Intermolecular Forces of Attraction

1. Intermolecular Forces of Attraction Attractive forces that cause atoms or molecules to stick together

2. Mixtures • Elements or compounds blended together but not chemically combined

3. Mixtures • Can be solid, liquid, or gaseous mixtures • Solid mixture examples: • Jewelry gold, most rocks, steel, alloys • Liquid mixture examples • Beverages, sea water, solutions • Gaseous mixture examples • Air, car exhaust

4. Mixtures • Why do they form? What allows the parts to stay mixed? Why do they stay together? • The components of a mixture are held together by IMF’s – intermolecular forces of attraction

5. Intermolecular Forces All intermolecular forces are: • attractive forces • between molecules • do not make new compounds • Makes the molecules “sticky” • weaker than true “bonds”

6. Types of IMF’s • London (dispersion) forces • all molecules • weakest interaction • dipole-dipole forces • polar molecules • ion-dipole forces • Between polar molecules and ions • hydrogen bonding • H atoms w/ O, N, F not covalently bonded to it • strongest interaction

7. Polar forces • “Dipole - Dipole” interactions • positive ends attract negative ends of other molecules • about 1% as strong as a covalent bond

8. Dipole - Dipole interactions

9. London forces • Momentary dipole-dipole interaction • “instantaneous” dipole “induces” a dipole in a neighboring molecule • brief attractive force results

10. Step 1

11. Step 2

12. Result...

13. London forces • Strength depends on: • size of electron cloud • bigger = stronger • number of atoms in molecule • more atoms = more electrons = stronger

14. Ion – dipole forces • Between polar molecules and ions • Aqueous solutions of ions • Positive ends of polar molecules attract negative ions • Negative ends attract positive ions • Only important in aqueous solutions

16. Hydrogen Bonds • Special case of polar attraction • H atom is • 1)bonded to high EN atom and • 2)attracted to a high EN atom it is not bonded to (N, O, F) • 5 times stronger than regular polar attractions

17. Hydrogen Bonds

18. Physical property effects of Intermolecular Forces of Attraction …all come down to how sticky the molecules are toward each other…

19. The Key Idea… • The stronger the IMF’s, the tighter the molecules cling to each other, the harder it is to separate them.

20. The Key Idea… • In other words, the harder it is to separate molecules, the stronger their IMF’s

21. What does that mean…”separate the molecules”? Pull them apart: • Melting • Boiling • Physically move them apart • Peel them away. Wipe them off, etc…

22. Boiling Point Effects

23. Boiling Point Effects

24. Boiling Point Effects

25. Intermolecular Forces of Attraction Attractive forces that cause atoms or molecules to stick together

26. Types of IMF’s • London (dispersion) forces • all molecules • weakest interaction • dipole-dipole forces • polar molecules • ion-dipole forces • Between polar molecules and ions • hydrogen bonding • H atoms w/ O, N, F not covalently bonded to it • strongest interaction

27. The Key Idea… • The stronger the IMF’s, the tighter the molecules cling to each other, the harder it is to separate them.

28. Solubility of a solid in liquid General rule: Like dissolves Like • Polar solutes dissolve in polar solvents • Sugar (polar solute) in H2O(polar solvent) • Ionic solids dissolve in polar solvents • NaCl (ionic solid) in H2O (polar solvent) • Nonpolar solutes dissolve in nonpolar solvents • Dried paint (nonpolar solute) in paint stripper – turpentine, etc. (nonpolar solvent)

29. Colligative properties of Solutions -Properties of solutions that are affected by the number of dissolved particles, not the identity of the particles Example: vapor pressure lowering The more particles (with little tendency to vaporize) that are added to a solvent, the lower the vapor pressure of the solution becomes

30. Colligative properties of Solutions Density of a solution The presence of dissolved particles(whose mass is greater than the mass of the solvent particles)in a solution causes the density of a solution to increase • Sugar water is more dense than pure water • Sugary sodas are more dense than diet sodas • it takes more sugar molecules than artificial sweetener molecules

31. Swimming in the Dead Sea

32. Colligative properties of Solutions Boiling point elevation: The presence of dissolved particles tends to increase the boiling point of a liquid Salt added to water before boiling to prepare food increases the temperature the water boils at, and thus cooks the food faster, as the water is hotter

33. Colligative properties of Solutions Freezing point depression The presence of dissolved particles tends to lower the freezing point of a liquid Salt or other substances are used to melt ice, because as they dissolve into the ice, they lower the freezing point of the ice, and thus even more of the ice melts

34. Compressibility • gases are compressible • empty space between molecules • Weak IMF’s won’t keep molecules together • liquids, solids aren’t • closely packed

35. Diffusion • The intermingling of molecules with each other • gases = rapid  much empty space • liquids = slowly  more closely packed • solids = negligible  locked in place

36. Cohesion/Adhesion • Cohesion = molecules sticking together • Adhesion = molecules sticking to a substrate

37. Viscosity • The resistance of a fluid to flow. • The stronger the IMF’s, the higher the viscosity

38. Surface Tension • Intermolecular attractions at the surface are only inward towards the substance

39. Surface Tension, continued • Molecules at the surface of a liquid act as a “skin” • the greater the IMF’s, the more pronounced the effect • liquids seek to lower their surface tension • water drops are spherical

40. Surface Tension, continued

41. Surface Wetting • The spreading of a liquid across a surface • must have adhesion  cohesion

42. Gasoline – a nonpolar hydrocarbon • Low surface tension • only London forces • low cohesion • spreads easily across most surfaces • does not effectively “wet” glass

43. H2O – a polar molecule that H- bonds • High surface tension • hydrogen bonding • High cohesive forces • High adhesive forces • will H-bond with O’s in glass • Good wetting agent for glass

44. Water beads on greasy glass • eliminates H-bonding to glass • low adhesion

45. Surfactants, used in soaps and detergents, drastically lower the surface tension of water • soapy water spreads more easily over dirty surfaces

46. Vaporization • The change of a liquid → gas is called vaporization • an example of separating molecules • Evaporation: happens at the exposed surface of a liquid • Boiling: happens within the liquid itself

47. Evaporation • Liquids exert vapor pressure due to the evaporated liquid • The stronger the IMF’s, the slower the evaporation rate, the lower the vapor pressure for a given temperature

48. Evaporation • The higher the T of a substance, the more molecules with enough energy to overcome cohesive forces/surface tension and evaporate • As liquid T , vapor P 

49. Temperature is ~ the average KE of a sample coolest warmest Abundance Energy Minimum E to evaporate

50. What happens... • To the evaporation rate • if the temperature = the boiling point ? • All the molecules, on average, have enough energy to vaporize • the liquid boils