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Intermolecular Forces

Intermolecular Forces. Questions. Why do some solids dissolve in water but others do not? Why are some substances gases at room temperature, but others are liquid or solid? What gives metals the ability to conduct electricity, what makes non-metals brittle? The answers have to do with ….

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Intermolecular Forces

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  1. Intermolecular Forces

  2. Questions • Why do some solids dissolve in water but others do not? • Why are some substances gases at room temperature, but others are liquid or solid? • What gives metals the ability to conduct electricity, what makes non-metals brittle? • The answers have to do with … Intermolecular forces

  3. Intermolecular Forces(IMFs) 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) • Generally, intermolecular forces are much weaker than intramolecular forces. • Intramolecular force • Ionic • covalent (network solids) • metallic • Intermolecular force • Ion-dipole • dipole - dipole • H-bonding • London dispersion forces

  4. Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Strong intermolecular forces High surface tension • Capillary Action: Spontaneous rising of a liquid in a narrow tube.

  5. Adhesion Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules attracted to glass Cohesion attracted to each other

  6. Properties of Water Surface Tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. The stronger the IMFs the molecules have with each other, the higher is its surface tension. This is because of the cohesive forces inside the liquid (Polar-Polar forces) become much greater than the adhesive forces between the liquid (polar) and the air (non-polar) molecules outside.

  7. Properties of Liquids Viscosity is a measure of a fluid’s resistance to flow. Strong intermolecular forces High viscosity

  8. Bond Polarity • Nonpolar Cl2(Electronegativity difference is zero) • Polar HCl (Electronegativity difference is not zero) • Ionic NaCl

  9. Ion-Dipole Interaction Types of Intermolecular Forces 1. Ion-Dipole Forces Attractive forces between an ion and a polar molecule

  10. Orientation of Polar Molecules in a Solid Types of Intermolecular Forces 2. Dipole-Dipole Forces Attractive forces between polar molecules

  11. or … … H H B A A A Types of Intermolecular Forces 3. Hydrogen Bond (strongest) The hydrogen bond is 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

  12. Hydrogen Bond

  13. Decreasing molar mass Decreasing boiling point Why is the hydrogen bond considered a “special” dipole-dipole interaction?

  14. Types of Intermolecular Forces 4. Dispersion Forces – van der Walls forces/London forces (weakest) Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules Non-polar molecules do not have dipoles like polar molecules.So London dispersion force exists in non polar molecule. ion-induced dipole interaction dipole-induced dipole interaction Dispersion forces usually increase with molar mass.

  15. Dispersion forces usually increase with molar mass. Intermolecular Forces 4. 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 • more diffuse electron cloud

  16. O O S What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

  17. Types of IMF • The hydrogen bonds in water explain its relatively high boiling point, considering that it is a small molecule. The H-bonds hold the water molecules together as a liquid, so you have to heat it a lot before it will change to a gas. Compare boiling points of these molecules: London Disp. London Disp. Dipole-Dipole London Disp./Dipole-Dipole/Hydrogen Bonding

  18. Water, H20

  19. Which is related to…

  20. And Glycerine (Glycerol)

  21. Compare the three structures Which of these molecules has the greatest polarity? Which will exhibit the strongest intermolecular forces?

  22. Ethanol. Polar or Non-Polar?

  23. Methanol. Polar or Non-Polar?

  24. Methane. Polar or Non-Polar?

  25. Chloro-Methane. Polar or Non-Polar?

  26. Sulfur Hexafluoride. Polar or Non-Polar?

  27. Nitrogen Trihidride, aka Ammonia. Polar or Non-Polar?

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