Ch. 10 in- Class Exercise

Transcription

1 Chemistry 123/125 Ch. 10 in- Class Exercise In a liquid, the molecules are very close to one another and are constantly moving and colliding. Molecules attract each other, and the force of attraction is strongest when the molecules are very close to one another. When a liquid evaporates, molecules in the liquid must overcome these intermolecular attractive 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. Figure1. H2O molecules in the liquid and gas phases. Attractive forces between water molecules are stronger in the liquid than in the gas because the molecules are very close to one another. Gaseous H2O molecules are moving fast enough to overcome the attractive forces that exist in the liquid.

2 The intermolecular forces that attract molecules to each other are much weaker than the bonds that hold molecules together (the intramolecular forces). For example, 463 kj are required to break one mole of O- H bonds in H2O molecules, but only 44 kj are needed to separate one mole of water molecules in liquid water. To a large extent, the boiling point of a liquid is determined by the strength of the intermolecular interactions in the liquid. These interactions are determined by the structure of the individual molecules. Table 1. Boiling points of selected compounds. Alkane MM (g/mol) bp ( C) Ketone MM (g/mol) bp ( C) CH3CH2CH CH3(C=O)CH propane acetone CH3(CH2)2CH CH3(C=O)CH2CH butane 2- butanone CH3(CH2)3CH CH3(C=O)(CH2)2CH pentane 2- pentanone CH3(CH2)4CH CH3(C=O)(CH2)3CH hexane 2- hexanone CH3(CH2)8CH3 decane CH3(C=O)(CH2)7CH3 2- decanone Alcohol MM (g/mol) bp ( C) CH3CH2CH2OH propanol CH3(CH2)2CH2OH butanol CH3(CH2)3CH2OH pentanol CH3(CH2)4CH2OH hexanol CH3(CH2)8CH2OH 1- decanol Alkanes are hydrocarbons containing only C- C and C- H single bonds. Ketones contain a C=O bond and alcohols contain an O- H bond.

3 1. How would you classify the O- H bonds within a water molecule? (i.e., what kind of intramolecular forces does water have?) 2. When water evaporates, are any of these bonds between O atoms and H atoms within a molecule broken? Explain. 3. On average, are the intermolecular forces, between water molecules, stronger in H2O(l) or in H2O(g)? Justify your answer. 4. For each type of compound in table 1 (alkane, ketone, alcohol), how does the boiling point change as the molar mass of the compound increases? alkanes: ketones: alcohols: 5. In general how do the intermolecular forces between molecules change as the molar mass of the molecule increases? Explain this trend.

4 6. Find an alkane, a ketone, and an alcohol with roughly the same molar mass (within 5 g/mol). List the three compounds below in order of increasing boiling point. 7. For each type of compound (alkane, ketone, alcohol) predict whether or not the compound is expected to be polar. Base you prediction on the type of intramolecular bond in the molecule and on its geometry. Show work. 8. Based on the information in table 1 (see question 6) and your answer to question 7, rank the three types of compounds in order of increasing strength of intermolecular interactions, for molecules of similar molecular weight.

5 9. In general, how does the strength of intermolecular interactions for polar molecules compare to that of nonpolar molecules? 10. Is the strength of intermolecular forces related to the type of intramolecular forces in the molecule (polar covalent or polar bonds)? Explain. 11. For each of the types of compounds in table 1, determine the strongest intermolecular force present in the liquid phase of this compound. alkanes: ketones: alcohols: 12. Draw the two possible Lewis structures for 1,2- dichloroethylene (C2H2Cl2). (Hint: the two carbons are the central atoms in the molecule and each carbon is bonded to one hydrogen and one chlorine). Which of the two compounds will have the higher boiling point?

6 13. Rank each of the following groups of substances in order of increasing boiling point. Explain your reasoning. NH3, He, CH3F, CH4 CH3Br, Ne, CH3OH, CH3CN 14. An unknown liquid has the molecular formula C2H6O2 and its boiling point is 198 C. Given the data in the following table, write the structural formula for this compound. Molecule MM (g/mol) bp ( C) CH methane CH3CH ethane CH3CH2CH propane CH3OH methanol CH3CH2OH ethanol CH3CH2CH2OH propanol CH3CH2OCH2CH3 diethyl ether Adapted from Chemistry A Guided Inquiry by Richard S. Moog and John J. Farrell, Preliminary Edition, published by John Wiley & Sons.