POGIL: Intermolecular Forces and Boiling Points

Transcription

1 Name Date Block POGIL: Intermolecular Forces and Boiling Points Model 1: Intermolecular Forces in Liquids and Gases Molecules attract each other, and the intermolecular force increases rapidly as the distance between the molecules decreases. In a liquid, the molecules are very close to one another and are constantly moving and colliding. When a liquid evaporates, molecules in the liquid must overcome these intermolecular forces and break free into the gas phase, where on average molecules are very far apart. For example, when water evaporates, rapidly moving H2O molecules at the surface of the liquid pull away from neighboring H2O molecules and enter the gas phase, as shown in Figure 1. Figure 1. H2O molecules in the liquid and gas phases. Critical Thinking Questions: 1. When water evaporates, are any bonds between H atoms and O atoms within a water molecule broken? 2. When water evaporates, are the intermolecular forces between water molecules broken? 3. Are intramolecular forces stronger than intermolecular forces? 4. On average, are the intermolecular forces stronger in liquid water or in gaseous water? 5. Explain your answer to question #4. Page 1 of 7

2 Model 2: Intermolecular Forces and Boiling Points To a large extent, the boiling point of a liquid is determined by the strength of the intermolecular forces (dipole-dipole, hydrogen bonding, and dispersion forces). These interactions are largely determined by the structure of the individual molecules. Tables 1, 2, and 3: Boiling Points of Selected Compounds Alkane Molecular Weight (g/mole) Boiling Point ( o C) CH3CH2CH propane CH3(CH2)2CH butane CH3(CH2)3CH pentane CH3(CH2)4CH hexane CH3(CH2)8CH3 decane Ketone Molecular Weight (g/mole) Boiling Point ( o C) CH3COCH acetone CH3COCH2CH butanone CH3CO(CH2)2CH pentanone CH3CO(CH2)3CH hexanone CH3CO(CH2)7CH3 2-decanone Alcohol Molecular Weight (g/mole) Boiling Point ( o C) CH3CH2CH2OH propanol CH3(CH2)2CH2OH butanol CH3(CH2)3CH2OH pentanol CH3(CH2)4CH2OH hexanol CH3(CH2)8CH2OH 1-decanol Figures 1, 2, and 3: General Structure of Selected Compounds butane 2-butanone 1-butanol Page 2 of 7

3 Critical Thinking Questions: 1. All alkanes contain only what two elements? 2. Are alkanes polar or nonpolar molecules? 3. What is the most important (strongest) intermolecular force present in alkanes? 4. All ketones contain a C=O group. Are ketones polar or nonpolar molecules? 5. What is the most important (strongest) intermolecular force present in ketones? 6. All alcohols contain an O-H group. Are alcohols polar or nonpolar molecules? 7. What is the most important (strongest) intermolecular force present in alcohols? 8. For each type of compound below, indicate how the boiling point changes as the molecular weight of the compound increases: a. Alkane b. Ketone c. Alcohol 9. Based on your answer to #8, how do the intermolecular forces between molecules change as the molecular weight increases? 10. Find an alkane, a ketone, and an alcohol with roughly the same molecular weight (within 5 g/mole). Rank these compounds in terms of lowest to highest boiling points. 11. Repeat #10 with at least TWO more sets of compounds. Page 3 of 7

4 12. Describe the general pattern that you can identify about the boiling points of alkanes, ketones, and alcohols that are roughly equal in molecular weight. 13. Rank the three types of compounds (alcohols, ketones, and alkanes) in terms of their strength of intermolecular interaction (from strongest to weakest), for molecules of roughly equal molecular weight. 14. Rank the three types of intermolecular forces (dipole-dipole, hydrogen bonding, and dispersion forces) in terms of their typical relative strengths (from strongest to weakest). 15. In terms of intermolecular forces, explain the general trend that you described in #13. Page 4 of 7

5 Model 3: Ionic Compounds and Melting and Boiling Points Ionic compounds are created when electrons are transferred from one atom to another to create oppositely charged ions that have full charges. Table 4: Melting and Boiling Points for Selected Ionic Compounds Formula/ Structure Charge on the cation Charge on the anion Formula Mass (amu) Melting Point ( C) Boiling Point ( C) NaCl CaF Al2O K2S CsCl Mel2ng and Boiling Points of Selected Ionic Compounds Temperature ( C) Mel-ng point Boiling Point Formula mass (amu) Page 5 of 7

6 Critical Thinking Questions: 16. How do the melting and boiling points for ionic compounds in Table 4 compare to the melting and boiling points of the molecular compounds in Tables 1,2, and 3? 17. Explain your answer to # Do ionic compounds follow the same trend between mass and boiling point that you found for molecular compounds in #8? 19. Compare the melting and boiling points of NaCl, CaF2, and Al2O3. Develop an explanation for why one ionic compound might have higher melting and boiling points than another. 20. Sodium chloride (NaCl) has higher melting and boiling points than cesium chloride (CsCl). Develop an explanation for why this is so. (HINT: Think about atomic radii and Coulomb s Law.) Page 6 of 7

7 Extension Questions Yes, you must answer these as well! 21. Circle the member of each pair that has the greater boiling point and briefly explain your reasoning. Your explanation must include the nature of the intermolecular forces involved. a. CH3CH2OH or CH3CH2CH3 b. NO or N2 c. H2S or H2Te d. LiF or RbI 22. Normal alkanes are hydrocarbons with unbranched carbon chains. At room temperature, ethane (C2H6) is a gas; hexane (C6H14) is a liquid; and octadecane (C18H38) is a solid. Describe the intermolecular forces present in each substance and explain the differences in their room-temperature phases. Page 7 of 7