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Transcript
CHEM 122 General Chemistry Summer 2014 Name: Midterm Examination 1 Multiple Choice Questions 1. If a solution of Water (H2O) and Methyl Amine (CH3NH2) were to form, the dominant Intermolecular Force between molecules of Water and Methyl Amine would be: a) b) c) d) 2. A solution prepared by mixing liquid Pentane (C5H12) and liquid Octane (C8H18) is expected to have an Enthalpy of Solution that is: a) b) c) d) e) 3. large and exothermic (Hsoln << 0) small and exothermic (Hsoln < 0) about zero (Hsoln ~ 0) ***** small and endothermic (Hsoln > 0) large and endothermic (Hsoln >> 0) Hpent ~ small (LDF) Hoctan ~ small (LDF) Hmix ~ small (LDF) Hsoln ~ small + small - small ~ 0 The Henry's Law Constant for O2 in Water is kO2 = 0.00128 M/atm. How many grams of O2 can dissolve in 2.00L Water if the partial pressure of O2 over the Water is 0.20 atm? (Hint: First calculate the solubility of the O2 in the Water.) a) b) c) d) 4. LDF Dipole-Dipole H-Bonding ***** Ion-Dipole 0.123g 0.105g 0.063g 0.016g ***** SO2 = kO2 PO2 = (0.00128 M/atm) (0.20 atm) = 2.56 x 10-4 M #g = (2.56 x 10-4 M) (2.00 L) (32 g/mol) = 0.016g When a diver dives to great depths, an excess of gaseous Nitrogen will dissolve in his blood. If he surfaces too rapidly, he can suffer from the Bends. This is because his blood is with respect to Nitrogen. a) saturated b) unsaturated c) super-saturated ***** 5. A solution is prepared by mixing 3.50g Naphthalene (moth-balls, C10H11) into 29.0g of Benzene (a hydrocarbon liquid, C6H6). What is the mole fraction of Benzene in this solution? a) b) c) d) 6. 0.978 0.933 ***** 0.871 0.711 # mol Nap = (3.50 g) (1 mol Nap / 131g) = 0.0267 mol Nap # mol Ben = (29.0 g) (1 mol Ben / 78g) = 0.3718 mol Ben xBen = 0.3718 / (0.3718 + 0.0267) = 0.933 Brass is a solution of Copper and Zinc. Red Brass is approximately 20% Zinc. What is the molality of this solid solution? a) b) c) d) 15.3 m 4.7 m 3.8 m ***** 0.25 m Assume 100 g Soln: 20g Zn & 80g Cu # mol = 20g / (65.4 g/mol) = 0.306 mol molality = 0.306 mol / (0.80 kg) = 3.8 m 7. While cooking, you add a "pinch" of Table Salt, NaCl, to a quart of water simmering in a sauce pan. Take the "pinch" of salt to be about 0.1 gram and the amount of water to be close to 1 L; ~1 kg. What is the elevation in the boiling point of this solution? For water, Kb = 0.521 oC/m. a) b) c) d) 1.003oC 0.1291oC 0.0214oC 0.0018oC ***** NaCl(aq) Na+(aq) + Cl-(aq) #mol NaCl = (0.1 g) (1 mol NaCl / 58.5g) = 0.00171 mol NaCl m = (0.00171 mol) / (1 kg) = 0.00171 m Tb = Kbm = (2) (0.521 oC/m) (0.00171 m) = 0.0018oC 8. Milk is a colloidal emulsion, meaning it is a phase. a) b) c) d) 9. gas; liquid liquid; solid liquid; liquid ***** solid; liquid dispersed into a Liquid Fat globules dispersed in an Aqueous Sol'n As a pan of water is heated, the concentration of the dissolved gases will: a) increase. b) decrease. ***** c) remain fairly constant. 10. Hydrogen reacts explosively with Oxygen to produce Water 2 H2(g) + O2(g) 2 H2O(g) The Rate of Consumption of O2 is: a) b) c) d) 11. one quarter that of H2. one half that of H2. ***** twice that of H2. four times that of H2. Rate = - (1/2) [H2] / t = - [O2] / t Chloroform reacts with Chlorine to form Carbon Tetrachloride: CHCl3 + Cl2 CCl4 + HCl The rate law for this reaction is: Rate = k [CHCl3] [Cl2]1/2 If the concentration of the Chlorine is increased by a factor of four, the reaction rate will increase by a factor of: a) b) c) d) 2. ***** 4. 8. 16. Rate ~ 41/2 x = 2 x 12. The decomposition of Nitramide occurs according to first-order kinetics with a rate constant of 0.00563 min-1: NH4NO2aq) N2O(g) + H2O(l) How long will it take for a solution that is 0.105M Nitramide to decompose to 0.041M? a) b) c) d) 23.4 min 97.1 min 167 min ***** 245 min [Nit] = [Nit]o exp(-kt) (0.041 M) = (0.105 M) exp( - (0.00563 min-1) t) t = 167 min 13. The following reaction is Second Order: 2 NO2(g) 2 NO(g) + O2(g) with a rate constant k = 0.543M-1sec-1. If we start with [NO2]o=0.01M, how long will it take for the concentration of the reactant to be cut in half? a) b) c) d) 2892 sec 593 sec 184 sec ***** 34 sec 1 / [N2O5] = 1 / [N2O5]o - kt 1 / (0.005 M) = 1 / (0.01 M) - (0.543 M-1sec-1) t t = 184 sec 14. The rearrangement of Methy Isonitrile has been extensively studied: CH3-NC(g) CH3-CN(g) Kinetic data at 189.7oC is plotted below. The Rate Constant k for this reaction is: a) b) c) d) 0.0002 min-1 0.0015 min-1 ***** 0.1133 min-1 0.3341 min-1 k = - slope 15. The bromination of nitric oxide: 2 NO + Br2 2 NOBr occurs according to the following rate law: Rate = k [NO]2 [Br2] The overall Order of this reaction is: a) b) c) d) 1st 2nd 3rd ***** 4th order = 2 + 1 = 3 Short Answer 1. Initial rate data is provided for the following reaction: 2 NO + H2 [NO]o (M) 6.4 x 10-3 12.8 x 10-3 6.4 x 10-3 Trial #1 Trial #2 Trial #3 N2O + H2O [H2]o (M) 2.2 x 10-3 2.2 x 10-3 4.4 x 10-3 rateo (M/sec) 2.60 x 10-5 10.4 x 10-5 5.20 x 10-5 Determine the Reaction Order with respect to NO and H2. Write the resulting Rate Law for this reaction. (Show your work.) Assume Rate = k [NO]m [H2]n Then Rate2 / Rate1 = ([NO]2 / [NO]1)m (10.4 / 2.60) = (12.8 / 6.4)m 4 = 2m m = 2 Rate3 / Rate1 = ([H2]3 / [H2]1)n (5.20 / 2.60) = (4.4 / 2.2)n 2 = 2n n = 1 Finally, Rate = k [NO]2 [H2] 2. For the decomposition of N2O5, we have: 2 N2O5 4 NO2 + O2 Time (sec) 0 600 1200 1800 [N2O5] (M) 1.65 x 10-2 1.24 x 10-2 0.93 x 10-2 0.71 x 10-2 Calculate the reaction rate during the first and last time intervals. Comment on your results. Rate = - (1/2) [N2O5] / t 1st Interval Rate = - (1/2) ((1.24 - 1.65) x 10-2 M) / (600 - 0 sec) = 3.42 x 10-6 M/sec Last Interval Rate = - (1/2) ((0.71 - 0.93) x 10-2 M) / (1800 - 1200 sec) = 1.83 x 10-6 M/sec The rate has decreased as expected when the concentration of the reactants decreases. 3. Amnesic Shellfish Poisoning is a result of eating shell which contain high levels of Domoic Acid. How were the harvesters of Blue Muscles on Prince Edward Island advised to avoid raising shellfish with high levels of Domoic Acid and why does this remedy work? Sol'n: Wait a sufficient amount of time after a Red Tide before harvesting the shellfish. Domic Acid is very hydrophillic. So it will not absorb into the fleshy parts of the muscle. It will instead be expelled from the digestive tract of the muscle with the water. 4. Polymers are high molecular weight molecules. 6.053g of poly(Vinyl Alcohol) in a 100.0mL solution has an Osmotic Pressure of 0.272 atm at 25oC. a) What is the Molarity of this solution? C = / RT = (0.272 atm) / (0.08206 L atm/K mol) (298.15 K) = 0.011 M b) What is the number of mole poly(Vinyl Alcohol) dissolved in this solution? # mol = (0.0111 M) (0.1000 L) = 0.00111 mol c) What is the MW of the polymer? MW = 6.053 g / 0.00111 mol = 5.4 x 103 g/mol 5. Why do we expect the Rate of a chemical reaction to decrease as the reaction proceeds? Why might this not happen? As the molecules get further apart, because they are more dilute, they collide less frequently and therefore should react more slowly. This may not happen if the reaction relies on some hidden variable or if the molecules collide and react in a way not explained by the overall reaction stoichiometry. Appendix Constants NA = 6.022045 x 1023 entities/mole kB = 1.380662 x 10-23 J/K c = 2.99792458 x 108 m/sec h = 6.626176 x 10-34 J sec R = 8.314 J/K mol = 0.08206 L atm/K mol 1 amu = 1.6605 x 10-24 g