Intermolecular Forces IMF Part I: Types of IMF Effect of IMF on Physical Properties based on Chap. 11 Se

1. 1 Intermolecular Forces (IMF) Part I: Types of IMF Effect of IMF on Physical Properties(based on Chap. 11 Sec 1-3 of Brady & Senese) Dr. C. Yau Fall 2011

2. 2 Review of the Postulates of the KMT KMT = Kinetic Molecular Theory KMT provides an explanation on the behavior of an ideal gas: A gas consists of an extremely large number of very tiny particles that are in constant, random motion. The gas particles themselves occupy a net volume so small in relation to the volume of their container that their volume can be considered negligible. The particles collide with each other and with the walls of the container in perfectly elastic collision. They move in straight lines between collisions neither attracting nor repelling each other.

3. 3 Parameters of a Gas When determining the amount of gas, one must specify all of the following (the parameters of the gas): P = Pressure V = Volume T = Temperature

4. 4 Review of the Postulates of the KMT The KMT explains: Boyle�s Law (P and V of a gas is inversely proportional when T is kept constant). Gay-Lussac�s Law (P and T of a gas is directly proportional). Increase in T increases the kinetic energy (KE) of the particles resulting in a higher frequency of collisions with the walls of the container, and thus increasing the P. Charles� Law (V is proportional to T when P is kept constant.

5. 5 View of the physical states at the particulate level

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7. 7 Changes in Physical State at the Particulate Level SOLID ??? LIQUID ??? GAS As heat is added to a solid, the energy is absorbed as it� weakens the attractive forces between the particles (allowing them to move away from each other), and increases the kinetic energy of the particles (causing them to move faster). As a gas, the particles no longer have any attractive forces between them. These attractive forces are called �intermolecular forces� (often abbreviated IMF)

8. 8 Intermolecular Forces This is only a summary of what is covered on the blackboard. Refer to your lecture notes for details. Types of Intermolecular Forces: 1) Dipole Forces (or dipole-dipole attraction) 2) London Forces (or London Dispersion Forces, or Dispersion Forces) 3) Hydrogen Bonding (or Hydrogen Bond) Other related forces: 4) Ion-dipole interaction 5) Ion-induced dipole interaction

9. 9 Relative Strengths of Intermolecular Forces Dipole Forces: The greater the difference between the electronegativities of the atoms, the larger the dipole, and the stronger is the dipole force. However, one must be careful to compare only molecules of comparable size because a variation in size has a separate effect on the strength of the IMF.

10. 10 Relative Strengths of Intermolecular Forces London Forces: Effect of Size: The larger the particle, the more polarizable it is, the stronger is the London forces. Effect of # of Atoms: The more atoms in the molecule, the larger the surface for interaction, the stronger is the London forces. Effect of Shape: The more compact the molecule, the less surface for interaction, the weaker is the London forces.

11. 11 Polarizability Learn the term �polarizable.� It refers to how easily the electron cloud surrounding a particle can be distorted by an outside charge. It is how �fluffy� the electron cloud is. A large particle tends to have �fluffier� electron clouds as the electrons are further from the nucleus and therefore less tightly bound. It is said to be more polarizable. It is more easily polarized, more easily form an induced dipole.

12. 12 What has IMF to do with BP? If compound A has stronger IMF than compound B, which would have the higher BP? A or B ? Ans. A has the higher BP.

13. 13 Effect of # Atoms on IMF Strength

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16. 16 Effect of Shape on IMF Strength

17. 17 H-bonding is of particular significance to us

18. 18 Significance of H-bonding Enzymes are biological catalysts that keep us alive. They work only if they maintain their special shapes. These shapes are often held in place by H-bonding. Example: Proteins are made of polypeptides, which are chains of amino acids. Hair protein is made of polypeptides exist in a shape known as ?-helix.

19. 19 ?-Helix of Hair Protein

20. 20 ?-Helix of Hair Protein

21. 21 ?-Pleated Sheets of Silk

22. 22 What holds the 2 strands in DNA is purely H-bonding! Significance of H-bonding

23. 23 Of the 3 IMF, if we compare molecules of comparable size (so that London forces are about the same for all)� London forces < dipole forces < H-bonding

24. 24 How do they compare with ionic bonds and covalent bonds? Dipole forces are about 1-4% in strength, compared to covalent bonds. H�Cl H�Cl H�Cl Where are the dipole forces in the structures shown above? Where are the covalent bonds? H-bonding is 5 to 10 times stronger than dipole forces ) 5-10% of of covalent bonds.

25. 25 Determination of Which IMF is in Effect Dipole forces: exhibited by polar molecules London forces: exhibited by all substances (polar and nonpolar molecules, ions, atoms) H-bonding: exhibited by compounds containing H-F, H-O or H-N bonds �.. Ionic bond: exhibited by ionic compounds Covalent bond: exhibited by molecular compounds (made of all nonmetals)

26. 26 Overall Picture: Relative Strengths of All Substances of comparable size

27. 27 Which of the following is not likely to have hydrogen bonding? Hint- sketch the molecules! CH3CO2H CH3OH CH3OCH3 All of these exhibit hydrogen bonding Ans. C is not likely to have H-bonding.

28. 28 Learning Check Identify the kinds of intermolecular forces present in the following compounds and then rank them in order of increasing boiling point: H2S CH3OH CBr4 Ne Chem FAQ: How can I predict boiling point trends from the relative strengths of intermolecular forces?Chem FAQ: How can I predict boiling point trends from the relative strengths of intermolecular forces?

29. 29 Arrange the following in terms of increasing strength of intermolecular forces: CO2, CH4, HF

30. 30 Effect of IMF on Physical Properties Compressibility: Gases are compressible, but not solids or liquids. Why? Surface tension: Gases have no surface tension. Water has a high surface tension. WHY?

31. 31 Wetting of a surface by a liquid: "Wetting" refers to the spreading of a liquid across a surface. In the lab, if water forms beads of droplets on the sides of a glass container (such as a buret or grad cylinder) it means the glassware is dirty! Why? If the glassware is clean, the water form a thin film across the surface and no droplets are observed. Effect of IMF on Wetting Ability

32. 32 Wetting Ability of Water

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34. 34 Wetting Ability Surfactants substances that have polar and non-polar characteristics improve a liquid�s wetting properties allow non-polar substances to dissolve in polar solvents Detergents contain surfactants, which causes water to be come �wetter� and allow detergents to spread out better on the surfaces we are trying to clean.

35. 35 Effect of IMF on Viscosity

36. 36 Solubility �Like dissolve like�- the more similar the polarity of two substances, the greater their ability to interact with each other rather This explains why oil and water don�t mix: H2O molecules form very strong H-bonding to each other. Oil is made of mostly hydrocarbons (nonpolar) and are attracted to each other by London forces.

37. 37 Solubility Why does table salt dissolve well in water but not in oil? What are the attractive forces in table salt? When table salt is added to water, what new forces are formed? What kind of IMF are in oil? What happens when table salt is added to oil?

38. 38 Role of IMF in How We Wash Dishes

39. 39 Effect of IMF on Evaporation What do you think an increase in IMF would do to the rate of evaporation? Increase the rate of evaporation Decrease the rate of evaporation Ans. B. Decrease the rate of evaporation.

40. 40 Understanding Evaporation Evaporation depends on... Surface area Temperature Strength of IMF

41. 41 Understanding E of Evaporation

42. 42 Understanding E of Evaporation At a higher temperature, more molecules have beyond the activation energy and they break off all IMF to form a gas. This means that the liquid would evaporate faster at a higher temperature.

43. 43 Understanding E of Evaporation