Chapter 12 - Chapter 12 Intermolecular Attractions and the...

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Chapter 12 Intermolecular Attractions and the Properties of Liquids and Solids In this chapter we will try to relate the characteristics of individual molecules with physical properties, i.e. relate our understanding of what is occurring at a molecular level with what we observe in the real world (physical properties). Why Gases differ from Liquids and Solids In Chapter 11, we studied some general laws about gases, and found them to be true for all gases. There are no such general laws for liquids and solids! Why??? The Kinetic-Molecular Description of Liquids and Solids Gases A collection of widely separated molecules The kinetic energy of the molecules is greater than any intermolecular attractive forces The lack of any significant attractive force between molecules allows a gas to expand to fill its container If attractive forces become large enough, then the gases exhibit non-ideal behavior Liquids The intermolecular attractive forces are strong enough to hold molecules close together Liquids are more dense (and so less compressible) than gases Liquids have a definite volume, independent of the size and shape of their container The attractive forces are not strong enough, however, to keep neighboring molecules in a fixed position and molecules are free to move past or slide over one another Thus, liquids can be poured …they assume the shape of their containers Solids The intermolecular forces between neighboring molecules are strong enough to keep them locked in position Solids (like liquids) are not very compressible due to the lack of space between molecules If the molecules in a solid adopt a highly ordered packing arrangement, the structures are said to be crystalline Due to strong intermolecular forces between neighboring molecules, solids are rigid CHM 111 Lecture notes Page 1
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Chapter 12 The physical properties of a substance depend upon its physical state . The different physical states of water … steam (gas), liquid water and ice (solid) … all have the same chemical properties, but their physical properties are considerably different. Covalent bonds determine: Intermolecular forces influence: * Molecular shape * Physical properties * Bond energies * Chemical properties The state of a substance depends on the balance between the kinetic energy of the individual particles (molecules or atoms) and the intermolecular forces Intermolecular Forces Kinetic energy Dominate Dominates Solid Liquid Gas Condensed Phases Intermolecular forces try to draw Kinetic energy keeps the molecules apart the particles together and moving around. It is a function of the temperature of the substance Changing the state of a substance Temperature Heating and cooling can change the kinetic energy of the particles in a substance, and so, we can change the physical state of a substance by heating or cooling it. o
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This note was uploaded on 04/28/2008 for the course CHEM 111 taught by Professor Zombeck during the Winter '07 term at Saginaw Valley.

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Chapter 12 - Chapter 12 Intermolecular Attractions and the...

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