Transcript Chemistry of the Environment
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.
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.
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.
Composition of the Atmosphere
• The atmosphere is comprised chiefly of N 2 ( 78.1%) and O 2 ( 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, CO 2 , Ne, He, CH 4 , Kr, H 2 , and others.
• The N 2 has an extremely strong triple bond (941 kJ/mol) that makes it very unreactive.
• The O 2 has a much weaker double bond (495 kJ/mol) that makes it much reactive.
Outer Regions
• Beyond the stratosphere, there are relatively few molecules of N 2 and O 2 .
• But, these outer regions form the defense shield from extreme radiation – radiation that if allowed to reach the surface would render our planet lifeless.
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. • O 2 + h n 2 O ; D H = +495 kJ/mol • PI is the loss of an electron from an atom or molecule.
• O 2 + h n O 2 + + 1e -
Outer Regions
• Recall from Ch. 7 that E = h n 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:
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 O 2 equal.
and O concentrations are about • Because of its higher bond energy, N 2 does not undergo PD readily – very little atomic N is found in the atmosphere.
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.
Outer Regions
• Some of the important PI’s are: • • • N 2 + h n O 2 + h n O + h n N O O + 2 2 + + + 1e + 1e + 1e , IE = 1495 kJ , IE = 1205 kJ , IE = 1313 kJ • These processes absorb radiation in the high energy region of the U.V. spectrum.
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 O 2 still occurs down to about 30km.
The atomic oxygen from 30 – 90km undergoes frequent collisions with O 2.
O + O 2 O 3 *
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 O 2 N 2 molecule.
or • The rate of O 3 formation depends on two opposing factors.
Ozone
• Factor number one is the concentration of O atoms. This increases with altitude.
• Factor number two is the concentrations of O, O 2 , and N 2 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.
Ozone
• • Once formed, ozone can absorb radiation and undergo PD like O 2 .
However, due to its weaker bond, it can absorb photons of 1140nm or shorter.
• The strongest and most important absorptions are from __________.
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.
Depletion of Ozone
• Rowland and Molina discovered in 1974 that Chlorine atoms from chlorofluorocarbons (CFC’s) can destroy ozone molecules.
• CFC’s (ex. CFCl 3 and CF 2 Cl 2 ) have longed been used as refrigerants and blowing agents are virtually unreactive and insoluble in water.
Depletion of Ozone
• • • In the stratosphere, PD of the CFC occurs.
• CFCl 3 + h n CFCl 2 + 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 + O 3 ClO + O 2 • Rate = k[Cl] 1 [O 3 ] 1 ; k = 7.2 x 10 9 /M s The ClO can regenerate more free Cl atoms.
• ClO + h n Cl + O
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.
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 CH 2 FCF 3 conditioning systems. in all new air • 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.
Chemistry of the Troposphere
• Man has also had a profound impact on the lowest layers of the atmosphere.
• The pollutants SO 2 , CO, NO, and O 3 related to man. are all directly • Rising CO 2 levels, produced by man, have been of concern by many as well.
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 SO 2 .
Sulfur and Acid Rain
• • • • The SO 2 When SO is further oxidized in the air to SO 3 mixes with water, H 2 SO 4 3 .
is produced.
Natural rain water has a pH of about 5.5 (due to dissolved CO 2 ).
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.
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.
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 CaCO 3 can be added to the furnace gases and when heated it decomposes to CaO. The CaO then reacts with the SO 2 to form CaSO 3 .
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 O 2 to attach in the lungs.
Carbon Monoxide
• The affinity for CO versus O 2 about ____ times greater.
in the heme unit is • 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.
Nitrogen Oxides
• Nitrogen oxides are the primary component of what is often referred to as “smog”. • Under normal conditions, N 2 (K = 10 -15 at 300K) and O 2 never react. • However, when an electric discharge (lightning) occurs or in an automobile engine, the two will react to form NO x .
• N 2 + O 2 2 NO ; D H = +180.8kJ
• K = 0.05 @ 2400K
Nitrogen Oxides
• Once released, the NO can further oxidize in air to produce NO 2 .
• 2 NO + O 2 2 NO 2 • K = 10 12 @ 300K ; D H = -113.1 kJ • NO • 2 NO 2 can then undergo PD to produce NO and O.
+ h n NO + O ; l = <393nm • The O atom combines with O 2 as described earlier.
to produce ozone
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?
“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.
“Greenhouse” gases
• Climate Quiz Question #1 • The primary greenhouse gas in the atmosphere is: • • • • • A) CO 2 B) H 2 O C) CH 4 D) N 2 O E) O 3
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 CO about 35% over the last 40 years or so to about 390 ppm.
2 levels have slowly risen by • There is much debate over the effect of increasing CO 2 in the atmosphere.
Greenhouse Gases
• Climate Quiz Question #2 • Expressed as a percentage, what is the amount of CO 2 in the atmosphere?
• A) 0.39% • B) 3.9% • C) 0.039% • D) 0.0039%
Greenhouse gases
• Rising carbon dioxide levels can not play as large a role in global warming due to its logarithmic relationship.
Greenhouse gases
• Computer models developed by some scientists show that global temperatures are predicted to increase by as much as 3 o C 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 CO 2 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!
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.
La Nina and El Nino
Solar Cycles
Solar Cycles
Loehle Temperature Reconstruction
Recent Trend in Global Temperatures
Benefits
• Proponents of the Kyoto Protocols fail to acknowledge potential benefits of increased CO 2 - like plants grow faster and longer growing seasons. • Some estimates put increased food production due to higher CO 2 levels by 15% since 1945.
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
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.
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, Br 2 , 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.
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.
Freshwater
• • • • Life in the water found in streams and lakes depends largely on the amount of dissolved O 2 . At 20 o C, water can hold up to 9ppm of O 2 . Fish require a minimum of 5ppm in order to live.
Other aquatic life – including bacteria and algae also compete for the available O 2 .
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.
Municipal Water
• Water in our homes originates from lakes, streams, or underground aquifers. • This water must be treated before consumption. This is generally a five step process .
• Coarse filtration • Sedimentation of smaller particles • Sand filtration • Aeration • Sterilization by chlorine