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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.

  15. Types of Intermolecular Forces 4. Ion-Dipole Forces • Most commonly found in solutions. Especially important for solutions of ionic compounds in polar liquids. • A positive ion (cation) attracts the partially negative end of a neutral polar molecule. • A negative ion (anion) attracts the partially positive end of a neutral polar molecule.

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