainwater has a pH of about 5.6, owing to the presence of carbonic acid. Th e solubility of carbon dioxide in water has a signifi cant impact on the earth’s carbon cycle. Dissolved CO₂ can be incorporated into carbonate sediments in oceans and lakes. Th e solubility of CO₂ in blood as carbonic acid is one of the most important buff ering systems in the human body. Th e pH of blood is about 7.4. If the pH of blood falls below 6.8 or above 7.8, critical problems and even death can occur. Th ree primary buff er systems control the pH of blood: carbonate, phosphate, and proteins. Th e primary buff er system in the blood involves carbonic acid and bicarbonate. Th e carbonic acid neutralizes excess base in the blood and the

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bicarbonate ion neutralizes excess acid. Th e excessive amount of bicarbonate in the blood means that blood has a much greater capacity to neutralize acids. Many acids accumulate in the blood during strenuous activity such as lactic acid. Excretion of bicarbonate through the kidneys and the removal of carbon dioxide through respiration also regulate the carbonic acid/bicarbonate blood buff er.

Carbon dioxide is a greenhouse gas, which makes it a major concern in the global warm-ing and climate change controversy. Carbon dioxide is the most important anthropogenic greenhouse gas and is thought to be responsible for roughly 60% of recent global warming.

Th e atmosphere’s CO₂ concentration has risen from about 280 ppmv (parts per million by volume) to 370 ppmv in the last 160 years. Th is increase is due primarily to the burning of fossil fuels. Land clearing has also contributed to increased CO₂ levels. Th e greenhouse eff ect continues to be a controversial topic in the early 21st century. Th e controversy has shifted from uncertainty concerning whether humans aff ect the earth’s climate to how much humans are aff ecting the climate and what action, if any, should be taken. A major diffi culty surround-ing the greenhouse eff ect is separatsurround-ing human infl uences from the climate’s natural variability.

Although there are still skeptics, the consensus seems to be that humans are aff ecting the cli-mate, but exactly how, and to what extent, continues to be debated. Th e issue is complicated by numerous factors. Th e oceans play a major role in the carbon cycle because they have a great capacity to absorb carbon dioxide. Vegetation also aff ects global carbon concentrations through photosynthesis. Some studies have indicated that primary productivity increases with elevated CO₂, but others show no increase.