Chemistry in Context: Chapter 4: Energy, Chemistry, and Society Practice Problems: All Ch. 4 problems with the blue codes or answers on Page

Transcription

1 Chemistry in Context: Chapter 4: Energy, Chemistry, and Society Practice Problems: All Ch. 4 problems with the blue codes or answers on Page Figure 4.4 on Page 144 Annual U. S. Energy Consumption Energy and Society Energy drives industrial and economic progress; energy production and use correlates well with gross national product, life expectancy, infant mortality, and literacy. Energy use has increased in stages from the earliest hominids to 2000 years ago (discovery of fire and domestication of animals) to the Industrial Revolution (use of coal) and to the present day (petroleum refining). Fig. 4.4 on Page 155 (a) U.S.A. (b) World -More hydroelectric power used in the world than the U.S. -Geothermal, wind, and solar energy sources are grouped under the Other category

2 Conservation of Energy Energy is neither created nor destroyed (First Law of Thermodynamics); it is converted from one form to another. Energy of universe is constant. Sun is the ultimate MAJOR source. Fossil fuels store chemical energy that can be converted to heat or electrical energy. Fig. 4.2 on Page 153 Definition of Energy Energy is defined as the capacity to do work (W) W = applied force (f) x distance (d) Movement of an object over a specified distance constitutes work. Kinetic energy refers to the energy of a moving object. Heat Energy Heat Energy Thermal energy or heat can be determined indirectly by knowing the specific heat capacity (C H ) of the object heated and its temperature (T) that is characterized by the average kinetic energy of atoms or molecules. Heat (kj) = C H (kj/g- C) x mass (g) x T ( C) Temperature is commonly measured in K or C by scientists but the F is popular among meteorologist. K = C and C = ( F-32)/1.8 As heat is applied, atoms or molecules gain kinetic energy and move further apart and eventually overcome the intermolecular forces during melting and boiling.

3 Fig. 4.5 on Page 156 Schematic Drawing of a Bomb Calorimeter Units of Measurement for Energy Joule and calories are the more common units for energy joule (J) = 1 calorie One calorie is the amount of heat needed to raise the temperature of 1 gram of pure liquid water by 1 C Temperature is a property that determines the direction of heat flow; heat flows from a hotter to a colder body. Units of Measurement for Energy Measure of heat or thermal energy 1 kcal = 1000 calories 1 Calorie = 1 kcal (Cal cal) Calorie = food or nutritional calorie Other energy units are British Thermal Units, ergs, & foot-pounds. Energy of fuels are usually expressed as kj/mole or kcal/mole. Combustion Chemical reaction of fuel with oxygen to form products and energy. Potential energy of reactants > than that of products CH O 2 CO H 2 O + Energy Heat of combustion refers to the amount of heat given off when a substance is burned in oxygen.

4 Chemical Reaction Atoms are rearranged in reactants to form new products with different structures; this involves the breaking and making of chemical bonds Energy is needed to initiate a reaction usually (spark plug, match). Two types of chemical reactions are classified according to the energy levels of products relative to reactants. Figure 4.6 on Page 157 Heat of combustion is negative for exothermic reaction Exothermic Reaction Chemical or physical change that releases heat. Energy required for bond breaking is smaller than the energy released by bond formation. Energy (products)<energy (reactants) Energy change (E products -E reactants ) is negative. Endothermic Reaction Chemical or physical change that absorbs energy. Energy required for bond breaking is greater than the energy released by bond formation. Energy (products)>energy (reactants) Energy change (E products -E reactants ) is positive.

5 Bond Energy (BE) Amount of energy needed to break a specific chemical bond Single < double < triple (BE trend) BE increases as the atomic number of the bonded homonuclear atoms decreases for H, C, N, and O. BE increases as the polarity of a bond increases from iodine to fluorine in their bonds to H, C, N, and S. Table 4.1 on Page 159 -Net energy change is also called heat or enthalpy of reaction ( H)( -Bond breakage requires energy (+ve or up) -Bond formation releases energy (-ve or down) Fig. 4.8 on Page 161 Activation Energy & Fuels Activation energy (AE) is the energy needed to start a reaction. Slow reactions, higher AE, poor fuels. Fast reactions, low AE, explosions result, poor fuels. Useful fuels, moderate reactions and AE.

6 Figure 4.11 page 163 Rate of Reaction Increase surface area, smaller particles (e.g. fluidized-bed power plant less pollutants) Increase temperature to overcome activation energy barrier Use a catalyst to lower AE via alternate reaction pathways. Solar Radiation as an Energy Source Plants captured sunlight via chlorophyll for photosynthesis. Photosynthesis converts solar energy to chemical energy in the form of lipids, carbohydrates, proteins, and cellulose. Prehistoric photosynthesis also provided plant matter that were turned into fossil fuels under extreme heat and pressure below ocean floor or sedimentary formation. Photosynthesis Energy (hν) + 6 CO H 2 O C 6 H 12 O O 2 Energy = 2800 kj per mole of C 6 H 12 O 6 or 15.5 kj/g of C 6 H 12 O 6 Photosynthesis is endothermic since it requires solar energy. Provides food from crops as well as fuel in the form of bioethanol, biodiesel, and fossil fuels.

7 Respiration C 6 H 12 O O 2 6 CO H 2 O kj Respiration is the reverse of photosynthesis and is equivalent to combustion reaction. Energy released sustains various activities of our bodies. Properties of Coal Better fuel than wood with energy of 30 kj/g compared to kj/g). Mixture of compounds approximated by C 135 H 96 O 9 NS Si, Na, Ca, Al, Ni, Cu, Zn, As, Pb, Hg. Anthracite and bituminous coal have more energy than lignite due to the lower oxygen %. Coal reserves are times that of petroleum; coal gasification needed. Table 4.2 on Page 166 Ethanol 29.7 Petroleum 48 Disadvantages of Coal Disadvantages of Coal Underground mining is dangerous present day > 100,000 workers killed in coal mines Cave-ins, fires, explosions, poisonous gases, and respiratory diseases. Strip mining causes environmental damage in the form of erosion. Coal is a dirty fuel and emits soot, SO x, NO x, and CO 2 (greenhouse gases). Coal cannot be pumped readily like petroleum and gas.

8 Figure 4.12 on Page 168 Oil embargo Petroleum Major energy source in US Petroleum can be more easily collected and transported than coal. Energy content at 48 kj/g is higher than that of coal. Gasoline from petroleum is the fuel of choice for automobile or internal combustion engines. Low sulfur and other elements. Figure 4.14 page 169 Refining Process Refining Process Fractions = compounds with similar properties Distillation is used to fractionate or separate fractions of hydrocarbons according to their boiling points (bp). Distillation towers allow fractions to be collected at various heights. Hydrocarbons have higher bp as their size or molecular weight increases.

9 Figure 4.15 Page 170 Petroleum Uses Liquefied petroleum gas (LPG) is used as fuels and feedstocks for making plastics. Gasoline and diesel are used as motor fuels. Gas oil is used for home heating and as a feedstock for cracking into gasoline fraction. Lubricating oil stock (C16-C20). Asphalt and waxes (>C20 or bp>370 C) Solvents and starting materials for many products Figure 4.16 on Page 171 Natural Gas Natural Gas Major component is methane, CH 4. Heats 2/3 homes in US and widely used in gas stove or range for cooking. Compressed natural gas is used as a transportation fuel but lacks driving range and refueling stations. Clean burning fuel with little SO 2 and NO x emissions; produces 30% and 43% less CO 2 than oil and coal, respectively, on a per joule basis.

10 Petroleum Refining Cracking or conversion of large gas oil molecules (C15- C18) into smaller gasoline molecules (C5-C12) allows more gasoline to be produced from petroleum. C 16 H 34 C 8 H + 18 C 8 H 16 C 16 H 34 C 5 H + 12 C 11 H 22 &. Reforming improves the octane rating of gasoline by removing hydrogen atoms and converting hydrocarbon chains into ring structures. Cracking and reforming are carried out with catalyst to reduce energy requirements of thermal cracking. Isomers The formula C 8 H 18 is shared by 18 different structures or isomeric compounds. Isomers have similar chemical and physical properties. Octane is linear but isooctane is branched Octane boils at 125 C vs. isooctane at 99 C. Octane is susceptible to pre-ignition and engine knock but not isooctane. Fig on Page 173 Octane Rating Octane Rating Isooctane is used as a standard for measuring smoothness of engine combustion for gasoline fuels. Isooctane = 100; heptane = 0 87 octane = 87% isooctane and 13% heptane. Reform octane to isooctane using catalysts (Pt, Pd, Rh, Ir) to avoid the need for tetraethyl lead, Pb(C 2 H 5 ) 4 ; 1976 phaseout. Lead oxide emitted into environment is a neurotoxin and a catalyst poison.

11 Isomers of Octane Page a) Fill in hydrogen atoms to show C8H18 b) Any duplications of structures? c) C-C-C bond angle? d) C-C-C bond angle constant in all structures? e) Write the structural isomer of one more isomer. Oxygenated Gasolines Oxygenated additives improve octane rating and reduce CO formation. Clean Air Act Amendments require MTBE, methanol, and ethanol to be blended at 2.7% O for winter gasoline. Reformulated gasoline (RFG) with a lower % of volatile and reactive hydrocarbons has been used to curb ozone and CO pollution; RFG has benzene < 1% & oxygenate > 2%. Risk: MTBE leaks into groundwater Synthetic Gasoline Synthetic Gasoline C + H 2 O CO + H 2 CO & H 2 are referred to as water gas and used for producing synthetic gasoline (C5- C8) or CH 4. Fischer-Tropsch synthesis uses iron or cobalt catalyst. Feasible in countries (South Africa) where oil and natural gas are scarce but coal is plentiful.

12 Biomass Biomass materials are renewable. Burning wood; but trees use CO 2 and combustion generates CO 2. Wood-burning stoves are still common in developing countries. Soot particles affecting human health is generated. Ethanol Fermentation of starch or sugar by yeast C 6 H 12 O 6 2 C 2 H 5 OH + 2 CO 2 C 2 H 5 OH +3 O 2 2 CO H 2 O kj C 2 H 4 + H 2 O C 2 H 5 OH (non-renewable) Gasohol 10% ethanol and 90% gasoline Ethanol (29.7 kj/g) has a lower energy density than gasoline (octane value of 47.8 kj/g) Agricultural interests; food vs. fuel. Acetaldehyde in smog is a respiratory irritant. Ethanol producers will gain from MTBE ban. Resource Recovery from Garbage 140 power plants in US burn garbage to generates electricity; one truckload (27,000 lb) 21 barrels of oil. Targets energy recovery and waste disposal. Japan & Germany rely more on energy-to-waste technology; 10 % residue to landfill. Rural China & India ferment animal and vegetable wastes into biogas ( 60% CH 4 ) 2 cows produce enough CH 4 for 1 family s energy needs for light and cooking fuel. Operations of a Power Plant Figure 4.23 on Page 181 Chemical energy in fossil fuels is transformed into heat energy in the boiler that vaporizes liquid water to steam for turning the turbine. The shaft of turbine is connected to a large coil of wire that rotates in the magnetic field of the generator, thereby producing electricity.

13 Table 4.4 on Page 182 Overall efficiency = 0.60 x 0.90 x 0.75 x 0.95 x 0.98 (electrical heat) = 0.34 Example: Heat needed = heat used x overall efficiency Heat used = (3.4 x10 7 kj) / 0.34 = 1.0 x 10 8 kj Mass of CH4 needed = 1.0 x 10 8 kj x 1g/50.1 kj = 2.0 x10 6 g CH4 Transforming Energy The overall efficiency of energy conversion is the product of individual steps of transformation including the efficiencies of theoretical maximum, boiler, turbine, electrical generator, and power transmission. The theoretical maximum efficiency of power plant is determined by the conversion of heat to work as given by Theo. Eff. = (Highest T - Lowest T)/Highest T = (823K - 303K)/ 823K = 63% Fuel combustion at elevated pressure helps raise the boiling point and compresses steam. Figure 4.22 on Page 181 Energy Conservation Energy Conservation Fossil fuels are limited; conventional oil reserves will last 43 years at current rate of consumption, heavy crude, bitumen, and oil shale will last another 170 years. World oil demand increasing at > 2% per year; faster increase in developing nations. Oil is a valuable feedstock for chemicals. Increase power plant efficiency to %. Increase the fuel efficiency (mpg) of cars. Require SUVs and pickup trucks to meet fuel economy standards of passenger cars.

14 Figure 4.13 on Page 168 Fig on Page 186 Regional Sources of Oil Imported by the U.S. in 2000; recent shift to Western hemisphere from the volatile Middle East Figure 4.25 on Page 187 Fig on Page 190

15 Energy Conservation Energy Conservation New designs for appliances and cars Green chemistry concepts in production. Recycling of metals such as aluminum. Energy efficient light bulbs Mass transportation Smart buildings Automation using computers