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AP unit 4-2 Intermolecular force and phase change

AP unit 4-2 Intermolecular force and phase change. States of Matter. Because in the solid and liquid states particles are closer together, we refer to them as condensed phases . Molecule distance: gas>liquid>gas. The States of Matter.

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AP unit 4-2 Intermolecular force and phase change

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  1. AP unit 4-2 Intermolecular force and phase change

  2. States of Matter Because in the solid and liquid states particles are closer together, we refer to them as condensed phases. • Molecule distance: gas>liquid>gas

  3. The States of Matter • The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: • The kinetic energy of the particles. Formula? • Intermolecule force: the strength of the attractions between the particles

  4. Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.

  5. Intermolecular Forces They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

  6. Intermolecular Forces These intermolecular forces as a group are referred to as van der Waals forces.

  7. van der Waals Forces • Dipole-dipole interactions • Hydrogen bonding • London dispersion forces

  8. Ion-Dipole Interactions • A fourth type of force, ion-dipole interactions are an important force in solutions of ions. • The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

  9. Dipole-Dipole Interactions • Molecules that have permanent dipoles are attracted to each other. • The positive end of one is attracted to the negative end of the other and vice-versa. • These forces are only important when the molecules are close to each other.

  10. Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point.

  11. London Dispersion Forces • At that instant, then, the helium atom is polar • Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.

  12. London Dispersion Forces London dispersion forces are attractions between an instantaneous dipole and an induced dipole. • Synonyms: “London forces”, “dispersion forces”, and “dispersion-interaction forces”

  13. London Dispersion Forces • These forces are present in all molecules, whether they are polar or nonpolar. But it plays a major role in the nonpolar molecules. • The tendency of an electron cloud to distort in this way is called polarizability.

  14. Factors Affecting London Forces • The shape of the molecule: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane). • This is due to the increased surface area in n-pentane.

  15. Factors Affecting London Forces • The molecular weight: strength of dispersion forces tends to increase with increased MW. • Larger atoms have larger electron clouds, which are easier to polarize.

  16. How Do We Explain This? • The nonpolar series (SnH4 to CH4) follow the expected trend. • The polar series follows the trend from H2Te through H2S, but water is quite an anomaly.

  17. Hydrogen Bonding • The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. • We call these interactions hydrogen bonds.

  18. Hydrogen Bonding Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.

  19. Relative Magnitudes of Forces The types of bonding forces vary in their strength as measured by average bond energy. Ionic bonds Covalent bonds (400 kcal/mol) Strongest Weakest Ion- dipole interactions Hydrogen bonding (12-16 kcal/mol ) Dipole-dipole interactions (2-0.5 kcal/mol) Londonforces (less than 1 kcal/mol)

  20. Summarizing Intermolecular Forces

  21. Page 427: 11.9, 11.11, 11.13, 11.17 Homework

  22. Intermolecular Forces Affect Many Physical Properties of liquid

  23. Boiling point increases with intermolecular attractive forces

  24. Viscosity • Resistance of a liquid to flow is called viscosity. • It is related to the ease with which molecules can move past each other. • Viscosity increases with stronger intermolecular forces and decreases with higher temperature. • Higher T  faster molecules move

  25. Viscosity of Glycerol as a Function of Temperature

  26. Surface Tension Surface tension results from the net inward force experienced by the molecules on the surface of a liquid. • Water surface due to surface tension • Hg surface lack of surface tension

  27. Page 428 11.19, 11.20, 11.25 Homework

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