The Behavior and Applications of Gases

329670_ch10.qxd 07/20/2006 11:32 AM Page 394 The Behavior and Applications Contents and Selected Applications 10.1 The Nature of Gases of Gases 10.2 Production of Hydrogen and the Meaning of Pressure 10.3 Mixtures of Gases—Dalton’s Law and Food Packaging Chemical Encounters: Dalton’s Law and Food Packaging 10.4 The Gas Laws—Relating the Behavior of Gases to Key Properties Chemical Encounters: Balloons and Ozone Analysis 10.5 The Ideal Gas Equation 10.6 Applications of the Ideal Gas Equation Earth’s atmosphere is a very thin layer of gases (only 560 km thick) that surrounds Chemical Encounters: Automobile Air Bags Chemical Encounters: Acetylene the planet. Although the atmosphere makes up only a small part of our planet, 10.7 Kinetic-Molecular Theory it is vital to our survival. 10.8 Effusion and Diffusion 10.9 Industrialization: A Wonderful, Yet Cautionary, Tale Chemical Encounters: Ozone Chemical Encounters: The Greenhouse Effect 394 329670_ch10.qxd 6/19/06 1:45 PM Page 395 Our planet is surrounded by a relatively thin layer known as an atmosphere. It supplies all living organisms on Earth with breathable air, serves as the vehicle to deliver rain to our crops, and protects us from harmful ultraviolet rays from the Sun. Although it does contain solid particles, such as dust and smoke, and liquid droplets, such as sea TABLE 10.1 Composition of Dry Air spray and clouds, much of the atmosphere we live in is Percent Parts per Million actually a mixture of the gases shown in Table 10.1. These Component by Volume by Volume gases, and the rest of the components in the atmosphere of N2 78.08 780,800 planet Earth, are vital to our survival. O2 20.95 209,500 In addition to forming the atmosphere surrounding Ar 0.934 9,340 planet Earth, gases are vital to our society in many other CO2 0.033 330 ways. Gases that are used in manufacturing, medicine, or Ne 0.00182 18.2 He 0.000524 5.24 anything else related to the economy are called industrial CH4 0.0002 2 gases, shown in Table 10.2. Some, such as nitrogen and oxy- Kr 0.000114 1.14 gen, can simply be separated from the air. Others, including H2 0.00005 0.5 sulfur dioxide, hydrogen, and chlorine, are produced by N2O 0.00005 0.5 chemical reactions. No matter what uses we make of them, Xe 0.0000087 0.087 < × −5 < nearly all of these common gases behave in about the same O3 1 10 0.1 CO <1 × 10−6 <0.01 way when present at low density. The fact that almost all NO <1 × 10−6 <0.01 gases share this characteristic enables us to work with them predictably and successfully. Source: Chemical Rubber Company. Handbook of Chemistry and Physics, 70th ed. CRC Press: Boca Raton, FL, 1990. TABLE 10.2 Industrial Uses of Some Common Gases U.S. Production (2004) in Metric Gas Use Tons (1 metric ton = 1000 kg) Cl2 Preparation of bleaches and cleansing agents, 12.2 million purification of water, preparation of pesticides CO2 Refrigeration, preparation of beverages, inert 8.1 million (estimated) atmosphere in chemical reactions and food packaging, fire extinguishers H2 Hydrogenation of oils in food and other 17.7 bcm (gas + liquid) unsaturated organic molecules, energy source (bcm = billion m3) in vehicles, petroleum-refining reactions, manufacturing of resins used in plastics NH3 Fertilizer and preparation of other fertilizers 10.8 million N2 Inert atmosphere in chemical reactions and food 30.3 million (not incl. NH3) packaging, electronics, and metalwork; manufacture of ammonia; refrigerant O2 Oxidizer in many chemical processes, such as 25.5 million production of steel and acetylene; oxidizer in reaction of fuels in rockets; in hospitals (breathing); pulp and paper industries for bleaching SO2 Production of sulfuric acid, bleaching agent in food 300,000 and textile industries, controls fermentation in wines Source: Chemical and Engineering News, July 11, 2005. 395 329670_ch10.qxd 6/19/06 1:45 PM Page 396 396 Chapter 10 The Behavior and Applications of Gases 10.1 The Nature of Gases Many of the chemists in the eighteenth century became aware of a very interest- ing phenomenon as they worked with gases. They noted that dissimilar gases appeared to behave in relatively similar ways. Why is this possible? It all comes down to the common features of gases. Under ambient conditions, electrostatic interactions make water a liquid and sodium chloride a solid. Gases have few such interactions, and when they occur, they are often weak and fleeting. In fact, the atoms or molecules of gases most often behave as individual units. Gases that behave as though each particle has no interactions with any other particles are called ideal gases; their properties are listed in Table 10.3. Most gases behave nearly ideally under normal conditions, so unless otherwise noted, the quantitative rela- tionships we will discuss assume ideal behavior. Notably, deviations from ideal behavior occur when gases are examined under conditions that encourage the interactions between molecules of a gas: high pressures and low temperatures. Gases have an exceedingly low density—that is, the particles are relatively far apart. For example, at 25°C, liquid water has a density of 1.00 g/mL, whereas dry air at sea level has a density of 0.00118 g/mL. The low density of gases means that they can be significantly compressed in order to save space during shipping, as shown in Figure 10.1. Solids and liquids are nearly incompressible and have volumes that change very little with temperature and pressure. This difference, too, is related to the nature of gases. Because gas molecules are far apart compared to molecules within a liquid or solid, the volume of a gas can be greatly affected by changes in temperature and pressure. As we will discuss later, we make use of the compressibility of gases, and the energy exchanges that accompany it, in applications from refrigeration to rocketry. FIGURE 10.1 Gases can be significantly compressed, which makes it possible to ship large quantities of industrial gases via truck in compressed gas cylinders. Liquid Gas Smaller volume Larger volume more dense less dense Gas molecules occupy a much larger volume than the same number of molecules of a liquid. This results in a much lower density for gases than for liquids. TABLE 10.3 The Ideal Gas The ideal gas is one that follows each of these rules: • The individual gas particles do not interact with each other. • The individual gas particles are assumed to have no volume. • The gas strictly obeys all of the simple gas laws. • The gas has a molar volume of 22.414 L at 1 atm of pressure and 273.15 K. 329670_ch10.qxd 6/19/06 1:45 PM Page 397 10.2 Production of Hydrogen and the Meaning of Pressure 397 Gases are highly compressible compared to liquids and solids. Solid Liquid Gas Tightly packed Loosely packed Widely spaced least compressible slightly compressible highly compressible 10.2 Production of Hydrogen and the Meaning of Pressure Power supply Nearly 18 billion cubic meters of hydrogen gas were produced in the United States in 2004. Because hydrogen gas is a very minor compo- nentof air(seeTable10.1),itmustbegeneratedviachemicalprocesses. One method is the decomposition of water by electrolysis,inwhichan Hydrogen electric current passing through a solution causes a chemical reaction. gas The resulting hydrogen gas can be collected separately from the oxygen gas. Let’s assume that the process takes place at constant temperature and that the collection vessel,which has a constant volume,is empty as hydrogen begins to enter, as shown in Figure 10.2. What happens as H2 flows in? The molecules begin to collide with the walls of the container and with each other, as shown in Fig- ure 10.3. The force that each molecule applies during these collisions is equal to the mass of the particle times its acceleration: m Force = mass × acceleration = kg × = kg·m·s−2 s2 − The SI unit of force is the newton (N), which is equal to 1 kg·m·s 2. Because each of us has mass and is accelerated toward the center of 9.81 m the Earth by the pull of gravity ,eachofusexertsaforceon FIGURE 10.2 s2 Our hypothetical reaction set-up for the electrolysis of water.The hydrogen gas can be isolated from the reaction and sent to the collection chamber. FIGURE 10.3 Molecules entering a vacant container collide with each other and with the walls of the container. The molecules have mass, + acceleration, and a resulting force. = H2 molecule Hydrogen and oxygen can be produced by the electolysis of water. 329670_ch10.qxd 6/19/06 1:45 PM Page 398 398 Chapter 10 The Behavior and Applications of Gases the floor upon which we are standing. In describing our own mass × ac- 9.8 m/s2 celeration, we most often speak of force by using the familiar unit pounds, where 1 lb = 4.47 N. EXERCISE 10.1 Calculating Force Suppose that your mass is 55 kg and you are on the Moon, where the acceleration 1.6m due to gravity is about , roughly one-sixth that on Earth. What force would s2 you exert on a scale (“How much would you weigh”) in newtons and in pounds? The force of gravity on a person. First Thoughts Mass is a measure of the amount of substance, whereas force is the effect of acceleration, in this case as gravity, on the mass.

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