Lecture_Outline_11 - 1 Intermolecular Forces Liquids and Solids Chapter 11 Intermolecular Forces Lecture Outline 11.1 A Molecular Comparison of Gases

Lecture_Outline_11 - 1 Intermolecular Forces Liquids and...

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Intermolecular Forces, Liquids, and Solids 1 1 Chapter 11: Intermolecular Forces Lecture Outline 11.1 A Molecular Comparison of Gases, Liquids and Solids Physical properties of substances are understood in terms of kinetic-molecular theory. Gases are highly compressible and assume the shape and volume of their containers. Gas molecules are far apart and do not interact much with one another. Liquids are almost incompressible; they assume the shape, but not the volume of the container. Liquid molecules are held together more closely than gas molecules, but not so rigidly that the molecules cannot slide past each other. Solids are incompressible and have a definite shape and volume. Solid molecules are packed closely together. The molecules are so rigidly packed that they cannot easily slide past each other. Solids and liquids are condensed phases . Solids with highly ordered structures are said to be crystalline. Converting a gas into a liquid or solid requires the molecules to get closer to each other. We can accomplish this by cooling or compressing the gas. Converting a solid into a liquid or gas requires the molecules to move further apart. We can accomplish this by heating or reducing the pressure on the solid. The forces holding solids and liquids together are called intermolecular forces. Physical properties of liquids and solids are due to intermolecular forces . These are forces between molecules. 11.2 Intermolecular Forces The attraction between molecules is an inter molecular force. Intermolecular forces are much weaker than ionic or covalent bonds. When a substance melts or boils, intermolecular forces are broken. When a substances condenses, intermolecular forces are formed. Boiling points reflect intermolecular force strength. A high boiling point indicates strong attractive forces. Melting points also reflect the strength of attractive forces. A high melting point indicates strong attractive forces. van der Waals forces are the intermolecular forces that exist between neutral molecules. These include London-dispersion forces, dipole-dipole forces, and hydrogen-bonding forces. Ion-dipole interactions are important in solutions. These are all weak (<15% as strong as a covalent or ionic bond) electrostatic interactions. Ion-Dipole Forces An ion-dipole force is an interaction between an ion (e.g., Na + ) and the partial charge on the end of a polar molecule/dipole (e.g., water). It is especially important for solutions of ionic substances in polar liquids. Example: NaCl ( aq ) Dipole-Dipole Forces Dipole-dipole forces exist between neutral polar molecules. Polar molecules attract each other.

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