Ch7[1] - CHAPTER Fossil Fuel Energy, C02 Emissions, and...

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Unformatted text preview: CHAPTER Fossil Fuel Energy, C02 Emissions, and Global Warming Problem 7-1 A doubling ofV means that, at time t, V = 2V0. Substituting for V in the equations: _ k V — Voe‘ . _ kl .. 2V0 — Oe ore‘“ = 2 Taking the natural logarithm of both sides, then kt = ln(2) = 0.69 sot = 0.69/k Thus the doubling time is 0.69/k. By substitution for k of the values 0.04, 0.03, 0.015, and 0.01, we ob! tain doubling times of 17, 23, 46, and 69 years respectively. A ten—fold increase corresponds to V = lOVO, so by substitution into the original formula of this rela— tionship and k = 0.02, then we have: 10VO = Voeoloz‘ so 0.02t = ln(10) = 2.30 and t = 115 years Finally, ifV = 10V0 and t = 50, thus again by substitution we can find the annual fractional increase, k: 10V0 = V085“ 50 50k = in (10) = 2.30 and k = 0.046 Thus the compounded rate was 4.6% annually. 63 64 (hapler 7 Problem 7-2 The molar masses of carbon and of CO2 are respectively 12.01 and 44.01, so the carbon content of 730 grams of carbon dioxide is (12.01 /44.01) X 730 g = 199 g. Problem 7—3 The balanced equations for the three reactions are shown in the text. Since AH = Sum AHf. values for products — Sum AHf values for reactants then for methane AH = AH,(COZ) + 2AH, (H20) — AH,(CH4) — ZAHAOZ) By substitution of the values given, and recalling the AHf = O for 02, thus AH = 493.5 + Z X (—2858) — (—74.9) — Z x (O) = —890.21<J mol’l Thus because 1 mole of CO2 is produced in this process, the moles of COZ per kilojoule of heat is 0.001 12. Similarly, the AH and moles COZ/k] values for CH2 and carbon are —658.7 and 000152, and 493.5 and 0.00254 respectively. Problem 7 4 The oxidation numbers of the carbons in C02, C, CH2, and CH4 are +4, 0, —2, and —4 respectively, because those of oxygen and hydrogen respectively are —2 and +1, and so to make the total numbers sum to zero, the value in CO2 must counteract the —4 from the two oxygens, etc. Thus the change in oxidation state in going from C to COZ is 4, from CH, to COZ is 6, and from CH4 to CO2 is 8. The ratio of changes, and thus the ratio of oxygen required, is therefore 4:6:8, or by division of each by the common factor of two, the ratio is 223:4. Problem 7-5 The unique isomers can be generated by first considering that all three methyl groups are on adjacent carbons, thus yielding 1, 2, 3—trimethyl benzene. The next possibility is two adjacent methyl groups, which yields only one unique position for the third, giving the 1, 2, 4 isomer (see following). Finally there is only one isomer possible when no two methyl groups are adjacent. Fossil Fuel Energy, CO2 Emissions, and Global Warming 65 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 1,2, 3— 1,2,4— 1,3,5— trimethyl benzene trimethyl benzene trimethyl benzene Problem 7-6 First compute the mass in grams of the C02, and then using the density, convert it to volume in cm‘: 10'2 tonnes X 1000 kg/tonne X1000 g/ng 1 cm3 /g = 1X 1018 cm3 Now convert the volume in cm3 to km3: 1 X 1018 cm3 X (0.01 m / cm)3 X (0001 km / ml3 = 1000 km3 The dimension of the cube is the cube root of its volume, so the length of each side is (1000 km3)”3 = 10 km. Problem 7-7 In the reaction, one mole of CO2 requires one mole of CaCO3. Since the molar masses of CO2 and of CaCO3 are 44.01 and 100.09 grams respectively, then the ratio of the masses of CaCO3 to COZ that react is 100.09 / 44.01 = 2.274. Thus the mass of CaCO3 that will react with 1000 kg of CO2 is 2.274 X1000 kg = 2274 kg, i.e., 2274 tonnes. Green Chemistry Questions 1. 2. Alternative reaction conditions for green chemistry. (b) 1. Prevention of waste. 5. The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary whenever possible, and should be innocuous when they are used. 7. A raw material feedstock should be renewable rather than a depleting one whenever technically and economically practical. 9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. 10. Chemical products should be designed so that at the end of their function they do not persist in the environment but break down into innocuous degradation products. 66 Chapter 7 Made from annually renewable resources (corn, sugar beets, eventually waste biomass will be used). Production of PLA consumes 20—50% less fossil fuel resources than petroleum’based polymers. Uses natural fermentation to produce lactic acid, uses no organic solvents or other hazardous substances in the production of PLA. Uses catalysts, resulting in reduced energy consumption and resource consumption. High yields of >95% are obtained. Use of recycle streams help to reduce waste. PLA can be recycled (converted back to monomer via hydrolysis, then repolymerized to produce Virgin polymer), i.e., closed»loop recycling. FLA can be composted (biodegradable); complete degradation occurs in a few weeks under nor— mal composting conditions. Growing crops, whether they are used to produce food or chemicals, require fertilizers and pesti’ cides. Energy is needed to plant, cultivate and harvest; to produce, transport, and apply fertilizers and pesticides; to make and run tractors; to transport seeds, biomass, monomers, and polymers. Use of land to produce crops for chemicals also removes land that could be used to produce food and animal feed. Additional Problems The US. and EU. fractions decreased slightly over the period, whereas that from China/lndia/ Other Developing Countries increased significantly. Region 1990 2004 US. 24% 22% EU. 18% 1600 China/India/Other DC. 30% 41% Consider the release of an equal amount of heat by the burning of oil—i.e., polymeric CH,— and of natural gas; thus, the amounts of O2 consumed are equal: 4CH,+6oZ ——> 4co,+4n,o 3CH,+6o2 ——> 3co,+6H,o Thus, combustion of three moles of CH4 “saves” 1 mole of C0,, while still producing the same heat as burning oil. But the release of every one mole of CH4 is equivalent to releasing 23 moles of C03, so releasing 1/23 mole of CH4 would produce the equivalent greenhouse effect as one mole of C02. Fossil Fuel Energy, CO2 Emissions, and Global Warming 67 Therefore, you cannot release more than 1/23 mole CH4 when burning 3 moles of it, so the fracv tion (maximum) of release is: 1/23 3+ 1/23 = 0.014 i.e., 1.4% is the maximum release The unbalanced reaction for the production of hydrogen and carbon dioxide from methane gas using steam is presumably CH4 + H20 —> H, + co Balancing the oxygen atoms, then the hydrogen, gives the balanced equation CH4 + 2 H20 —> 4 H2 + COZ Hydrogenation of CH to give CH2 corresponds to CH + V; HZ —> CHZ If we multiply this reaction by 8 so as to use all the hydrogen produced in the previous reaction, and then add the reactions together, we obtain the overall reaction: 8CH + CH4 + ZHZO —> SCH, + co, Z. 5 metric tonnes = 5000 kg = 5.0 X 106 g If this was converted to dry ice, the volume would be 5.0 X 106 g /1.56 g cm'3 = 3.2 X 106 cm3 The equation relating volume to radius ofa sphere is: V = 4/3 Tltr3 .‘.r = (3V/41t)” = (3 x3.2 ><106 cm3/4 >< 3.142)l/3 = (7.65 x 105 anal/3 = 91.5 cm .'.d = 183 cm = 1.83 m The rationale behind this slogan is that local produce requires much less transportation to arrive at the market, as compared to produce from other areas. Much of the produce in a North American supermarket is shipped by truck from California, Florida, and even Central and South America. The amount of fossil fuels burned by the transport trucks (for example) used to deliver this produce is huge. It is clear that if people bought local produce only, then the amount of fossil fuel consumed would be significantly decreased, as would the C02 emission associated with this fossil fuel consumption. Thus, as long as we accept that increased CO2 emissions are contributing to climate change in the form of global warming, then taking the advice of this sign could indeed “help stop climate change.” The problem of course is that much of the produce available simply cannot be grown in all areas. (a) The fraction from heavy oil will be about 0.16, i.e., 16%. (b) The fraction from natural gas liquids will be about 0.22, i.e., 22%. The fraction of all kinds predicted for 2050 is about 0.40, i.e., 40%. ...
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Ch7[1] - CHAPTER Fossil Fuel Energy, C02 Emissions, and...

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