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Intermolecular Forces

Intermolecular Forces. Courtesy: labinitio.com. A phase is a homogeneous part of a system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases. Solid phase - ice. Liquid phase - water. Characteristic Properties of

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Intermolecular Forces

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  1. Intermolecular Forces Courtesy: labinitio.com

  2. A phase is a homogeneous part of a system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases Solid phase - ice Liquid phase - water

  3. Characteristic Properties of Gases, Liquids, and Solids • Gases: • Assumes the volume and shape of its container. • Low Density • Very Compressible • Very Free Motion

  4. Liquids: • Has a definite volume but assumes the shape of its container. • High Density • Only Slightly Compressible • Molecules slide past one another easily.

  5. Solids: • Has definite volume and shape. • High Density • Virtually Incompressible • Molecules vibrate about a fixed position.

  6. Intermolecular Forces • Intermolecular forces are attractive forces between molecules. • Intramolecular forces hold atoms together in a molecule. • Intermolecular vs Intramolecular • 41 kJ to vaporize 1 mole of water (inter) • 930 kJ to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force boiling point melting point DHvap DHfus DHsub Generally, intermolecular forces are much weaker than intramolecular forces.

  7. or … … H H B A A A Types of Intermolecular Forces 1. Hydrogen Bond (strongest) The hydrogen bondis a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. IT IS NOT A BOND. A & B are N, O, or F

  8. Figure (a): The polar water molecule. Figure (b): Hydrogen bonding among water molecules. Note that the small size of the hydrogen atom allows for close interactions.

  9. Hydrogen Bonding Electrostatic potential diagrams – red indicates the most electron-rich region and blue indicates the most electron-poor region.

  10. Hydrogen bonding has very important effect on physical properties. • Note that the nonpolar tetrahedral hydrides of Group 4A (14) show a steady increase in boiling point with molar mass (i.e., going down the group). • But in the other groups the lightest member has the highest boiling point.

  11. Why do H2O, HF, and NH3 break the trend of decreasing molar mass = decreasing boiling point? Unusually strong hydrogen bonding forces due to two factors: relatively large electronegativity values which leads to especially polar X-H bonds small size of the element, which allows for the close approach of the dipoles, strengthening the intermolecular forces. Hydrogen bonding is so strong it takes a great amount of energy to overcome attraction and separate the molecules to produce the gaseous state.

  12. Types of Intermolecular Forces 2. Dipole-Dipole Forces Attractive forces between polar molecules

  13. Types of Intermolecular Forces 3. London Dispersion Forces – (weakest) Attractive forces that arise as a result of temporary dipolesinduced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction

  14. Intermolecular Forces 3. London Dispersion Forces Continued Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. • Polarizability increases with: • greater number of electrons Dispersion forces usually increase with molar mass.

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