Danby Kang, Homework 2 Due: Sep 14 2007, 10:00 pm Inst: Walker This print-out should have 22 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. The due time is Central time. CHAPTER 8 001 (part 1 of 1) 2 points On a relative basis, the weaker the intermolecular forces in a substance 1. the larger its molar heat capacity as a liquid. 2. the higher its melting point. 3. the more it deviates from the ideal gas law. 4. the greater its vapor pressure at a particular temperature. correct 5. the larger its heat of vaporization. Explanation: Weak intermolecular forces suggest that it is easy to cause molecules of the substance to be pulled away from each other. Thus it will take a relatively low temperature to both melt and boil the substance. Weaker intermolecular forces also mean that the substance will have a relatively high vapor pressure at a given temperature (which would cause the compound to boil at a lower temperature, since the normal boiling point would occur when the substance's vapor pressure reaches 760 torr.) 002 (part 1 of 1) 2 points Which of the following is expected to boil at the highest temperature? 1. C3 H8 2. CH4 3. C5 H12 correct 4. C4 H10 5. C2 H6
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Explanation: The boiling point tends to increase with increasing molecular weight. C5 H12 has the biggest MW, so it should boil at the highest temperature. 003 (part 1 of 1) 2 points Consider the following statements: I) The vapor pressure of ethyl alcohol increases as temperature increases. II) Liquids with low molecular weight exhibit relatively high vapor pressures, liquids with high molecular weight show relatively low vapor pressures, and solids do not have vapor pressures. III) A liquid boils at a higher temperature at sea level than on top of a mountain. IV) The temperature of boiling water inside a pressure cooker at sea level is above 100 C. Which response contains all of the TRUE statements, and no others? 1. II and IV only 2. III only 3. I and IV only 4. I, III, and IV only correct 5. I and II only Explanation: Solids can have vapor pressures. CO2 sublimes (s g) and therefore has a vapor pressure. When temperature increases, molecules have greater energy to overcome intermolecular forces/bonds and will therefore have increased vapor pressures. Atmospheric pressure is greater at sea level. Molecules require more energy (higher temperatures) to vaporize when pressure is high. 004 (part 1 of 1) 5 points Liquid metallic zinc (Zn) has a vapor pressure of 40 torr at 673 C and 100 torr at 736 C.
Kang, Danby Homework 2 Due: Sep 14 2007, 10:00 pm Inst: Walker Predict the normal boiling point of zinc. 1. 906 K 2. 1252 K 3. 1076 K 4. 1452 K 5. 1183 K correct Explanation: T1 = 673 C + 273 = 946 K P1 = 40 torr T2 = 736 C + 273 = 1009 K P2 = 100 torr We must first calculate Hvap, Zn : ln P2 P1 = Hvap, Zn R 1 1 - T1 T2 Thus T2 = 1183.45 K
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005 (part 1 of 1) 2 points At 75 C, the vapor pressures of water, methanol, diethylether, and benzene are 300, 1126, 2680, and 644 torr, respectively. Which of these is above the boiling point at 75 C and at atmospheric pressure?
1. None of these 2. methanol and diethyl, ether correct 3. water 4. methanol, diethyl, and benzene
Hvap, Zn
P2 R ln P1 = 1 1 - T1 T2 100 torr J ln mol K 40 torr = 1 1 - 946 K 1009 K = 115421 J/mol 8.314
5. diethylether Explanation: Atmospheric pressure is 760 torr. A substance will boil when its vapor pressure exceeds atmospheric pressure. Methanol and diethylether have vapor pressures above 760 torr. 006 (part 1 of 3) 2 points The following is the phase diagram for a given substance.
400 100
The normal boiling point is at 760 torr, standard atmospheric pressure. When the vapor pressure of Zn equals 760 torr, Zn will boil. We need to calculate the temperature at which this occurs: P1 = 100 torr P2 = 760 torr T1 = 736 C + 273 = 1009 K T2 = ? 1 1 R P2 - = ln T1 T2 Hvap, Zn P1 1 1 R P2 = - ln T2 T1 Hvap, Zn P1 1 = 1009 K J 8.314 mol K ln 760 torr - 115421 J/mol 100 torr -1 = 0.000844989 K
pressure (atm)
50 1 0.1
II
I III
0.03 100 150 200 temperature (K) 250 300
What is the normal melting point of this substance?
Kang, Danby Homework 2 Due: Sep 14 2007, 10:00 pm Inst: Walker 1. 136 K correct 2. 315 K 3. 107 K 4. 292 K 5. 216 K Explanation: Follow the 1 atm line horizontally to the solid/liquid line and read from the temperature scale 136 K. 007 (part 2 of 3) 2 points In the previous phase diagram, the three zones that are depicted as I, II, and III are which of the following? 1. I-vapor; II-solution; III-blend
pressure (atm)
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6. vaporization 7. sublimation and vaporization 8. fusion 9. melting and vaporization correct 10. deposition Explanation: The solid/liquid line is crossed, so melting occurs. Then the liquid/gas line is crossed, so vaporization occurs. 009 (part 1 of 3) 2 points Refer to the following phase diagram for the next 3 questions.
400 100 50 1 0.1
2. I-liquid; II-solid; III-gas correct 3. I-gas; II-solid; III-plasma 4. I-liquid; II-gas; III-solid 5. I-solid; II-liquid; III-gas Explanation: Zone II is solid, zone III is gas, and zone I is liquid. 008 (part 3 of 3) 2 points Referring to the same phase diagram, the substance is heated from 100 K to 300 K at a pressure of 50 atm. Which of the following happens to the substance? 1. condensation and vaporization 2. sublimation 3. condensation 4. solidification 5. fusion and sublimation
0.03 100 200 300 temperature (K) 400 500
What is the critical temperature of this substance? 1. 475 K correct 2. 140 K 3. 185 K 4. 525 K 5. 320 K Explanation: The liquid/gas equilibrium line ends at the critical point which corresponds to a critical temperature of 475 K and a critical pressure
Kang, Danby Homework 2 Due: Sep 14 2007, 10:00 pm Inst: Walker of 85 atm. 1. CCl4 , SiCl4 , CH4 , SiBr4 . 010 (part 2 of 3) 2 points What is the state of the substance when it is at 1 atm and 250 K? 1. gas 4. SiBr4 , SiCl4 , CCl4 , CH4 . 2. liquid correct 3. solid/liquid equilibrium 4. solid 5. its critical point Explanation: Starting with the region adjacent to the pressure axis, the ordering is solid, liquid, and gas going clockwise. The intersection of 1 atm and 250 K is in the bottom portion of the liquid region. 011 (part 3 of 3) 2 points When the solid phase of this substance is subjected to a considerable amount of pressure it will 1. melt. correct 4. positive ions move. 2. remain a solid. 5. negative ions move. 3. sublime. 4. go supercritical. Explanation: The solid/liquid equilibrium line WILL ultimately be crossed, as you go to higher and higher pressures. As you cross the line the solid phase will MELT into the liquid phase. 012 (part 1 of 1) 4 points Arrange the CCl4 molecules , SiCl4 , CH4 , SiBr4 in order of increasing molar enthalpy of vaporization. Hint: the molecular mass of SiBr4 is higher than SiCl4 . Explanation: 2. SiBr4 , SiCl4 , CH4 , CCl4 . 3. CH4 , CCl4 , SiCl4 , SiBr4 . correct
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Explanation: In general, the heavier the molecule, the more energy is required to get that molecule into the gas phase, and the higher the boiling point; and the greater the molar enthalpy of vaporization. 013 (part 1 of 1) 2 points If you melt an ionic solid, it conducts electricity because 1. positive ions move in one direction and negative ions in the opposite direction. correct 2. positive and negative ions move together. 3. only electrons are free to move.
014 (part 1 of 1) 2 points Water causes many electrolytes to dissociate 1. because water molecules are dipoles and the dipoles orient in an energetically favorable manner to solvate the ions. correct 2. because of repulsive interactions between ions in the crystalline state. 3. because the dispersion forces between ion and solvent are strong. 4. because it undergoes hydrogen bonding to large halide ions.
Kang, Danby Homework 2 Due: Sep 14 2007, 10:00 pm Inst: Walker Explanation: The more negative end of the water molecule orients toward the cation and the more positive end orients toward the anion; the ions remain apart. 015 (part 1 of 1) 2 points Based on the information Ion radius species A Li+ Fe3+ K+ 0.9 0.64 1.52
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All pairs of liquids will dissolve completely in each other if the resulting energy change is not too unfavorable because 1. Henry's Law says ideal solutions are always completely miscible. 2. the change in enthalpy always favors complete miscibility. 3. Raoult's Law says ideal solutions are always completely miscible. 4. the change in entropy always favors complete miscibility. correct 5. the change in surface tension always favors complete miscibility. Explanation: The dissolving process brings together unlike particles, thus decreasing the order in the system. 017 (part 1 of 1) 2 points Hydrogen chloride in the gaseous state is in the form of covalent molecules, yet in aqueous solution it behaves as a strong electrolyte. The explanation for this solution behavior is that 1. at lower temperatures, HCl becomes ionic. 2. HCl forms ionic crystals in the solid state. 3. there is more than one form of gaseous HCl. 4. although it is covalent, the molecule of HCl is polar, and stays in solution as a polar molecule, conducting electricity. 5. HCl reacts with water to form ions. correct Explanation: The more negative end of the polar water molecule attracts the H end of the HCl. H+
which is the correct order for the magnitudes of the heat of hydration? Answer in increasing magnitude of heats of hydration. 1. K+ , Li+ , Fe3+ correct 2. Li+ , K+ , Fe3+ 3. K+ , Fe3+ , Li+ 4. Fe3+ , Li+ , K+ 5. Li+ , Fe3+ , K+ Explanation: Hydration energy (or heat of hydration) is the energy involved in the hydration of one mole of gaseous ions. Hydration energy is related to the charge-to-radius ratio, which is a measure of the charge density of the ion. The larger the charge density, the larger is the hydration energy. Charge-to-radius ratios are 1 = 1.11 0.9 3 Fe3+ = = 4.69 0.64 1 = 0.66 K+ = 1.52 Li+ = Increasing magnitude means smallest to largest, so K+ < Li+ < Fe3+ . 016 (part 1 of 1) 2 points
Kang, Danby Homework 2 Due: Sep 14 2007, 10:00 pm Inst: Walker is transferred to a water molecule where it is better accommodated due to the greater electronegativity of O. The equation is HCl + H2 O - H3 O+ + Cl- 018 (part 1 of 1) 2 points Which of the following is the best explanation of why sugar is not very soluble in gasoline? Note that all of the statements below are true. 1. The disorder of the system would increase if the sugar dissolved in gasoline. 2. The attractive forces between gasoline molecules are weaker than the forces between gasoline and sugar molecules. 3. There is hydrogen bonding in the sugar crystal, but not between sugar and gasoline. correct 4. There are dipole-dipole interactions between sugar molecules but not between gasoline molecules. 5. Sugar does not ionize in gasoline. Explanation: The sugar molecule has several polar OH groups which are strongly attracted to polar water molecules but poorly attracted to nonpolar gasoline (hydrocarbon) molecules. 019 (part 1 of 1) 2 points Which of the following alcohols would be the least miscible with water? 1. propanol (CH3 CH2 CH2 OH) 2. hexanol (CH3 CH2 CH2 CH2 CH2 CH2 OH) correct 3. pentanol (CH3 CH2 CH2 CH2 CH2 OH) Explanation: Water is a polar liquid and for another liquid to be miscible with water it too must be polar. Least miscible means liquid which does not mix with water; i.e., which is non-polar.
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Hexanol has the longest non-polar region, so it will be least miscible. 020 (part 1 of 1) 2 points Calculate the concentration of argon in lake water at 20 C. The partial pressure of argon is 0.0090 atm and Henry's constant is 0.0015 mol L-1 atm-1 . 1. 1.5 10-3 M 2. 6.0 M 3. 9.0 10-3 M 4. 1.4 10-5 M correct 5. 0.17 M Explanation: 021 (part 1 of 1) 2 points Consider a series of chloride salts (MCl2 ). As the charge-to-size ratio of M2+ (decreases, increases) the hydration energy of M2+ (decreases, increases, does not change) in magnitude and the crystal lattice energy of MCl2 (decreases, increases, may increase or decrease) in magnitude. 1. decreases; increases; increases 2. decreases; decreases; increases 3. increases; decreases; increases 4. increases; increases; decreases 5. increases; does not change; may increase or decrease 6. increases; decreases; decreases 7. increases; increases; increases correct Explanation: As charge-to-size ratio increases, hydration energy and crystal latice energy also increase. 022 (part 1 of 1) 5 points
Kang, Danby Homework 2 Due: Sep 14 2007, 10:00 pm Inst: Walker The heat of vaporization of water is 9.73 kcal/mol. At what pressure (in torr) would pure water boil at 97 C? 1. 683 torr correct 2. 1366 torr 3. 342 torr 4. 512 torr 5. 911 torr Explanation: Here we use the Clausius-Clapeyron equation: ln P2 P1 = Hvap R 1 1 - T1 T2 P2 = (760 torr)e-0.106449 P2 = 683.256 torr
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Don't forget to convert from C to K: K = C + 273 We also need to convert the unit of Hvap into cal/mol, so it will match the unit of R. Here you should remember that the normal boiling point of water occurs at 760 torr (1 atm) at 100 C. T1 = 100 C = 373 K T2 = 97 C = 370 K R = 1.987 cal/mol K Hvap = 9.73 kcal/mol = 9730 cal/mol Substituting these values into the ClausiusClapeyron equation and solving for P2 , we have 9730 cal mol = 1.987 cal mol K 1 1 - 373 K 370 K = -0.106449 1000 cal 1 kcal
ln
P2 760 torr
P2 = e-0.106449 760 torr