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General Chemistry Principles, Patterns, and Applications, 2011

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The kinetic molecular description of liquids must take into account both the nonzero<br />

volumes of particles <strong>and</strong> the presence of strong intermolecular attractive forces.<br />

C O N C E PTUAL P R OBLEMS<br />

1. A liquid, unlike a gas, is virtually incompressible. Explain what this means using macroscopic <strong>and</strong> microscopic<br />

descriptions. What general physical properties do liquids share with solids? What properties do liquids share<br />

with gases?<br />

2. Using a kinetic molecular approach, discuss the differences <strong>and</strong> similarities between liquids <strong>and</strong> gases with<br />

regard to<br />

a. thermal expansion.<br />

b. fluidity.<br />

c. diffusion.<br />

3. How must the ideal gas law be altered to apply the kinetic molecular theory of gases to liquids? Explain.<br />

4. Why are the root mean square speeds of molecules in liquids less than the root mean square speeds of<br />

molecules in gases?<br />

11.2 Intermolecular Forces<br />

L E A R N I N G O B JE C T I V E<br />

1. To describe the intermolecular forces in liquids.<br />

The properties of liquids are intermediate between those of gases <strong>and</strong> solids but are more similar to solids. In contrast<br />

to intramolecular forces, such as the covalent bonds that hold atoms together in molecules <strong>and</strong> polyatomic<br />

ions, intermolecular forces hold molecules together in a liquid or solid. Intermolecular forces are generally much<br />

weaker than covalent bonds. For example, it requires 927 kJ to overcome the intramolecular forces <strong>and</strong> break both<br />

O–H bonds in 1 mol of water, but it takes only about 41 kJ to overcome the intermolecular attractions <strong>and</strong> convert 1<br />

mol of liquid water to water vapor at 100°C. (Despite this seemingly low value, the intermolecular forces in liquid<br />

water are among the strongest such forces known!) Given the large difference in the strengths of intra- <strong>and</strong><br />

intermolecular forces, changes between the solid, liquid, <strong>and</strong> gaseous states almost invariably occur for molecular<br />

substances without breaking covalent bonds.<br />

Note the Pattern<br />

The properties of liquids are intermediate between those of gases <strong>and</strong> solids but are more similar to solids.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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