Chemistry of Atmosphere SCI 1 PDF

SCI 1 Chemistry of Atmosphere 2 Learning Outcomes Students should be able to meet the following intended learning outcomes: Explain the different regions of the atmosphere based on temperature profile. Explain the temperature changes in the atmosphere. Discuss the composition of the atmo...

SCI 1 Chemistry of Atmosphere 2 Learning Outcomes Students should be able to meet the following intended learning outcomes: Explain the different regions of the atmosphere based on temperature profile. Explain the temperature changes in the atmosphere. Discuss the composition of the atmosphere. Calculate concentrations from partial pressure. Explain the photochemical reactions in the atmosphere. Calculate the wavelength required to break a bond. Discuss the chemical processes involve in the stratosphere Discuss the chemical reactions involve in the depletion of the ozone layer. Discuss the chemical processes involve in the burning of fossil fuels resulting to acid rain. Discuss the chemical processes involve in the emission of nitrogen oxides resulting to photochemical smog. Discuss the role of greenhouse gases on global warming and climate change.  Earth is the only planet in the solar system with an atmosphere that can sustain life. The blanket of gases not only contains the air that we breathe but also protects us from the blasts of heat and radiation emanating from the sun. It warms the planet by day and cools it at night. Earth’s atmosphere is the thin envelope of air that surrounds our planet is a mixture of gases, each with its own physical properties. The temperature of the atmosphere varies with altitude. Figure 1. Temperature vs Altitude in the Atmosphere 4 Regions of the Atmosphere based on its Temperature Profile 1. Troposphere - the layer closest to Earth's surface, about 10 km above the surface reaching about 215 K - It is 4 to 12 miles (7 to 20 km) thick and contains half of Earth's atmosphere - Air is warmer near the ground and gets colder higher up. - contains about 75% of all of the air in the atmosphere, - It contains most of our weather clouds, rain, snow. - the temperature gets colder as the distance above the earth increases, by about 6.5°C per kilometer. - The lowest part of the troposphere is called the boundary layer 2. Stratosphere - the second layer - It starts above the troposphere and ends about 31 miles (50 km) above ground and reaching a maximum of 275 K. - Ozone is abundant here and it heats the atmosphere while also absorbing harmful radiation from the sun. - The air here is very dry, and it is about a thousand times thinner here than it is at sea level, because of this, this is where jet aircraft and weather balloons fly. 3. Mesosphere - Starts at 31 miles (50 km) and extends to 53 miles (85 km) high. - Mesopause is the top of the stratosphere, the coldest part of Earth's atmosphere, with temperatures averaging about minus 130 ℉ (minus 90 ℃) 4. Thermosphere - the layer above mesopause. - a region in which temperatures again increase with height. It extends from about 56 miles (90 km) to between 310 and 620 miles (500 and 1,000 km) - Temperatures can get up to 2,700 degrees F (1,500 C) at this altitude. - is considered part of Earth's atmosphere, but air density is so low that most of this layer is what is normally thought of as outer space - above 80 km of this layer is also called “ionosphere” - The ionosphere reflects and absorbs radio waves, allowing us to receive shortwave radio broadcasts in New Zealand from other parts of the world. - Aurora Borealis and Aurora Australis occur in this region. Figure 3. Aurora Borealis (left) and Aurora Australis(right) 5. Exosphere - the outermost layer of the Earth’s atmosphere. It starts at an altitude of about 500 km and goes out to about 10,000 km. - is extremely thin and is where the atmosphere merges into outer Space - It contains mainly oxygen and hydrogen atoms, but there are so few of them that they rarely collide - they follow "ballistic" trajectories under the influence of gravity, and some of them escape right out into space. 6. Magnetosphere - the earth behaves like a huge magnet - It traps electrons (negative charge) and protons (positive), concentrating them in two bands about 3,000 and 16,000 km above the globe - the Van Allen "radiation" belts. - this where charged particles spiral along the magnetic field lines, The suffix –pause is used to denote the boundaries between adjacent regions. The boundaries are important because gases mix across them relatively slowly. Tropopause—the boundary between the stratosphere and troposphere. The top of the troposphere is called the tropopause. This is lowest at the poles, where it is about 7 - 10 km above the Earth's surface. It is highest (about 17 - 18 km) near the equator. Stratopause—the boundary between the mesosphere and the stratosphere. Mesopause—the boundary between the mesosphere and the thermosphere; the coldest region in the atmosphere. The troposphere and the stratosphere together account for 99.9% of the mass of the atmosphere, 75% of which is the mass in the troposphere. The thin upper atmosphere plays an important roles in determining the conditions of life at the surface. Composition of the Atmosphere The earth’s atmosphere is constantly bombarded by radiation and energetic particles from the Sun. This barrage of energy has profound chemical and physical effects, especially in the upper regions of the atmosphere, above 80 km. According to NASA, the gases in Earth's atmosphere include: Nitrogen — 78 percent Oxygen — 21 percent Argon — 0.93 percent Carbon dioxide — 0.04 percent Trace amounts of neon, helium, methane, krypton and hydrogen, as well as water vapor Figure 4. Composition of the atmosphere The Major Components of Dry Air near Sea Level Example 1. A 2.0 L container is pressurized with 0.25 atm of oxygen gas and 0.60 atm of nitrogen gas. What is the total pressure inside the container? Calculating the mole fraction The mole fraction is a way of expressing the relative proportion of one particular gas within a mixture of gases. This is done by dividing the number of moles of a particular gas i by the total number of moles in the mixture. 𝑋𝑖 = (𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠𝑖 ) / (𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑔𝑎𝑠) Example 2. A 3.0 L container contains 4 mol He, 2 mol Ne, and 1 mol Ar. What is the mole fraction of neon gas? Calculating partial pressure The partial pressure of one individual gas within the overall mixtures, 𝑃𝑖 , can be expressed as follows 𝑃𝑖 = 𝑃𝑡𝑜𝑡𝑎𝑙𝑋𝑖 where 𝑋𝑖 is the mole fraction Example 3. A mixture of 2 mol H2 and 3 mol He exerts a total pressure of 3 atm. What is the partial pressure of He? Example 4. What is the concentration, in parts per million, of water vapor in a sample of air if the partial pressure of the water is 0.80 torr and the total pressure of the air is 735 torr. Example 5. Deep-sea divers must use special gas mixtures in their tanks, rather than compressed air, to avoid serious problems, most notably a condition called “the bends.” At depths of about 350 ft, divers are subject to a pressure of approximately 10 atm. A typical gas cylinder used for such depths contains 51.2 g of 𝑂2 and 326.4 g of He and has a volume of 10.0 L. What is the partial pressure of each gas at 20.00°C, and what is the total pressure in the cylinder at this temperature? Photochemical Reactions in the Atmosphere Two kinds of Chemical Changes in the Upper Atmosphere 1. Photodissociation 2. Photoionization  The sun emits radiant energy over a wide range o wavelengths. Electromagnetic radiation can be pictured as a stream of photons. The energy of each photon is given by 𝐸 = ℎ𝑣, where ℎ is Planck constant (6.626 𝑥 10−34 𝐽⁄𝑠) and 𝑣 is the radiation frequency. For a chemical change to occur when radiation strikes atoms or molecules, two conditions must be met: 1. The incoming photons must have sufficient energy to break a chemical bond or remove an electron from the atom or molecule. 2. The atoms or molecules being bombarded must absorb these photons Photodissociation is the rupture of chemical bond resulting from absorption of a photon by a molecule. No ions are formed when the bond between two atoms is cleaved by photodissociation, instead half the bonding electrons stay with one atom and half stay with the other atom. The result is two electrically neutral particles. - One of the most important processes occuring above an altitude of about 120 km is photodissociation of the oxygen molecule. - The minimum energy required to cause this change is determined by the bond energy (or dissociation energy) of 𝑂2, 495 kJ/mol. - Bond energy is a specific quantity of energy that must be added to break the bond. Calculating the wavelength required to break a bond Example 6. What is the maximum wavelength of light, in nanometers that has enough energy per photon to dissociate 𝑂2 molecule? Solution: First calculate the energy required to break the 𝑂2 double bond in one molecule and then find the wavelength of a photon of this energy. The dissociation energy of 𝑂2 is 495 kJ/mol. Using this value and Avogadro’s number, we can calculate the amount of energy needed to break the bond in a single 𝑂2 molecule. Second, use the Planck relationship, has this amount of energy: 𝐸 = ℎ𝑣 to calculate the frequency 𝑣 of a photon that Third, Use the relationship between frequency and wavelength to calculate the wavelength of the light: λ = (c / v) Photoionization occurs when a molecule in the upper atmosphere absorbs solar radiation and the absorbed energy causes an electron to be ejected from the molecule. The molecule then becomes a positively charged ion. For photoionization to occur, therefore, a molecule must absorb a photon, and the photon must have enough energy to remove an electron. The result is formation of a cation. Photoionization Reactions for Four Components of the Atmosphere Photons of any wavelength shorter than the maximum lengths given in the table have enough energy to cause photoionization. All of these high-energy photons are filtered out of the radiation reaching Earth because they are absorbed by the upper atmosphere. Ozone in the Stratosphere Ozone, 𝑂3 forms a kind of layer in the stratosphere, where it is more concentrated than anywhere else. Ozone and oxygen molecules in the stratosphere absorb ultraviolet light from the Sun, providing a shield that prevents this radiation from passing to the Earth's surface. - It is the key absorber of photons having wavelengths ranging from 240 nm to 310 nm, in the ultraviolet region of the electromagnetic spectrum. - Ozone in the upper atmosphere protects us from these harmful high-energy photons which would otherwise penetrate to Earth’s surface. Ozone and its Depletion In 1995 the Nobel Prize in Chemistry was awarded to F. Sherwood Rowland, Mario Molina and Paul Crutzen for their studies of ozone depletion. In 1970, Crutzen showed that naturally occurring nitrogen oxides catalytically destroy ozone. Rowland and Molina recognized in 1974 that chlorine from chlorofluorocarbons (CFCs) may deplete the ozone layer. Chlorofluorocarbons These substances, principally 𝐶𝐹𝐶𝑙3 and 𝐶𝐹2𝐶𝑙2, do not occur in nature and have been widely used as propellants in spray cans, as refrigerant and air-conditioner gases, and as foaming agents for plastics. They are virtually unreactive in the lower atmosphere. Furthermore, they are relatively insoluble in water and are therefore not removed from the atmosphere by rainfall or by dissolution in the oceans. Unfortunately, the lack of reactivity that makes them commercially useful also allows them to survive in the atmosphere and to diffuse into the stratosphere. It is estimated that several million tons of chlorofluorocarbons are now present in the atmosphere. Sulfur Compounds and Acid Rain Sulfur compounds, chiefly sulfur dioxide, 𝑆𝑂2, are among the most unpleasant and harmful of the common pollutant gases. Combustion of coal accounts for majority of 𝑆𝑂2 released in the atmosphere. The coal-burning electrical power plants, which generates our electricity. The extent to which 𝑆𝑂2 emissions are a problem when coal is burned depends on the amount of sulfur in the coal. Low - sulfur coal is in greater demand and is thus more expensive, and has a lower heat content per unit mass. Sulfur dioxide is harmful to both human health and property Acid Rain Scientists have discovered that air pollution from burning of fossil fuels is the major cause of acid rain. The main chemicals in air pollution that create acid rain are sulfur dioxide ( 𝑆𝑂2) and nitrogen (N𝑂𝑥). Effects of Acid Rain 1. Acid rain weakens plants by washing away the protective film on leaves, and stunts its growth. 2. Acid rain can change the composition of soil and bodies of water, making them uninhabitable for local animals and plants. 3. Acid rain causes respiratory problems to humans and animals. 4. Acid rain can deteriorate limestone and marble buildings and monuments, like gravestones. Nitrogen Oxides and Photochemical Smog Nitrogen oxides are a group of seven gases and compounds composed of nitrogen and oxygen. The two most common and hazardous nitrogen oxides are nitric oxide and nitrogen dioxide. Nitrous oxide, commonly called laughing gas, is a greenhouse gas that contributes to climate change. Nitrogen oxides are emitted from vehicle exhaust, and the burning of coal, oil, diesel fuel, and natural gas, especially from electric power plants. They are also emitted by cigarettes, gas stoves, kerosene heaters, wood burning, and silos that contain silage. Photochemical Smog Photochemical smog is a type of smog produced when ultraviolet light from the sun reacts with nitrogen oxides in the atmosphere. It is visible as a brown haze, and is most prominent during the morning and afternoon, especially in densely populated, warm cities. Cities that experience this smog daily include Los Angeles, Sydney, Mexico City, Beijing, and many more. Automobile exhaust consists mainly of NO, CO and various unburned hydrocarbons. These gases are called primary pollutants because they set in motion a series of photochemical reactions that produce Secondary pollutants. It is the secondary pollutants – chiefly 𝑁𝑂2 and 𝑂3 – that are responsible for the buildup of smog.