Intermolecular Forces Liquids and Solids

1. Intermolecular ForcesLiquids andSolids

2. How Gecko can climb upside down on the ceiling? • Gecko are capable of climbing vertical or even upsidedown, utilizing micrometer size hairs to adhere to surfaces: Intermolecular force. • New research showed gecko can turn on and off the “stickiness”.

3. Properties of the three Phases of Matter • Fixed = keeps shape when placed in a container • Indefinite = takes the shape of the container Tro: Chemistry: A Molecular Approach, 2/e

4. Three Phases of Water Densities of ice and liquid water >> Density of steam Densities and molar volumes of ice and liquid water >> steam Density of ice < Density of liquid water. vital to the development of life as we know it. Tro: Chemistry: A Molecular Approach, 2/e

5. Molecular Features of Solids • The particles are packed close together and fixed in position • though they may vibrate • Incompressible • retaining their shape and volume when placed in a new container, and prevents the solid from flowing Tro: Chemistry: A Molecular Approach, 2/e

6. Solid: Crystalline vs. Amorphous • Crystalline solids have their particles arranged in an orderly geometric pattern. Examples: salt and diamonds • Amorphous solids have particles that do not show a regular geometric pattern over a long range. Examples: plastic and glass Tro: Chemistry: A Molecular Approach, 2/e

7. Molecular features of Liquids • The particles are closely packed, but have some ability to move around • incompressible • take the shape of their container and to flow • Yet don’t have enough freedom to escape or expand to fill the container Tro: Chemistry: A Molecular Approach, 2/e

8. Molecular feature of Gases • complete freedom of motion • not held together • constantly flying/colliding • Large space between the particles • compared to the size of the particles • Molar volumegas (Liter/mol) l >> Molar volumeliquid or Molar volumesolid • Compressible: Because there is a lot of empty spaces between molecules • Expand to fill and take the shape of their container, and will flow or escape (“antigravitiy”) Tro: Chemistry: A Molecular Approach, 2/e

9. Kinetic – Molecular Theory • What state a material is in depends largely on two major factors 1. the amount of kinetic energy the particles possess 2. the strength of attraction between the particles • These two factors are in competition with each other Tro: Chemistry: A Molecular Approach, 2/e

10. Degrees of Freedom • Particles may have one or several types of freedom of motion • and various degrees of each type • Translationalfreedom: the ability to move from one position in space to another • Rotationalfreedom: the ability to reorient the particles direction in space • Vibrational freedom: the ability to oscillate about a particular point in space Tro: Chemistry: A Molecular Approach, 2/e

11. States and Degrees of Freedom • Gas particles has the highest freedom of motion • their kinetic energy overcomes the attractive forces between the molecules • Solid particles has the least freedom: locked in place, cannot move around • do vibrate, not enough kinetic energy to overcome the attractive forces • Liquid particles have moderate freedom – can move around a little within the structure of the liquid • enough kinetic energy to overcome some of the attractive forces, but not enough to escape each other

12. Phase Changes Changing the material’s state requires changing the amount of kinetic energy of the particles • Melting of Solids: Heating gives Particles enough kinetic energy to partially overcome the attractive forces • Boiling of Liquids: Heating gives Particles enough kinetic energy to completely overcome the attractive forces • the stronger the attractive forces, the higher you will need to raise the temperature Tro: Chemistry: A Molecular Approach, 2/e

13. Phase Changes Tro: Chemistry: A Molecular Approach, 2/e

14. The Nature of Attractive Forces • Electrostatic forces • The strength varies, some are small and some are large • The strength depends on the kind(s) of particles • The stronger the attractive forces between the particles, the more they resist moving (“sticky”) • though no material completely lacks particle motion Tro: Chemistry: A Molecular Approach, 2/e

15. Intermolecular Attractions • The strength of the attractions between the particles of a substance determines its state • At room temperature, moderate to strong attractive forces result in materials being solids or liquids • Stronger the attractive forces leads to higher boiling point of the liquid and higher melting point of the solid • other factors also influence the melting point Tro: Chemistry: A Molecular Approach, 2/e

16. Why Are Molecules Attracted to Each Other? • Due to attractive forces between opposite charges • + ion to − ion • + end of polar molecule to − end of polar molecule • H-bonding especially strong • even nonpolar molecules will have temporary charges • Larger charge = stronger attraction • Longer distance = weaker attraction Tro: Chemistry: A Molecular Approach, 2/e

17. Trends in the Strength of Intermolecular Attraction • The stronger the attractions between the atoms or molecules, the more energy it will take to separate them • Boiling a liquid requires enough energy to overcome all the attractions between the particles • However, not breaking the covalent bonds • The higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces Tro: Chemistry: A Molecular Approach, 2/e

18. Kinds of Attractive Forces • Dispersion forces:Temporary polarity in the molecules due to unequal electron distribution • Dipole–dipole attractions: Permanent polarity in the molecules due to their structure • Hydrogen bonding: An especially strong dipole–dipole attraction results when H is attached to an extremely electronegative atoms F, O, N. Tro: Chemistry: A Molecular Approach, 2/e

19. Dispersion Forces • Cause: Fluctuations in the electron distribution in atoms and molecules result in a temporary dipole • region with excess electron density has partial (─) charge • region with depleted electron density has partial (+) charge • Exists among All molecules and atoms Tro: Chemistry: A Molecular Approach, 2/e

20. Dispersion Forces: Instantaneous Dipoles Nonpolar Somewhat polar Polar

21. Size of the Induced Dipole Strength of the induced dipole depends on: • Polarizability of the electrons (“fluidity of electron clouds”) • Size of atom • Molar mass = stronger attractions • Shape of the molecule • more surface-to-surface contact = larger induced dipole = stronger attraction Tro: Chemistry: A Molecular Approach, 2/e

22. Effect of Molecular Sizeon Size of Dispersion Force The Noble gases: nonpolar atomic elements molar mass increases  #electrons increases.  strength of dispersion forces increases.  the higher the boiling point will be. Tro: Chemistry: A Molecular Approach, 2/e

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24. Boiling Points of Nonpolar n-Alkanes Tro: Chemistry: A Molecular Approach, 2/e

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26. Molecular ShapeAffects Dispersion Force the larger surface-to- surface contact between molecules in n-pentane results in stronger dispersion force attractions Tro: Chemistry: A Molecular Approach, 2/e

27. Alkane Boiling Points • Branched chains have lower BPs than straight chains • The straight chain isomers have more surface-to-surface contact Tro: Chemistry: A Molecular Approach, 2/e

28. Practice – Choose the Substance in Each Pair with the Higher Boiling Point a) CH4 C4H10 b) C6H12 C6H12 Tro: Chemistry: A Molecular Approach, 2/e

29. Practice – Choose the Substance in Each Pair with the Higher Boiling Point Both molecules are nonpolar larger molar mass a) CH4 CH3CH2CH2CH3 b) CH3CH2CH=CHCH2CH3 cyclohexane Both molecules are nonpolar, but the flatter ring molecule has larger surface-to-surface contact Tro: Chemistry: A Molecular Approach, 2/e

30. Dipole–Dipole Attractions • Polar molecules have a permanent dipole • because of bond polarity and shape • dipole moment • as well as the always present induced dipole • The permanent dipole adds to the attractive forces between the molecules • raising the boiling and melting points relative to nonpolar molecules of similar size and shape Tro: Chemistry: A Molecular Approach, 2/e

31. Effect of Dipole–Dipole Attraction on Boiling and Melting Points Tro: Chemistry: A Molecular Approach, 2/e

32. replace with the figure 11.8 Tro: Chemistry: A Molecular Approach, 2/e

33. or Practice – Choose the substance in each pair with the higher boiling point a) CH2FCH2F CH3CHF2 b) Tro: Chemistry: A Molecular Approach, 2/e

34. or Practice – Choose the substance in each pair with the higher boiling point a) CH2FCH2F CH3CHF2 more polar b) polar nonpolar Tro: Chemistry: A Molecular Approach, 2/e

35. Hydrogen Bonding • When H is bonded to F, O, N (very electronegative), bonding electron is pulled away from H • O─H, N─H, or F─H • With its electron is pulled away, the nucleus of H becomes deshielded (“stripped of electron”) • exposing the H proton • The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules Tro: Chemistry: A Molecular Approach, 2/e

36. HF, H2O, and NH3 have unusually high boiling points due to hydrogen bonding Polar molecules XH3, H2X, and HX have both dispersion forces and dipole–dipole attractions, have higher boiling points than the corresponding Group 4 molecules. For nonpolar molecules XH4 only dispersion forces exists: higher molar mass cause higher boiling point Tro: Chemistry: A Molecular Approach, 2/e

37. H-Bonding HF Water Tro: Chemistry: A Molecular Approach, 2/e

38. H-Bonds • Hydrogen bonds are very strong intermolecular attractive forces • stronger than dipole–dipole or dispersion forces • Substances that can hydrogen bond will have higher boiling points and melting points than similar substances that cannot • But hydrogen bonds are not nearly as strong as chemical bonds • 2 to 5% the strength of covalent bonds Tro: Chemistry: A Molecular Approach, 2/e

39. Effect of H-Bonding on Boiling Point Tro: Chemistry: A Molecular Approach, 2/e

40. Example: One of these compounds is a liquid at room temperature (the others are gases). Which one and why? MM = 30.03 Polar No H-Bonds MM = 34.03 Polar No H-Bonds MM = 34.02 Polar H-Bonds Formaldehyde: dispersion forces: MM 30.03, trigonal planar dipole–dipole: very polar C=O bond uncancelled H-bonding: no O–H, N–H, or F–Htherefore no H-bond Only the hydrogen peroxide also has additional hydrogen bond attractions Tro: Chemistry: A Molecular Approach, 2/e

41. Practice – Choose the substance in each pair that is a liquid at room temperature (the other is a gas) a) CH3OH CH3CHF2 b) CH3-O-CH2CH3 CH3CH2CH2NH2 can H-bond can H-bond Tro: Chemistry: A Molecular Approach, 2/e

42. Attractive Forces and Solubility • Solubility depends, in part, on the attractive forces of the solute and solvent molecules • like dissolves like • miscible liquids will always dissolve in each other • Polar substances dissolve in polar solvents • hydrophilic groups = OH, CHO, C=O, COOH, NH2, Cl • Nonpolar molecules dissolve in nonpolar solvents • hydrophobic groups = C-H, C-C • Many molecules have both hydrophilic (like water) and hydrophobic (dislike water) parts – solubility in water depends Tro: Chemistry: A Molecular Approach, 2/e

43. Immiscible Liquids Pentane, C5H12 is a nonpolar molecule. Water is a polar molecule. The attractive forces between the water molecules is much stronger than their attractions for the pentane molecules. The result is the liquids are immiscible. Tro: Chemistry: A Molecular Approach, 2/e

44. Dichloromethane (methylene chloride) Water Ethanol (ethyl alcohol) Polar Solvents Tro: Chemistry: A Molecular Approach, 2/e

45. Nonpolar Solvents Tro: Chemistry: A Molecular Approach, 2/e

46. Ion–Dipole Attraction • In a mixture, ions from an ionic compound are attracted to the dipole of polar molecules • The strength of the ion–dipole attraction is one of the main factors that determines the solubility of ionic compounds in water Tro: Chemistry: A Molecular Approach, 2/e

47. Practice – Choose the substance in each pair that is more soluble in water a) CH3OH CH3CHF2 b) CH3CH2CH2CH2CH3 CH3Cl can H-bond with H2O more polar Tro: Chemistry: A Molecular Approach, 2/e

48. Summary • Dispersion forces: the weakest of the intermolecular attractions, present in all molecules and atoms, increases with molar mass • Polar molecules also have dipole–dipole attractive forces Tro: Chemistry: A Molecular Approach, 2/e

49. Summary (cont’d) • Hydrogen bonding: the strongest of the intermolecular attractive forces apure substance can have, present when a molecule has H-O, H-N, H-F • only example of H bonded to F is HF • Ion–dipole attractions: present in mixtures of ionic compounds with polar molecules, the strongest intermolecular attraction, especially important in aqueous solutions of ionic compounds Tro: Chemistry: A Molecular Approach, 2/e

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