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Understanding Intermolecular Forces and Their Effects on Properties

Explore the different types of intermolecular forces and their impact on various properties such as boiling point, melting point, viscosity, vapor pressure, and surface tension.

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Understanding Intermolecular Forces and Their Effects on Properties

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  1. Intermolecular Forces Microscopic Properties Rule the Macroscopic World

  2. What causes them? • Two things affect intermolecular forces: • Charge • Distance • Bigger distance equals LESS attraction! • More charge equals MORE attraction! • Coulomb’s Law • F ~ charge/distance2 • Distance has more effect since it is squared.

  3. IMF’s versus Bonds • Bonds are permanent attractions, IMF’s are temporary

  4. Types of Bonds • Covalent • Sharing of electrons in permanent tug of war. • Ionic • Stealing electrons and then being attracted by opposite charges

  5. Types of IMF’s • Ion Dipole • Hydrogen Bonding • Dipole-Dipole • London Dispersion forces

  6. Ion Dipole Interactions • Between an ion and a dipole! • Ion = permanently charged particle • Dipole = a polar molecule • Almost exclusively seen in aqueous solutions.

  7. Dipole-Dipole Interactions • Between two dipoles! • Dipoles are polar molecules that have no net charge, but the charge is distributed unequally. • Positive end of one attracts negative end of the other.

  8. London Dispersion Forces • ALL covalent compounds experience this force. • It is between the electrons in one atom and the nucleus of another atom. • More mass and/or more atoms means more dispersion forces.

  9. Be Very Careful! • More mass doesn’t cause an increase in LDF! • More mass infers that there are more P and N, which MEANS there are more electrons. • It is an increase in the number of e- which causes a larger e- density which leads to a stronger temporary dipole. •  You can use molar mass to figure it out since a larger molar mass implies a bigger density but you can't use the words molar mass in your justificationin a Free Response Question

  10. Hydrogen Bonding • Occurs in molecules in which hydrogen is having FON! • The most electronegative atoms are F, O and N. With a hydrogen, there is excessive polarization. • This excessive polarization attracts the lone pair of electrons.

  11. Strengths of IMF’s • Strongest: Ion-Dipole (very strong) Hydrogen Bonding (strong) Dipole-Dipole Dispersion Forces (very weak)

  12. Effects of IMF’s • More IMF’s or stronger IMF’s affect the following properties: • Boiling point • Melting Point • Viscosity (thickness) • Vapor Pressure • Surface tension

  13. Boiling Point • Stronger IMF • Higher boiling point • Weaker IMF • Lower boiling point • Compare CS2 to CO2

  14. CS2 to CO2 Both are linear Both are nonpolar covalent Both have LDF CS2 has stronger LDF – larger molecule means larger e- density

  15. Melting Point • Stronger IMF • Higher melting point • Weaker IMF • Lower melting point • Think of red rover… • Compare HBr to Cl2

  16. HBr to Cl2 • Both are liner • HBr has LD, Dipole • Cl2 has LD, nonpolar • HBr has stronger IMF – higher melting point

  17. Viscosity • Stronger IMF • Higher viscosity • Weaker IMF • Lower viscosity • More attached to each other, the hard to flow, like solids vs. liquids. • Compare CH3OCH3 to CH3CH2OH

  18. CH3OCH3 to CH3CH2OH CH3OCH3 LD, Dipole CH3CH2OH LD, HB Therefore CH3CH2OH has stronger IMF and a higher viscosity

  19. Vapor Pressure the pressure exerted by a vapor, particularly a vapor in contact with its liquid form • Stronger IMF • Lower vapor pressure • Weaker IMF • Higher vapor pressure • Again…think red rover. • Compare H2S to H2O2

  20. H2S to H2O2 • H2S LD, Dipole • H2O2 LD, HB • H2O2 has stronger IMF therefore lower vapor pressure

  21. Surface Tension • Resistance of a liquid to increase in its surface area • Stronger IMF • Higher surface tension • Weaker IMF • Lower surface tension • Compare H2CO to H2O

  22. H2CO to H2O • H2CO LD, Dipole • H2O LD, HB • Hydrogen bonding is a stronger IMF than Dipole, therefore H2O has a stronger surface tension

  23. Summing it Up • Stronger IMF • Higher boiling point • Higher melting point • Higher viscosity • Higher surface tension • Lower vapor pressure • Weaker IMF • Lower boiling point • Lower melting point • Lower viscosity • Lower surface tension • Higher vapor pressure

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