Sample Question Solutions for the Chemistry of Environment Topic Exam

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Deirdre Blair
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1 Name (Lat, Firt): ID Number: Sample Quetion Solution for the Chemitry of Environment Topic Exam 1. The earth tratophere contain a region of high ozone (O3) concentration called the ozone layer. The ozone layer i very important in protecting the earth and it organim from the harmful UV radiation of the un. It doe o by aborbing ultraviolet light and undergoing variou chemical reaction, thu preventing the ray from paing through the Earth atmophere. However, due to certain man-made pollutant, it wa found that the amount of ozone wa depleting at an alarming rate. a. Ozone i produced by the combination of an oxygen atom (O) with oxygen ga (O2) through the following reaction: O + O 2 O 3 (Equation 1) Uing thi information and the knowledge of the charge of ozone, draw two reonance tructure for the molecule ozone (O3). b. Oxygen radical, aka oxygen atom, are formed by the following reaction: O 2 2 O (Equation 2) O 3 O + O 2 (Equation 3) Photolyi, or photodiociation, i the chemical reaction in which the bond in a chemical compound are broken down by the aborption of high-energy photon. The equation 2 and 3 can thu be modified: O 2 + hν 2 O (Equation 2.5) O 3 + hν O + O 2 (Equation 3.5) Conidering thi information, along with the function of the ozone layer and the tructure of oxygen radical (O), explain why there i a high concentration of 1

2 Name (Lat, Firt): ID Number: ozone in the ozone layer a compared to on the earth urface. Ue chemical reaction in your explanation if appropriate. Oxygen radical are very untable (incomplete octet) thu have low concentration normally. However, in the high atmophere region aka the ozone layer, there i more UV radiation and the following reaction can occur, generating more oxygen radical: O 2 + hν 2 O (Equation 2.5) O 3 + hν O + O 2 (Equation 3.5) Thee oxygen atom can then react and form ozone: O + O 2 O 3 (Equation 1) c. Before the oxygen atom can react with an O2 molecule and make ozone, it mut loe ome energy and go to a more table electronic configuration. It doe thi by colliding with another molecule, which will leave with exce kinetic energy (E K ). i. (1) Write the initial electronic configuration of unimolecular oxygen and (2) the more table electronic configuration of oxygen following it colliion with another molecule and ubequent energy lo. Oxygen goe from: [He]2 2 2p x 2 2p y 2 2p z 0 to [He]2 2 2p x 2 2p y 1 2p z 1 Minimize electron-repulion ii. Equation 1 from above of ozone production can then be modified by the following addition O + O 2 O 3 + E K where E K tand for the exce energy of the reaction which i converted to kinetic energy of molecule, or heat. (1) I thi reaction endothermic or exothermic? (2) Draw reaction coordinate diagram of thi reaction. Label the energie of the reactant, the energie of the product, and ΔG clearly on the diagram. 2

3 Name (Lat, Firt): ID Number: Exothermic d. The concentration of ozone in the tratophere i contant. Thi mean that there i a dynamic equilibrium between the rate of ozone formation and the rate of ozone detruction. The reaction mechanim, named the Chapman Cycle, i categorized by the following four tep: 1. O 2 2 O 2. O 3 O + O 2 (fat) 3. O + O 2 O 3 (fat) 4. O + O 3 2 O 2 (low) i. Manmade pollution ha increaed the rate of ozone detruction relative to formation, cauing a decreae in overall ozone level. The propoed reaction mechanim for ozone detruction i defined by tep 2-4 above. Uing thoe equation, write the overall reaction for the detruction of ozone. Step 2 and 3: O 3 O 2 + O Step 4: O 3 + O 2 O 2 Overall reaction i 2 O 3 (g) 3 O 2 (g) ii. Determine the rate law for ozone detruction. What order of reaction i thi? Step 2 and 3: O 3 O 2 + O (equilibrium) r₁ = k₁+[o₃] - k₁ [O₂][O ] = 0 3

4 Name (Lat, Firt): ID Number: Solve for [O ] = (k₁+/k₁ )[O₃]/[O₂] and plug into Step 4: O 3 + O 2 O 2 r = r₂ = k₂[o₃][o ] r = k[o₃]²/[o₂] (rate-determining tep) with k = k₂(k₁+/k₁ ) overall firt order reaction e. Ozone depletion i caued by chemical found in refrigerator and air conditioner which are called chlorofluorocarbon (CFC). They contain chlorine atom which are releaed from the compound after encountering UV radiation. Thee Cl atom (aka Cl radical) can then detroy ozone molecule via the following reaction: Cl + O 3 ClO + O 2 ClO + O. Cl + O 2 i. Write the overall reaction of thi reaction mechanim. Which pecie act like a catalyt? O 3 + O. 2 O 2 Cl radical i a catalyt. ii. Thi reaction can alo occur with other halogen radical, uch a bromine which i actually more efficient at detroying ozone than Cl. However, on the other hand, fluorine atom are not very efficient at detroying ozone. Uing your prior knowledge and the information provided in thi quetion, explain why thi may be. It i important to note that Cl atom can be removed from the atmophere through reaction that form reervoir pecie uch a hydrogen chloride (HCl). F atom are removed more efficiently from the atmophere ince they react to form trongly bound HF in the Earth' tratophere. The other reervoir pecie, uch a HCl and HBr, have weaker bond. Thu, Cl and Br radical are more likely to exit in the atmophere than F radical. 4

5 Name (Lat, Firt): ID Number: 2. In 2015 about 29% of the United State energy came from the combution of natural ga. Overall natural ga i compoed of a range of hydrocarbon, while containing trace amount of nitrogen, carbon dioxide, and many other compound. With a much higher energy value than other ubtitute including coal many ee the ue of natural ga a a good tranition toward a more renewable energy ource. However, natural ga i till known to have negative impact on the environment releaing large quantitie of CO 2 and carbon monoxide (CO) into the atmophere. a. Carbon monoxide i a colorle, odorle ga that i toxic to human in concentration above 35ppm and a contant product of incomplete combution. i. Draw the mot table Lewi dot tructure of thi molecule. Reonance and non-zero formal charge mut be included for full credit. ii. On the Lewi tructure above ue an arrow to draw the general direction of the net dipole, indicating which end repreent the negative end of the dipole. iii. Explain if the Lewi tructure you drew agree with an electronegativity argument? No thi doe not agree with electronegativity ince the oxygen i more electronegative than carbon and therefore hould have a partial negative charge if relying only on electronegativity value. b. For part b, you will be operating a teady tate oxy-combution proce. In thi proce natural ga which we are auming for implicity to be only methane and pure oxygen are combined and combuted reulting in CO 2 and water vapor. While oxy-combution ha many benefit it unfortunately burn at a much higher temperature than regular combution with air. A a reult, there i a recycled tream of CO 2 preent to prevent the exceively high temperature which could 5

Name (Lat, Firt): ID Number: potentially melt the proce equipment. Remember the ytem below i at teady tate meaning that all the ma entering the ytem i equal to all the ma exiting the ytem i.e. no accumulation of ma within the ytem.

6 Name (Lat, Firt): ID Number: potentially melt the proce equipment. Remember the ytem below i at teady tate meaning that all the ma entering the ytem i equal to all the ma exiting the ytem i.e. no accumulation of ma within the ytem. For thi problem aume that the ma flow rate of CH 4 i entering the combutor i 1.73 kg/. Aume that the combution of the reaction i perfectly toichiometric and that the only product are CO 2 and water vapor. All anwer involving calculation hould be given with unit (kg/). i. Determine (ma flow rate of O 2 ) into the toichiometric combutor. RXN: CH 4 + 2O 2 CO 2 + 2H 2 O 1.73 kg CH4 2 mol O2 1 mol CH4 32g/(gmol) 6.92kg O2 = 16g/(gmol) ii. Determine (ma flow rate of CO 2 ) into the combutor o that the inlet ga (carbon dioxide and oxygen mixture) ha an oxygen mole fraction of kg O2 kgmol 32kg = CO2, IN = 0.2 CO2, IN = 0.865kgmol 44kg 1kgmol = kg CO2 iii. Determine the fraction of CO 2 in the product tream mut be recycled in order to achieve thi 0.2 mole fraction of oxygen in the inlet tream. Ue SS aumption: 6

7 Name (Lat, Firt): ID Number: kg CO2 CO2, IN = CO2, Out = CO2, IN + CO2, RXN kg CO2 CO2, out = CO2, RXN 1.73 kg CH4 1 mol CO2 44g/(gmol) kg CO2 = 1 mol CH4 16g/(gmol) kg CO kg CO kg CO2 CO2, out = = kg CO kg CO2 % recycled = = 88.89% iv. An alternative combutor ued for the combution of natural ga i an air blown combutor which ue air intead of pure oxygen a it i much cheaper ince air i free. While the oxy-combutor above needed a recycle tream of CO 2 to prevent exceively high temperature, why would an air blown combutor not need one? There are molecule other than O 2 in air uch a N 2 which aborb a lot of the heat releaed. c. A eriou problem with natural ga i fugitive methane emiion. Once in the atmophere methane i aid to be 30 time better at trapping heat than CO 2 but luckily ha a much horter life pan. The bigget ink for thee methane molecule are in atmophere where they can react with variou radical uch a OH or Cl which can abtract hydrogen. i. Write two eparate reaction of methane with a OH and a Cl radical. OH Rxn: CH 4 + OH CH 3 + H 2 O Cl Rxn: CH 4 + Cl CH 3 + HCl 7

8 Name (Lat, Firt): ID Number: ` Photo credit: Univerity of Texa Chemitry Department ii. Calculate the change in enthalpy for both reaction above H = Bond Broken Bond Formed OH RXN: H = -50kJ/mol Cl RXN: H = -19kJ/mol iii. Which reaction i more exothermic? Explain? The hydroxyl radical rxn i more exothermic due to it releaing more heat than the chlorine radical reaction. ( H OH < H Cl ) iv. For thee reaction above doe the entropy of the univere increae or decreae? The entropy of the univere i alway increaing. 8

9 Name (Lat, Firt): ID Number: 3. A radiation worker walk down a half way to find that drop from a uranium fuel rod have melted through the ceiling. He i 50 meter away from the naty pool of radioactive death. The uranium pool ha a decay rate of 10 curie (aume 1 curie=3.7*10 18 decay per econd). For implicity ake aume that only gamma ray get to the worker, and that the gamma decay rate i the ame a the uranium decay rate. a. Calculate how many gamma ray hit the radiation worker per econd. You can model the radiation worker a a 1 by 1 by 1-meter cube. You may alo aume that the uranium pool act like a point ource of radiation. To calculate the angler correction all you need to do i divide the amount of gamma ray emitted by the ource that reache the ame ditance a the radiation worker by 4pi*ditance quared, and then multiple by the area of interet. 1 4πd Q ST d 1 4π 50 Q m Q 1m 1m = SU gamma ray per econd b. Calculate how much energy i depoited into the radiation worker per econd. Aume for now that all gamma ray have energie of 1.6*10-13 J. Alo aume that if they hit the radiation worker all of their energy will be depoited in the radiation worker. gamma ray SU XSY J 1 gamma ray = Q J c. Calculate the Rad per econd that the radiation worker i expoed to (1 Rad=90erg/g of material, 1 erg=1*10-7 joule) (aume the denity of human are 1g/cm^3). 1 meter = 100cm Ma of human = Volume Denity 9

10 Name (Lat, Firt): ID Number: Ma of human = 100cm 100cm 100cm 1g 1cm Y Ma of human = 10 a g 1890J human cube 1 10c erg 1g 1 human cube 1 J 90erg 10 a = 210 Rad g d. Calculate the Rem (Rad equivalent man) per econd. Rem i calculated by multiplying the Rad by the quality factor, the purpoe of thi converion i to take into account the difference in tiue damage done by different form of radiation. A table of quality factor i placed below. 210Rad 1 = 210Rem e. Calculate how long a peron would have to be expoed to a 1 Curie neutron flux (aume all neutron have 1*10-13 J and 1 J=6.42*10 15 kev) in order to accumulate the ame number of Rem a the radiation worker received if he tayed in the hallway for 1 hour. 10Curie before 1 Curie now 210 Rem 60 1min 60min 1hr = 75600Rem 1 hr 1 Q factor before 10 Q factor now 1hr time it took before = 1hr 10

11 Name (Lat, Firt): ID Number: 11

4. The following graphs were prepared from experimental data for a reactant, A. What is the correct order of A?

4. The following graphs were prepared from experimental data for a reactant, A. What is the correct order of A? Diebolt Summer 010 CHM 15 HOUR EXAM I KEY (15 pt) Part One: Multiple Choice. 1. Conider the reaction IO 3 + 5I + 6H + 3I + 3H O. If the rate of diappearance of I i 9.010-3 M/, what i the rate of appearance