Lecture 22sf - We have been studying Lewis structures of...

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We have been studying Lewis structures of covalently bonded molecules, such as H 2 and N 2 . These structures both have strong covalent bonds between the atoms making up the molecule. But are there any bonds between molecules? Does one H 2 molecule attract another H 2 molecule? Or does one N 2 molecule attract another N 2 molecule? For ideal gases, the answer is no. One of the tenets of the ideal gas model is that molecules exert no attractive forces on each other. They move around randomly, colliding often with elastic collisions, but except for these collisions, each molecule is completely independent of other molecules. However, gases are not ideal under all conditions. If we lower the temperature enough, any gas will liquefy or even solidify at still lower temperatures. The existence of these condensed states of matter (liquids and solids) is evidence of intermolecular attractions (attractive forces between molecules). Compare molar volumes of liquids and gases. Molar volume of H 2 O( ) : 18 mL Molar volume of any gas at STP: 22,400 mL What holds the mole of water molecules together in this relatively 18 mL volume? The general answer: attractive forces between molecules, also called intermolecular attractions. If such attractions did not exist, water or any other substance would not be a liquid or solid. A similar equivalent comparison is one of density. Density of H 2 O( ): 1.00 Density of air at room temperature and pressure: 1.21 or 0.00121 Liquids have much higher densities and much lower molar volumes than gases. The reason: intermolecular forces, resulting in molecules which are much closer together. Which are stronger: Covalent bonds or intermolecular forces?
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Intermolecular attractions are generally much weaker than covalent bonds. When we boil a liquid (change it from liquid to gas), we break the intermolecular attractive forces, but leave the covalent bonds intact. The gaseous molecules have the same Lewis structure as the liquid molecules, except the molecules are much farther apart in the gas, with no intermolecular attractions (if the gas is ideal). We can compare the energetics of breaking covalent bonds and intermolecular attractions. H () H 2 (g) → 2H(g) 436 (bond energy of H-H) H 2 ( ) → H 2 (g) 0.90 (heat of vaporization) N 2 (g) → 2N(g) 946 (bond energy of N N triple bond) N 2 ( ) → N 2 (g) 5.6 (heat of vaporization) H 2 O(g) → 2H(g) + O(g) 928 (bond energy of 2(H-O) H 2 O( ) → H 2 O(g) 40.7 (heat of vaporization) H 2 has a very low boiling point of 20 K, and a correspondingly low H vap of 0.90 kJ/mol. H 2 has very weak intermolecular attractions. N 2 boils at 77 K, higher than H 2 , and its H vap is likewise higher than H 2 (5.6 kJ/mol compared with 0.90 kJ/mol). This means that the intermolecular attractions in N 2 , though weak, are stronger than those in H 2 . For both H
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This note was uploaded on 04/06/2011 for the course CHEMISTRY 161 taught by Professor Seigel during the Fall '10 term at Rutgers.

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Lecture 22sf - We have been studying Lewis structures of...

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