Ch. 18 Chemistry of the Environment

1. Ch. 18 Chemistry of the Environment • Environmental issues are constantly in the news. • Both the atmosphere and the hydrosphere make life possible. Thus, maintaining these areas is important. • As citizen’s of both our country and the planet, we must make informed decisions about these issues.

2. Earth’s Atmosphere • The temperature of the Earth’s atmosphere varies greatly with altitude. • The atmosphere is divided into four regions based on these temperature regions: Troposphere, Stratosphere, Mesosphere, and Thermosphere.

4. Earth’s Atmosphere • In the troposphere, all of the “weather” that we experience occurs here. • In the stratosphere, the temperature increases with altitude. This prevents large scale mixing of the molecules. • Particulate matter deposited in the stratosphere has a tendency to remain there for long periods.

5. Composition of the Atmosphere • The atmosphere is comprised chiefly of N2 (78.1%) and O2 (20.9%). Note that these values are for dry air. • All of the other components add up to less than 0.1% and include Ar, CO2, Ne, He, CH4, Kr, H2, and others. • The N2 has an extremely strong triple bond (941 kJ/mol) that makes it very unreactive. • The O2 has a much weaker double bond (495 kJ/mol) that makes it much reactive.

6. Outer Regions • Beyond the stratosphere, there are relatively few molecules of N2 and O2. • But, these outer regions form the defense shield from extreme radiation – radiation that if allowed to reach the surface would render our planet lifeless.

7. Outer Regions • In the upper atmosphere, two important process take place: photodissociation (PD) and photoionization (PI). • PD is the rupturing of a bond when energy is absorbed. • O2 + hn 2 O ; DH = +495 kJ/mol • PI is the loss of an electron from an atom or molecule. • O2 + hn O2+ + 1e-

8. Outer Regions • Recall from Ch. 7 that E = hn and that n = c / l. Thus, shorter wavelengths of light produce higher energies. • Above 120km, the PD of molecular oxygen is an extremely important process. The minimum wavelength of light required for this process can be calculated by:

9. Outer Regions • This process absorbs the majority of high energy radiation from our Sun. • At 400km, only 1% of Oxygen exists in molecular form. • At 130km, the O2 and O concentrations are about equal. • Because of its higher bond energy, N2 does not undergo PD readily – very little atomic N is found in the atmosphere.

10. Outer Regions • In 1901, Marconi received a radio signal from 2900km away. • This led to the discovery that electrons are abundant in the upper atmosphere. • These electrons are due to PI processes that occur in the atmosphere.

11. Outer Regions • Some of the important PI’s are: • N2 + hn N2+ + 1e- , IE = 1495 kJ • O2 + hn O2+ + 1e- , IE = 1205 kJ • O + hn O+ + 1e- , IE = 1313 kJ • These processes absorb radiation in the high energy region of the U.V. spectrum.

12. Ozone • Ozone in the upper atmosphere is Earth’s final defense against harmful radiation. • Below 90km, most of the short wavelength (high energy) radiation has been absorbed. • PD of O2 still occurs down to about 30km. • The atomic oxygen from 30 – 90km undergoes frequent collisions with O2. O + O2 O3*

13. Ozone • When ozone is initially formed, it contains an excess of 105 kJ/mol that must be quickly transferred away or the ozone molecule will decompose. • This transfer of energy is accomplished by a collision of the ozone molecule with another O2 or N2 molecule. • The rate of O3 formation depends on two opposing factors.

14. Ozone • Factor number one is the concentration of O atoms. This increases with altitude. • Factor number two is the concentrations of O, O2, and N2 for the various molecular collisions. These decrease with an increase in altitude. • The highest rate of ozone formation is found at about 50km. The highest concentration of ozone is found at about 25km.

15. Ozone • Once formed, ozone can absorb radiation and undergo PD like O2. • However, due to its weaker bond, it can absorb photons of 1140nm or shorter. • The strongest and most important absorptions are from __________.

16. Ozone • Plants and animals could not survive if this radiation would reach the Earth’s surface. • The overall process of the formation and decomposition of ozone is a rather complex one. • Finally, the ozone cycle is a net exothermic reaction – this is why the temperature _____ with altitude in the stratosphere.

17. Depletion of Ozone • Rowland and Molina discovered in 1974 that Chlorine atoms from chlorofluorocarbons (CFC’s) can destroy ozone molecules. • CFC’s (ex. CFCl3 and CF2Cl2) have longed been used as refrigerants and blowing agents are virtually unreactive and insoluble in water.

18. Depletion of Ozone • In the stratosphere, PD of the CFC occurs. • CFCl3 + hn CFCl2 + Cl; l = <225nm • The chlorine atom has an odd electron which makes it extremely reactive (The term used for this species is free radical). • Cl + O3 ClO + O2 • Rate = k[Cl]1[O3]1 ; k = 7.2 x 109 /M s • The ClO can regenerate more free Cl atoms. • ClO + hn Cl + O

19. Depletion of Ozone • Depletion of ozone from Cl atoms has produced the thinning of the ozone layer at both poles – most notably the southern one during late winter / early spring. • This is due to an unrelated phenomenon called polar stratospheric cloud (PSC) formation. • PSC’s form during this time of year and seem to concentrate large quantities of Cl atoms.

20. Solution to the Problem • Elimination of CFC’s was seen as a priority. • CFC’s are no longer used as the blowing agent for the production of styrofoam. • CFC’s are being replaced by CH2FCF3 in all new air conditioning systems. • However, CFC’s are still used in older systems. • CFC’s also take years to diffuse into the stratosphere. • Recent reports and data show that the ozone depletion may be lessening.

21. Chemistry of the Troposphere • Man has also had a profound impact on the lowest layers of the atmosphere. • The pollutants SO2, CO, NO, and O3 are all directly related to man. • Rising CO2 levels, produced by man, have been of concern by many as well.

22. Sulfur and Acid Rain • Some sulfur compounds are generated naturally by bacteria and volcanic activity. • The large majority of sulfur compounds in the atmosphere are due to the combustion of coal and oil. • The amount of sulfur in coal and oil depends on their source. Oil from the Middle East and coal from the western U.S. is low in sulfur. • When coal or oil is burned, the sulfur is converted to SO2.

23. Sulfur and Acid Rain • The SO2 is further oxidized in the air to SO3. • When SO3 mixes with water, H2SO4 is produced. • Natural rain water has a pH of about 5.5 (due to dissolved CO2). • Acid rain results when its pH is lowered to 4.0 or lower. • The results of acid rain on lakes with relatively low levels of carbonates and bicarbonates is tremendous.

24. Sulfur and Acid Rain • Over years, the acid rain lowers the pH of these lakes until it too is below 4.0. • At a pH of below 4.0, microorganisms can no longer survive. This in turn disrupts the entire food chain and makes the lake a lifeless one. • Currently, there are over 300 lakes in New York that contain no life.

25. Sulfur and Acid Rain • Acid rain can also be destructive to stone and metal objects – including many historical buildings. • What can be done? • Remove the sulfur before burning. • Powdered CaCO3 can be added to the furnace gases and when heated it decomposes to CaO. The CaO then reacts with the SO2 to form CaSO3.

26. Carbon Monoxide • CO is formed by the incomplete combustion of hydrocarbons. • CO is fairly unreactive with a strong triple bond between the two atoms. • It does, however, have a lone pair of electrons on the C atom. This lone pair is very good at binding itself to metal atoms or ions. • Hemoglobin, found in red blood cells, contains four protein molecules (heme units). • Each heme unit contains an Fe center, which serves as the site for the O2 to attach in the lungs.

27. Carbon Monoxide • The affinity for CO versus O2 in the heme unit is about ____ times greater. • When CO is bound to the heme unit it is called carboxyhemoglobin (COHb). • Under normal conditions, a non-smoker would have about 0.3 – 0.5% COHb present in the bloodstream. • Exposure to even small sustained levels of CO can cause death over time.

28. Nitrogen Oxides • Nitrogen oxides are the primary component of what is often referred to as “smog”. • Under normal conditions, N2 and O2 never react. (K = 10-15at 300K) • However, when an electric discharge (lightning) occurs or in an automobile engine, the two will react to form NOx. • N2 + O2 2 NO ; DH = +180.8kJ • K = 0.05 @ 2400K

29. Nitrogen Oxides • Once released, the NO can further oxidize in air to produce NO2. • 2 NO + O2 2 NO2 ; DH = -113.1 kJ • K = 1012 @ 300K • NO2 can then undergo PD to produce NO and O. • NO2 + hn NO + O ; l = <393nm • The O atom combines with O2 to produce ozone as described earlier.

30. Nitrogen Oxides • Although ozone is essential in the stratosphere, it is a pollutant near the ground. This is because it is an extremely reactive molecule and is most harmful to asthmatics and the elderly. • What can be done?

31. “Greenhouse” Gases • In addition to screening out harmful radiation, our atmosphere also absorbs escaping infrared radiation (also called longwave radiation) • At night on a planet like Mars, the temperature drops over 100K. • At night on our planet, escaping infrared radiation is absorbed by greenhouse gases and then re-emitted.

32. “Greenhouse” gases • Climate Quiz Question #1 • The primary greenhouse gas in the atmosphere is: • A) CO2 • B) H2O • C) CH4 • D) N2O • E) O3

33. Greenhouse Gases • The chief “greenhouse” gas is ___________ (>90%). __________ absorbs a wide range of light in the infrared region (10k – 30k nm). • _____________ plays a secondary role in this as well (4 – 8%). • The atmospheric CO2 levels have slowly risen by about 35% over the last 40 years or so to about 390 ppm. • There is much debate over the effect of increasing CO2 in the atmosphere.

36. Greenhouse Gases • Climate Quiz Question #2 • Expressed as a percentage, what is the amount of CO2 in the atmosphere? • A) 0.39% • B) 3.9% • C) 0.039% • D) 0.0039%

37. Greenhouse gases • Rising carbon dioxide levels can not play as large a role in global warming due to its logarithmic relationship.

38. Greenhouse gases • Computer models developed by some scientists show that global temperatures are predicted to increase by as much as 3oC by the end of this century. • However, these models do not include many other variables such as increased cloud cover and have been wrong in all of their predictions. • In addition, models tend to treat CO2 as an exponential factor – and we know it is logarithmic. • Climate on Earth is one of the most complex non-linear system that is impossible to replicate!

39. Climate Forces • Climate is more likely driven by the Sun and Ocean changes. • Sun energy output varies over an eleven year cycle (sunspot activity) • Ocean temperature changes – La Nina, El Nino, PDO, AMO, etc. • Produces nearly all of the observed climate changes.

40. La Nina and El Nino

41. Solar Cycles

42. Solar Cycles

43. Loehle Temperature Reconstruction

44. Recent Trend in Global Temperatures

45. Benefits • Proponents of the Kyoto Protocols fail to acknowledge potential benefits of increased CO2 - like plants grow faster and longer growing seasons. • Some estimates put increased food production due to higher CO2 levels by 15% since 1945.

46. The World Ocean • Water is truly an amazing substance. • Unusually high melting and boiling point • A liquid over a moderate temperature • High heat capacity • Solid is less dense than the liquid phase • Highly polar which allows it to dissolve a wide variety of substances • Serves as a medium for many important reactions

47. The World Ocean • Water covers 72% of the Earth’s surface. • 97.2% of all water is found in the oceans. • Freshwater, found in streams, lakes, and groundwater, amounts to only 0.6% of the total water. • In many areas like California and Arizona, water is a scarce resource whose rights are often fought over.

48. The World Ocean • Seawater contains about 35g of dry salts per kilogram. • The major components are Cl, Na, sulfate, Mg, Ca, and K. • Only three substances are obtained commercially from seawater – NaCl, Br2, and Mg. • To make seawater drinkable, all of the salts must be removed. This can be done by either heating the seawater to evaporate the water or through a process known as reverse osmosis.

49. Freshwater • The average person uses about _________ for consumption and hygiene. • Agriculture and industry use even far greater amounts of water. • Freshwater is produced via the Water Cycle. • As the population grows, more and more emphasis must be placed on finding abundant sources of water.

50. Freshwater • Life in the water found in streams and lakes depends largely on the amount of dissolved O2. • At 20oC, water can hold up to 9ppm of O2. Fish require a minimum of 5ppm in order to live. • Other aquatic life – including bacteria and algae also compete for the available O2. • Nutrients like nitrates and phosphates can spur the rapid growth of algae, which can result in the eutrophication of a lake. • This leads to the death of all of the fish and plants in the lake.