2011, Robert Ayton. All rights reserved.

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1 Liquids, Solids, and Intermolecular Forces Outline 1. Phase Diagrams and Triple Point Diagrams 2. Intermolecular Forces Review 1. Phase Diagrams and Triple Point Diagrams Phase Diagram of Water Triple Point Diagram of Water Boiling Point is when the Vapor Pressure = Atmospheric Pressure If the Melting / Freezing Point line is below the temperature of the Triple Point, the substance will sink in its liquid form (solid will be more dense than the liquid).

2 2. Intermolecular Forces Covalent Network Metallic Ionic Covalent C (diamond) C (graphite) SiO 2 (quartz) Fe, Ag, Au, etc. Delocalized e- Metal / Nonmetal Cation / Anion Transferring e- Nonmetals Sharing e- Lattice Energy LE = k q q R 2 Polar Nonpolar Asymmetric due to lone pairs of e- on the central atom Symmetric Hydrogen Bonding H bonded to F, O, N NH 3, H 2 O, HF Dipole to Dipole PH 3, H 2 S, HCl London Dispersion Force Instantaneous Dipole Greater Molar Mass: Higher MP, BP, strength Greater the Dipole Moment Stronger Force, Higher BP, Higher MP, Lower Vapor Pressure

3 Phase Diagrams and Triple Point Diagrams Questions 1-2: Refer to the diagram below Liquids, Solids, and Intermolecular Forces Problem Set 1. The substance is at its normal melting point at time (A) T1 (B) T2 (C) T3 (D) T4 (E) T5 2. Which of the following best describes what happens to the substance between T4 and T5? (A) The temperature is increasing. (B) The vapor pressure is decreasing. (C) The average intermolecular distance is decreasing. (D) The molecules are leaving the liquid phase. (E) The solid and liquid phases are in equilibrium. 3. The phase diagram above is shown for a pure substance Z. The temperature of a sample of pure solid Z is slowly raised from 0 C to 80 C at a constant pressure of 0.5 atm. What is the expected behavior of the substance? (A) It first melts to a liquid and then boils at 80 C. (B) It first melts to a liquid and then boils at 30 C. (C) It remains a solid until the temperature is greater than 80 C.

4 (D) It melts to a liquid at a temperature of about 25 C and remains a liquid until the temperature is greater than 80 C. (E) It sublimes to a vapor at an equilibrium temperature of about 20 C. 4. Answer the following questions using correct chemical principles: a. When will a substance or solution begin to vaporize? b. What can be said about a substance which has a triple point temperature above its melting point? Intermolecular Forces 5. What type of intermolecular force is found within all molecules? What type of intermolecular forces are found within polar molecules? 6. Which of the following compounds is the most ionic? (A) SiCl 4 (B) ICl (C) PCl 3 (D) I 2 O (E) MgCl 2 7. Which of the following oxides is a gas at standard temperature and pressure? (A) Cs 2 O (B) NO 2 (C) Na 2 O 2 (D) SiO 2 (E) Fe 2 O 3 8. The boiling points of the elements in group 18 increase in the order of helium, neon, argon, krypton, and xenon. Which of the following statements accounts for this increase in boiling points? (A) The number of unshared pairs of electrons increases. (B) The ionization energy increases. (C) The London (dispersion) forces increases. (D) The dipole-dipole forces increases. (E) The hydrogen bonding forces increases. 9. Consider the statement: As H 2 O changes from a liquid form to a gaseous form, the H-O bonds of the water are breaking. Support or refute this statement with a complete explanation. 10. Of the following pure substance, which has the highest melting point? (A) Br 2 (B) Fe (C) NO 2 (D) C 6 H 6 (E) SiO Does the following contain (1) polar bonds and (2) an overall dipole moment?

5 a. CO 2 b. HCN c. H 2 d. CH 3 Cl e. F 2 f. Ne 12. Identify the molecular force in each molecule (identify all forces if there are more than one): a. Xe b. CH 3 OH, methanol c. NH 3 d. HF e. PH 3 f. CH 4, methane Compound Name Compound Formula ΔH vaporization (kj mol -1 ) Butane CH 3 CH 2 CH 2 CH Butanone CH 3 COCH 2 CH Butanol CH 3 CH 2 CH 2 CH 2 OH For each of the compounds below (using the information from the table above), explain why the compounds do not have the same value for the standard heat of vaporization (include specific information about both substances): a. Butane and Butanone b. Butanone and 1-Butanol 14. Use appropriate chemical principles to account for each of the following observations. In each part you must include specific information about each substance. a. At 25 C and 1 atm, F 2 is a gas whereas Br 2 is a liquid. b. The melting point of NaCl is 801 C, whereas MgS is greater than 2000 C. c. Ammonia, NH 3 dissolves readily in water, whereas PH 3 is moderately soluble. d. CH 3 OCH 3 is a liquid at room temperature whereas C 3 H 8 is a gas. e. KCl is a solid at room temperature whereas HCl is a liquid. 15. Compare the strength of the intermolecular forces listed below between H 2 O and H 2 S: a. Dipole to dipole intermolecular attractions b. London dispersion intermolecular attractions 16. At a pressure of 1 atm, the boiling point of NH 3 (l) is higher than NF 3 (l). a. Identify the hybridization in each molecule. b. Is the bond angle of each less than, greater than, or equal to 109.5? Justify your answer. c. Identify the intermolecular forces(s) in each substance. d. Account for the difference in the boiling points of each substance. 17. The melting point of CsBr is 636 C, whereas NaBr is 747 C.

6 a. Identify the bonding for both substances. b. Account for the difference in the melting points of these substances. 18. Support or refute this following statement (justify your response): a. As water reaches its boiling point of 100 C at 1 atm of pressure, the energy breaks the O-H bonds of the molecule.

7 19. Use appropriate chemical principles to account for each of the following observations. In each part you must include specific information about each substance. a. SiO 2 melts at 2230 C, whereas CO 2 melts at -78 C. b. The boiling point of Cl 2 (l) is higher than the boiling point of HCl. c. C 6 H 6 is not soluble in water whereas C 5 H 5 N is soluble. d. H 2 O beads up on a table whereas CH 3 COCH 3 just spreads out and evaporates easily. e. KCl melts at 770 C whereas ICl 3 melts at 63 C. 20. Use appropriate chemical principles to account for each of the following observations. In each part you must include specific information about each substance. a. Even though NH 3 and CH 4 have similar molar masses, NH 3 has a much higher boiling point of -33 C and CH 4 of -164 C. b. At standard temperature and pressure, hexane (C 6 H 14 ) is a liquid and ethane (C 2 H 6 ) is a gas. c. CaO melts at a much higher temperature than KF. d. Even though both are made up of nonmetals, C melts at a much higher temperature than F 2. e. LiF has a much higher melting point than CF 4. Free Response: 21. Use the principles of bonding and molecular structure to explain the following statements. a. The boiling point of argon is C, whereas the boiling point of neon is C. b. Solid sodium melts at C, but solid rubidium melts at C. c. A greater amount of energy is required to dissociate a CaO (s) crystal than to dissociate a KF (s) crystal. d. Molten NaF conducts electricity, but solid NaF does not. Substance Boiling Point ( C) Bond Length (pm) Bond Strength (kj mol -1 ) H N O Cl Explain the differences in the properties given in the table above for each of the following pairs of molecules. a. Explain the bond strengths of N 2 and O 2. b. Explain the bond lengths of H 2 and Cl 2. c. Explain the boiling points of O 2 and Cl 2. d. Use the principles of molecular bonding to explain why H 2 O is a liquid at standard temperature and pressure, whereas H 2 and O 2 are both gases. 23. Use the principles of bonding and molecular structure to explain the following statements. a. The angle between the N-H bonds in NH 3 is smaller than the angle between the C-H bonds in CH 4. b. I 2 (s) is insoluble in water, but it is soluble in benzene (C 6 H 6 ). c. Ne is a gas at room temperature whereas Fe is not. d. HF and HBr both have higher boiling points than HCl.

8 Reference: All chemical values such as melting point, boiling point, etc. taken from David R. Lide (ed), CRC Handbook of Chemistry and Physics, 84th Edition. CRC Press. Boca Raton, Florida, 2003