Gases in Earth's Atmosphere PDF

EXPERIENCE 4 Gases in Earth’s Atmosphere GO ONLINE to Explore and Explain atmospheric gases in...

EXPERIENCE 4 Gases in Earth’s Atmosphere GO ONLINE to Explore and Explain atmospheric gases in familiar scenarios. Composition of the Atmosphere Earth’s atmosphere is made up of many different gases. The atmosphere is primarily made up of nitrogen and oxygen, with small amounts of trace gases. The contribution each gas makes to the total atmospheric pressure is called the partial pressure. Composition of Dry Air The table and charts show the composition of dry air, or air that does not contain any water vapor. The table shows both the volume by percentage and the partial pressure of each gas. Major Ga Trace Ga (0.04%) 0.036% Argon Oxygen 0.93% 20.95% Trace gases Nitrogen 0.04% 78.08% 0.0018% 0.0005% 0.00017% Carbon Neon Helium Methane dioxide Copyright © Savvas Learning Company LLC. All Rights Reserved. Data from: SFSU Compostion of Dry Air Percentage of Partial Component Total Volume Pressure (kPa) The contribution of each Nitrogen 78.08 79.11 gas to the total pressure of the atmosphere is due Oxygen 20.95 21.22 to its percent volume. For example, nitrogen contributes Argon 0.93 0.95 (78.08 ÷ 100%) × 101.32 = 79.11. Carbon dioxide 0.04 0.04 and others Total 100.00 101.32 4 Gases in Earth’s Atmosphere 31  Dalton’s Law In 1801, chemist John Dalton recognized that at constant volume and temperature, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the component gases. This is called Dalton’s law, and it is expressed mathematically as follows: Ptotal = P1 + P2 + P3 +... For example, dry air is a mixture of a number of gases, (N 2, O2, CO2, Ar, etc.). The individual partial pressures of each gas can be added together to get a total atmospheric pressure, which at sea level equals 101.32 kPa. Air at High Altitude As you ascend a high mountain, you will notice that the air is thinner at high altitudes, making it more difficult to breathe. This is due to less oxygen being taken in for each breath. PO = 6.4 kPa 2 2 O2 8000 m Although the amount of 8 N2 oxygen is less, the ratio between oxygen and nitrogen in the atmosphere 6000 m does not change. Atmospheric pressure PO = 12.9 kPa 2 Altitude 4000 m The curved line on 2000 m the graph shows that PO = 21.2 kPa as altitude increases, 2 Copyright © Savvas Learning Company LLC. All Rights Reserved. atmospheric pressure 7 O2 decreases. 28 N2 0m 30 40 50 60 70 80 90 100 Atmospheric Pressure (kPa) 27 SEP Analyze and Interpret Data The table shows the pressure in a SCUBA diver’s lungs at various depths. Nitrogen narcosis is a problem for divers when the N2 partial pressure in their lungs exceeds 700 kPa. Complete the table and discuss with a classmate at what depth narcosis becomes an issue. Lung Air Pressure at Various Depths Underwater N2 Volume (%) Depth (m) Pressure (kPa) PN2 (kPa) 78.08 0 101.32 79.11 78.08 30.48 404.32 315.69 78.08 91.44 1010.32 788.86 32 Investigation 9 The Behavior of Gases  SAMPLE PROBLEM Using Dalton’s Law Air contains oxygen, nitrogen, argon, and trace amounts of other gases. What is the partial pressure of oxygen (P O 2) at 101.30 kPa of total pressure if the partial pressures of nitrogen, argon, and other gases are 79.10 kPa, 0.94 kPa, and 0.040 kPa, respectively? ANALYZE List the knowns and the unknown. Knowns Unknown P N 2 = 79.10 kPa PO 2 = ? kPa PAr = 0.94 kPa Pothers = 0.040 kPa Ptotal = 101.30 kPa CALCULATE Solve for the unknown. State Dalton’s law. Ptotal = PO + PN + PAr + Pothers 2 2 Rearrange the equation to isolate PO 2. PO = Ptotal − PN − PAr − Pothers 2 2 Substitute the known quantities into PO 2 = 101.30 kPa − 79.10 kPa − 0.94 kPa − 0.040 kPa the equation to find the partial pressure of oxygen. PO 2 = 21.22 kPa EVALUATE Does the result make sense? Copyright © Savvas Learning Company LLC. All Rights Reserved. The partial pressure of oxygen must be smaller than that of nitrogen. The other partial pressures are smaller, so the calculated answer of 21.22 kPa seems reasonable. 28 SEP Use Mathematics A gas mixture containing oxygen, nitrogen, and carbon dioxide has a total pressure of 32.9 kPa. If P O2 = 6.6 kPa and P N 2 = 23.0 kPa, what is P CO 2? P CO 2 = Ptotal − PO2 − PN2 = 32.9 kPa − 6.6 kPa − 23.0 kPa = 3.3 kPa. The partial pressure of CO2 is 3.3 kPa. GO ONLINE for more practice problems. 4 Gases in Earth’s Atmosphere 33  Diffusion Diffusion is the tendency of molecules to move toward areas of lower concentration until the concentration is uniform. Suppose you open a bottle of perfume in one corner of a room. At some point, a person standing at the opposite corner will be able to smell the perfume. Diffusion of Gas Molecules Low When a perfume bottle is opened, Concentration volatile, or odorant, molecules in perfume change to gas through The gas molecules have a evaporation and mix with the higher concentration and surrounding air. higher pressure when they leave the perfume bottle compared to the rest of High the room. Concentration Since there is a high concentration of odorant gas molecules in one corner of the room, the odorant molecules begin to diffuse and spread out through the air in the room to regions of low or no odorant concentration. Diffusion Eventually, the odorant gas molecules will be evenly dispersed throughout the room, such that the concentration of odorant molecules is at equilibrium and constant Copyright © Savvas Learning Company LLC. All Rights Reserved. everywhere. Diffusion and Particle Speed For two gases at the same temperature, the average kinetic Small particles move faster. energy (KE) is the same. If the A larger velocity results in mass (m) of one of the particles is a faster rate of diffusion. larger, then the velocity (v) of the particle must be smaller. Kinetic energy is related to mass and velocity by the equation KE = 12mv2. 34 Investigation 9 The Behavior of Gases  29 SEP Develop a Model The ideal gas law and the concept of diffusion are demonstrated many ways in everyday life. For example, an air pump draws in air during the upstroke. Describe what happens to the pressure and volume inside the pump. On the upstroke, the volume increases and the pressure temporarily decreases. Then, the air diffuses to fill the entire increased volume, increasing the pressure again. Graham’s Law Another process that involves the movement of molecules in a gas is called effusion. During effusion, a gas escapes through a tiny hole in its container. Scottish chemist Thomas Graham studied rates of effusion during the 1840s. From his observations, he proposed an explanation, now called Graham’s law of effusion, which states that the rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass (M). This law can also be applied to the diffusion of gases. Graham’s law can be written as follows for two gases, A and B: RateA RateB =√MB MA Effusion and Molecule Size A larger quantity of small molecules can move through a hole during a given amount of time than larger molecules can. This means small molecules Copyright © Savvas Learning Company LLC. All Rights Reserved. will effuse faster than large molecules. 30 SEP Develop Models Sketch two balloons of equal volume, one filled with helium and one filled with oxygen. Then, sketch the balloons after one week, modeling which balloon has deflated more. Student sketches should show that the effusion of the helium molecules is greater than the effusion of the oxygen molecules so that the helium ballon has a lesser volume after one week. 4 Gases in Earth’s Atmosphere 35 SAMPLE PROBLEM Comparing Effusion Rates How much faster does helium (He) effuse than nitrogen (N2) at the same temperature? ANALYZE List the knowns and the unknown. Knowns Unknown molar mass of He = 4.0 g/mol Ratio of effusion rates = ? molar mass of N2 = 28.0 g/mol CALCULATE Solve for the unknown. Start with the equation for Graham’s law RateHe √ MN 2 = of effusion. RateN2 √MHe Substitute the molar masses of nitrogen RateHe √28.0 g/mol and helium into the equation to find the = = √7.0 = 2.7 RateN2 √4.0 g/mol ratio of effusion rates. EVALUATE Does the result make sense? Helium atoms are less massive than nitrogen molecules, so it makes sense that helium effuses faster than nitrogen. 31 SEP Use Mathematics Calculate the ratio of the velocity of hydrogen molecules to the velocity of carbon dioxide molecules at the same temperature. Copyright © Savvas Learning Company LLC. All Rights Reserved. The molar mass of hydrogen molecules is 2.00 g/mol. The molar mass of carbon M dioxide is 44.00 g/mol. Rate √ CO √ = 44.0 = 4.69 H2 Rate CO 2 = 2 √MH 2 √2.00 32 SEP Use Mathematics Your friend says that argon gas effuses faster than oxygen gas because argon gas is monatomic and oxygen gas is diatomic. Do you agree? Explain your reasoning. Sample answer: I disagree. According to Graham’s law, oxygen will effuse faster than argon because oxygen (32 g/mol) has a lesser molar mass than argon (40 g/mol). GO ONLINE for more practice problems. 36 Investigation 9 The Behavior of Gases  Wind Wind Is the Diffusion of Air Wind is caused by the movement of air particles and can be explained using the ideal gas law and the concept of diffusion. For an open system like the atmosphere, the air will move from regions of high pressure to regions of low pressure. Air Heated From Below The sun’s rays Warmed Air Rises The density of the gas shine on the surface of Earth, warming decreases as the temperature increases at the air directly above the surface through constant pressure. This results in the expansion radiation. The air heats in an isobaric of the air directly above the ground. Since the air process, resulting in the expansion of the air above the ground is now less dense than the air directly above the ground. surrounding it, this section of air begins to rise. Wind Is Diffusing Air Relatively warm temperatures cause air to rise, reducing the pressure and air concentration near the ground. Relatively cold temperatures cause air to fall toward the ground, increasing pressure and air concentration. As a result, air diffuses from the high-pressure, cold region to the low-pressure, warm region. This movement of air is wind. Copyright © Savvas Learning Company LLC. All Rights Reserved. Wind Warm Air Cool Air Low Pressure High Pressure 4 Gases in Earth’s Atmosphere 37  Using a Mathematical Model to Explain Wind The intensive form of the ideal gas law is useful for mathematically modeling the behavior of air. Recall that density is related to pressure, temperature, the gas constant, and the molar mass of gas: P = RspecificT Looking at this equation, you may think that high temperature should result in high pressure. In reality, the pressure of a sample of air will actually stay constant; it is the density that changes according to the equation. The air behaves in a way similar to an isobaric system, such as the inside of a balloon. When the temperature of an air sample increases, density decreases. The warmer, less dense air will rise, leaving behind a low- pressure void. The pressure of the warmed air doesn’t change; the pressure of the void left behind is what is actually different. Air that was not heated will rush in to fill the void through diffusion, resulting in wind. Pressure and Temperature The pressure of a heated sample of air actually stays constant, consistent with Pressure is constant for heated the ideal gas law. It’s the empty space left behind by gas, like a balloon. Density the rising gas that results in a low-pressure region. decreases and the air rises. Copyright © Savvas Learning Company LLC. All Rights Reserved. This sample of air has pressure Pressure is low in the area left P and density , which is the behind by the rising gas. same as the surrounding air. 33 SEP Obtain and Evaluate Information Use an online resource and research why the Great Plains area is the windiest region in North America. From Chinook winds to blizzards and tornadoes, strong winds have always had a major role as part of the Great Plains story. Include the topography of the Great Plains as part of your explanation as to how these powerful winds are generated in this region. Sample answer: The Great Plains are at the center of the continent, so high pressure areas can develop easily. Also, the lack of mountains allows winds to develop across large areas if pressure gradients are present. 38 Investigation 9 The Behavior of Gases  Saturated Water Vapor Evaporation of water can happen until equilibrium is achieved between molecules going back and forth between the liquid and gas phases. Saturated vapor pressure is the partial pressure of water in the air at this equilibrium point. Humidity So far, air has been modeled as being dry. However, air almost always contains a varying amount of water vapor, which results from the evaporation of liquid water on Earth’s surface. The maximum pressure of water vapor in air is called saturated vapor pressure. The temperature at which the air is saturated with water vapor is called the dewpoint. If the air is cooled further than the dewpoint, then some of the water vapor will condense back into the liquid phase, resulting in cloud formation or rain. Relative humidity (RH ) is the percent ratio of actual water vapor partial pressure to the saturated vapor pressure. It can be expressed mathematically as follows: actual water vapor partial pressure RH = × 100 saturated vapor pressure Relative Humidity Saturated vapor 30ºC pressure varies with temperature. 20ºC 10ºC Copyright © Savvas Learning Company LLC. All Rights Reserved. Therefore, the relative humidity also varies with temperature. The diagram shows the amount of actual water vapor as a fraction of the amount possible. The Water actual amount of water vapor in the air Maximum vapor 65% 53% 28% water stays constant, going from 10°C to 30°C. relative relative relative However, the relative humidity decreases. vapor humidity humidity humidity possible 34 Predict As you descend a mountain, the total atmospheric pressure increases, while the partial pressure of water vapor remains constant. Use Dalton’s law to infer how the relative humidity would change. How would the relative humidity change if the air also got warmer? As atmospheric pressure increases, saturated vapor pressure also increases, so relative humidity decreases as you descend. The realtive humidity will decrease even more if the air temperature increases. 4 Gases in Earth’s Atmosphere 39 35 Predict Would you expect saturated vapor pressure to change more drastically from winter to summer over the equator or in the northern Pacific Ocean? Explain your answer. Sample answer: Saturated vapor pressure depends on temperature. If the temperature change from winter to summer is greater in the northern Pacific Ocean, then the saturated vapor pressure change should also be greater. Revisit INVESTIGATIVE GO ONLINE to Elaborate on and Evaluate your knowledge of gases in Earth’s atmosphere PHENOMENON by completing the class discussion and writing activities. In the modeling worksheet, you developed a model to explain how winds manifest as a result of the behavior of gases in the atmosphere. With a partner, complete the prompt below and then reevaluate your model. 36 SEP Construct an Explanation As air moves rapidly from high altitude up in the Santa Ana Mountains to low altitude by the Pacific Ocean, the relative humidity of the air dramatically decreases. Explain why. Warm air can hold much more water vapor than cold air, and air with a higher pressure can also hold much more water vapor than air with a lower pressure. Copyright © Savvas Learning Company LLC. All Rights Reserved. Therefore, air moving rapidly from the high-altitude, cooler Santa Ana Mountains will dramatically decrease in relative humidity as it moves to warmer, lower altitudes. 40