EM 105 Dr. Lammi Name: EXAM III Nov. 1, 2018 You may work until 10:50 to complete this exam. Please show all work in the space provided or on the attached scratch page. Remember to report your final answers with the correct number of significant figures, as appropriate. A Periodic Table, a table of thermodynamic data, and a sheet of helpful constants, conversion factors and equations are provided for your use. GD LUK!! 1. (4 pts) Which of the following is/are true at equilibrium? (ircle all that apply.) a. The rates of the forward and reverse reactions are equal. b. G < 0 c. The amounts of reactants and products are equal d. None of the above 2. (10 pts) Dr. anna and I are working with our research students to design and evaluate compounds to prevent a protein-aggregation process involved in Alzheimer s disease. Two compounds made by a Winthrop chemistry student are shown below. Note (1) that they differ only in the presence of -3 versus - groups and (2) that lone pairs are not shown. 3 3 3 3 I II a. hoose either of the structures above and show how this compound engages in hydrogen bonding with water molecules. Be sure to clearly show the atoms involved in these interactions. b. ne of the two compounds shown dissolves in water much more readily than the other. Which one do you think is more soluble in water and how can this be explained on the basis of intermolecular forces? 1
3. (18 pts) Ethylene (24) reacts with fluorine gas to form tetrafluoroethylene (F4), a buildingblock for polymers such as Teflon TM, according to the following reaction: 24 (g) + 6 F2 (g) 2 F4 (g) + 4 F (g) rxn =??? a. Please calculate rxn for this process from the thermochemical data below: Thermochemical data: 2 (g) + F2 (g) 2 F (g) (s) + 2 F2 (g) F4 (g) = -537 kj = -680 kj 2 (s) + 2 2 (g) 24 (g) = +52.3 kj b. Please calculate the amount of heat in kj that is absorbed or released in the complete reaction of 10.00 g of F2. 2
4. (33 pts) The U.S. produces approximately 2.6 billion gallons of methanol (3) each year. It is used in fuels, as a solvent for perfumes and dyes, and in the preparation of a wide range of other chemicals formaldehyde, plastics, paints, explosives, etc. A common preparation method involves reacting carbon monoxide and hydrogen gas as shown below: (g) + 2 2 (g) 3 (g) The following questions are related to this reaction (or these substances). Note that your answer to each part is independent of the others. a. (8 pts) Which of the reactants do you expect to have the higher molar entropy at 25? Briefly explain your choice, being sure to specifically discuss intermolecular forces and at least one other factor influencing molecular entropy. b. (5 pts) If all three gases are present in a mixture at 25, which molecules are moving at the fastest average speed? ow do you know? c. (10 pts) Using the thermodynamic data provided (p. 5), please calculate for this reaction in kj per mole of 3 formed. Is the reaction endothermic or exothermic? d. (10 pts) Suppose that an engineer in a chemical plant performs this reaction in a 1500.0- liter stainless steel vat at 25.0 and determines the pressure of methanol to be 25.4 atm. ow many moles of 3 were formed? 3
5. (20 pts) The over-the-counter remedy called milk of magnesia contains magnesium hydroxide, which neutralizes hydrochloric acid in the stomach. Suppose that you carry out the following reaction in a coffee-cup calorimeter to determine the heat flow involved: 2 l (aq) + Mg()2 (aq) Mgl2 (aq) + 2 2 (l) You add 250.0 ml of 4.00 M l to enough Mg()2 to make 500.0 total grams of solution. Initially, you measure a temperature of 23.6 ; after reaction is complete, the temperature is 50.3. alculate rxn in kj per mole of Mgl2 formed. The specific heat of solution 4.18 J/g. [int: Start by calculating the heat of reaction.] 6. (10 pts) Another important industrial process is the production of lime (or calcium oxide, a) from limestone (calcium carbonate, a3); the US produces approximately 20 million metric tons per year. Under typical industrial conditions, rxn = +178 kj for this process. a3 (s) a (s) + 2 (g) = +178 kj a. Please predict the sign of S for this reaction. Briefly explain your reasoning. [Note: No calculations are needed here.] b. Under what temperature conditions do you expect this process to be spontaneous? (hoose from: Low T, igh T, or All T). Explain your reasoning. You should refer to a mathematical equation in your answer, but you need not calculate anything. 4
7. (10 pts) Phosgene, l2, is used in the production of bulletproof glass. (A poisonous gas, it also was used as a chemical warfare agent during World War I.) Phosgene may be formed from carbon monoxide and chlorine gas according to the following equilibrium: (g) + l2 (g) l2 (g) KP = 0.10 a. Please write an expression for KP. b. Do reactants or products dominate at equilibrium? Explain your answer in a few words. Thermodynamic Data: Substance f (kj/mol) 3 (g) -201.0 (g) -110.5 2 (g) 0 5
BLANK SRAT PAGE If there is material to be graded here, make sure that it is clearly labeled, and that your name is written on top of this page. 6
onstants and onversion Factors: onstants, onversion Factors and Equations h = 6.626 10-34 J. s c = 2.9979 10 8 m s 1 J = 1 kg m2 s 2 NA = 6.022 10 23 R = 0.08206 1 atm = 760 Torr = 760 mm g = 1.013 bar L atm mol K 1 cal = 4.184 J = 1 10-3 al Equations: dd = mm V ν = cc λ Ephoton = hν EK (ejected electron) = Ephoton - φ EE = 2.18 10 18 J 1 n2 1 f n2 Ephoton = E i λ matter = h mmv MM i VV i = MM f VV f PV = nrt PM = drt PA = χaptotal χa = n n A total 1 2 3 E K = mv E 3 RT = K RT v 2 rms = 2 M G = - T S S rxn = Σ[nS m (products)] - Σ[nS m (reactants)] (similar for G f, f ) q = ms T qrxn = -qsoln = qp 7
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