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Class13 - Chemistry 83 Population Growth and the Need for...

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Unformatted text preview: Chemistry 83 Population Growth and the Need for Technologies ! “Go forth and multiply” Genesis 1:28 ! Population Curve up to year 2000 Joesten,”Essentials”,Saunders, 1993,324 Class13 1 Chemistry 83 Two common math models used to describe curves (Exponential - Jshaped) Increases by constant amount: 1, 2, 3…. (Linear straight line) Increases by constant multiple: 1, 2,4,8…. Two most common math models are linear and exponential (J-shaped). The preceding population curve appears to be Jshaped or exponential up to 2000. Cunningham/Saigo, Environmental Science, 6th, McGraw Hill, MA, 2001, 130 Population curve >2000 is expected to be s-shaped (sigmoidal) *Carrying capacity - the largest number of any given species that an ecosystem can support indefinitely. http://www.emc.maricopa.edu/faculty/farabee/biobk/expgrowth.gif Class13 2 Chemistry 83 Population projections are based on sigmoidal curve Projection raises two questions: (1) are we likely to exceed the carrying capacity by 2100? and 2) if not, can output of current technologies be increased or are new technologies needed? Turk, Intro to Envir. Studies, 3rd, Saunders, NY, 1989, 85 Exceeding carrying capacity has consequences Critical number Recovery Critical number *Critical numberminimum population required to sustain a species (also not known for humans). Unfortunately earth’s carrying capacity for humans is not known. Extinction Cunningham/Saigo, Environmental Science, 6th, McGraw Hill, MA, 2001, 131 Class13 3 Chemistry 83 Campbell made “estimates” of earth’s carrying capacity for humans ! •  Example problem: Annual rainfall over land portions of earth is 3 x 1017 L/yr. The average human in a developed country consumes 107 L of water/yr. Estimate the maximum carrying capacity for humans at the level of a developed country with respect to water (make same assumptions as Campell) •  3 x 1017 L/yr/107 L/yr/person 
 = 3 x 1010 persons = estimate of carrying capacity! Campbell, J.A., Chemical Systems, Freeman, CA, 1970, 16 Some estimates require chemical calculations Example problem: Photosynthesis annually produces 2.0 x 1015 lbs of sugar by the reaction, 6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g). The average human in a developed country consumes 2.1 x 105 lbs of O2 per year. Estimate the maximum carrying capacity of earth for humans at the level of a developed country with respect to oxygen. 2.0 x 1015 lbs x 6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g) 6 MW 6(44) 264 lbs 6 MW 6(18) 108 lbs 2.0 x 1015 lbs sugar = 180 lbs sugar 1 MW 1(180) 180 lbs 6 MW 6(32) 192 lbs x lbs oxygen 192 lbs oxygen x = 2.1 x 1015 lbs of oxygen 2.1 x 1015 lbs oxygen = 1.0 x 1010 persons = carrying capacity 5 lbs oxygen/person 2.1 x 10 Campbell, J.A., Chemical Systems, Freeman, CA, 1970, 16 Class13 4 Chemistry 83 Based on his estimates, Campbell identified possible “limiting” factors 6.9 B 2075. Campbell suggested in 1970 that 1) heat will limit the carrying capacity of earth to <10 billion humans, and 2) we should develop solar technologies to replace fossil fuels to minimize heat stress as population increases. Campbell, J.A., Chemical Systems, Freeman, CA, 1970, 16 Assuming the carrying capacity is not exceeded, can output of current technologies be increased or are new technologies needed? Miller, Living in Environment, 12th, Brooks/Cole,CA, 2002, 90 The uneven supply of water is already problematic in some countries. New technology needed? Class13 5 Chemistry 83 Fossil fuels are produced in a cycle that is so slow that it is already a non-renewable energy source 1 M years 1 M years Cunningham/Saigo, Environmental Science, 6th,McGraw-Hilll, NY, 2001, 68 New technologies must be developed for energy even if there is no increase in population. The stability of N2(g) makes upgrading the current technology problematic. New technology needed? ! www.h2ou.com/h2images/ NitrogenCycle-lgr-F.jpg Class13 6 Chemistry 83 The low solubility of solid calcium phosphate makes upgrading the current technology problematic. New technology needed? ! Turk/Turk, Environmental Science, 4th,Saunders, PA, 198878 Chemical Technologies and their Natural Limitations! Class13 7 Chemistry 83 What is a chemical technology (in Chem 83)? Any technology that uses a chemical reaction to convert resource (s) into products that are used (with or without further modification) by the public is a chemical technology (≈ 20% GDP) Outer Space Heat Components of Typical Chemical Technology Solar Energy Earth’s Environment Extra ction Reactants Waste Recycled Materials Chemical reaction Heat Materials to Waste provide man's + Materials + (Degradedneeds and wants (By-products) energy) Used Materials What are the natural limitations on energy transformations in chemical technologies? ! •  The maximum work that a given amount of energy can do is given by the first law of thermodynamics (Conservation of Energy)! •  The actual work that a given amount of energy can do is limited by the second law of energy (natureʼs heat tax)! •  Quantified by %-efficiency, which = 100(Actual work output)/(Energy Input) Class13 8 Chemistry 83 Hinrichs, Energy, 2nd, Saunders, NY,1996, 78-79 How does minimizing the number of energy conversions benefit overall efficiency? (98.4% of original chemical energy of coal is degraded to heat and dispersed!) Minimize number of conversions to maximize overall energy efficiency Hinrichs, Energy, 2nd, Saunders, NY,1996, 85 Class13 9 Chemistry 83 What are the natural limitations on matter transformations in chemical technologies? ! •  The maximum mass of product that can be obtained from a given reaction is limited by the Law of Conservation of Mass (calculated from coefficients of balanced chemical equation) ! •  The actual mass of product that can be obtained from a given reaction is limited by competing reactions such as: 2C + O2 → 2CO and C + O2 → CO2! •  Quantified by %-yield, which = 100(Actual yield)/(calculated yield) One common type of competing chemical reaction is called a reversible reaction •  Reversible reaction involves two reactions with one being the reverse of the other •  Reversible reactions are represented by one equation using a double arrow: N2O4 Forward 2 NO2 Reverse •  Rates of forward and reverse reactions are proportional to the concentrations of their respective reactants –  Rate (forward) = Ratef = kf[N2O4] –  Rate (reverse) = Rater = kr[NO2] Class13 10 Chemistry 83 When the rate of forward reac9on = rate of reverse reac9on, a reversible reac9on is said to be at equilibrium N2O4 2 NO2 Ratef = kf[N2O4]! (Concentra9ons decreasing) At equilibrium, the two react ­ ions nullify each other and there are no further net changes in concentra9ons Rater = kr[NO2]! (Concentra9ons increasing) McMurray/Fay, Chemistry,Pren9ce Hall, NJ, 1995, 498 At equilibrium, the concentrations of reactants and products no longer change with time - the overall reaction appears to stop N2O4 2 NO2 The concen ­ tra9on of the product at equilibrium = actual yield 0.034 M 0.013 M Actual yield McMurray/Fay, Chemistry,Pren9ce Hall, NJ, 1995, 497 Class13 11 Chemistry 83 •  Example problem: at 25 ºC and 1 atm pressure, the reaction, N2O4 2 NO2, produces 0.013 moles of NO2 from 0.040 moles of N2O4. Find the % yield. • % yield = (100)(actual yield/calculated yield) 0.040 moles N2O4 1 mole x 2 NO2 ; 0.040 moles/1 mole = x/2 moles 2 moles X = 0.080 moles NO2 = calculated yield % yield = (100)(0.013 moles/0.080 moles) = 16% •  Minimizing the number of steps in a multi-step chemical process maximizes the yield of the process since the total yield of a multi-step process is the product of the fractional yields of each step, e.g., a 2step reaction sequence having 90% yield at each step would have an overall yield of 81%! Class13 12 ...
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This note was uploaded on 10/19/2011 for the course CHEM 83 taught by Professor Bonk,j during the Fall '08 term at Duke.

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