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Packet 11

Packet 11. Liquids, Solids, and Intermolecular Forces. Concept Area I: Terminology. intermolecular forces induced dipole-induced dipole dipole-induced dipole dipole-dipole hydrogen-bonding ion-dipole miscible like dissolves like soap detergent surface tension viscosity

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Packet 11

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  1. Packet 11 Liquids, Solids, and Intermolecular Forces

  2. Concept Area I: Terminology • intermolecular forces • induced dipole-induced dipole • dipole-induced dipole • dipole-dipole • hydrogen-bonding • ion-dipole • miscible • like dissolves like • soap • detergent • surface tension • viscosity • capillary action • vaporization • vapor pressure • condensation • volatile and volatility • nonvolatile • heat of vaporization, ΔHvap • enthalpy • exothermic • endothermic • equilibrium • boiling point • normal boiling point • critical temperature, Tc • critical pressure, Pc • sublimation • deposition • melting point • melting / fusion • freezing • heat of fusion, ΔHfus • phase diagram • critical point • triple point • ionic • metallic • molecular • network • amorphous • crystalline • crystal lattice • unit cell CHE 170 Packet 11 - 2

  3. Concept Area II: Intermolecular Forces • You should be able to explain, recognize and give examples of the important interactions that occur between molecules: ion-dipole, hydrogen-bonding, dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole. • You should be able to explain why certain compounds do and do not form solutions based on the principle of “like dissolves like”. • You should understand and be able to describe how soap and detergent works. • You should understand the consequences of intermolecular forces: relative strengths, utility in “real-world” situations, trends in volatility, vapor pressure, boiling point. • You should be able to explain capillary action. CHE 170 Packet 11 - 3

  4. Intermolecular Forces Crossword Puzzle Handout Intra- versus Inter- molecular Forces • intramolecular forces: these are the forces, we call them bonds, that hold a compound together. They can be ionic or covalent bonds. • intermolecular forces: these are the forces that attract one compound to another; they are forces between different compounds. CHE 170 Packet 11 - 4

  5. States of Matter Let’s compare the three states of matter… most least volume distance between particles motion forces between particles Make sense? Now let’s take a closer look at forces… CHE 105 Packet 13 - 5

  6. Why are molecules/ions/atoms attracted to each other? • Attractions between two chemical species happen because opposites attract! For instance, • cations and anions • partially positive side of a polar molecule to the partially negative side of a polar molecule • even nonpolar molecules can and do have temporary charges that will attract to each other • Key points about attractions between two chemical species • larger the charge the stronger the attraction • greater the distance the weaker the attraction • However, these attractive forces between two chemical species are small relative to the bonding forces between atoms. • generally smaller charges • generally over much larger distances bond CHE 170 Packet 11 - 6 Tro page 392

  7. The weakest intermolecular force! Why do nonpolar covalent molecules stick together in liquids and solids? induced dipole-induced dipole Forcesaka: Dispersion / London / van der Waals Forces • We’ve studied how electrons behave and know that we can describe their location using probabilities. • Well, at any given time, the electrons can be where they want. Sometimes a group will congregate on one side… this makes it a bit polar –an induced dipole! This now “polar” speciescan cause another one to also “polarize” and thus they are mildly attracted! CHE 170 Packet 11 - 7 Tro: page 393

  8. CHE 170 Packet 11 - 8 top image: Tro page 393

  9. CHE 170 Packet 11 - 9

  10. What do we look for to determine strength of induced dipole? Induced dipole attraction is stronger when: • molar mass is more • molecular shape is elongated CHE 170 Packet 11 - 10 Tro page 394

  11. Why is I2 a solid and F2 a gas? CHE 170 Packet 11 - 11

  12. Why does octane have a higher m.p. & b.p. than isooctane when the have the same molar mass? CHE 170 Packet 11 - 12

  13. Are you ready to predict? • Should nitrogen or oxygen have a higher boiling point? • Should hexane, CH3CH2CH2CH2CH2CH3, or 2,2-dimethylbutane, , have a higher boiling point? CHE 170 Packet 11 - 13

  14. This intermolecular force is a bit stronger! How does a nonpolar covalent compound dissolve in a polar covalent compound? • Well, if two nonpolar molecules are attracted because of temporary dipoles…. • It stands to reason that if we add a nonpolar solutethat can be polarized to a polar solvent which has a permanent dipole, the polar solvent polarizes the nonpolar molecule so that it can be solvated or dissolve to form a solution! • Warning! Our text never mentions this one! CHE 170 Packet 11 - 14

  15. CHE 170 Packet 11 - 15

  16. This intermolecular force is even stronger! Dipole-Dipole Interactions • In molecules with a permanent dipole moment (polar molecules), molecules are always attracted to each other since positive attracts negative. • Thus, these are stronger attractions than those where a dipole must be induced. CHE 170 Packet 11 - 16 Left image: Tro page 395

  17. This intermolecular force is even stronger! Hydrogen-Bonding:a special case of dipole-dipole • Some polar molecules have an even stronger permanent dipole; and thus, the intermolecular force is stronger. • We can recognize one partner in an H-bond because they will always have: • a hydrogen bonded to a nitrogen – strong • a hydrogen bonded to an oxygen – stronger • a hydrogen bonded to a fluorine – strongest • Since a hydrogen always participates in an H-bond, that’s how it got its name. • Just remember, an H-bond is an intermolecular force despite the name! CHE 170 Packet 11 - 17

  18. Hydrogen-Bonding:a special case of dipole-dipole CHE 170 Packet 11 - 18 Left image: Tro page 397

  19. Hydrogen-Bond Givers and Receivers Most molecules that can H-bond both “give” and “receive” an H-bond. However, some molecules can only “receive” and H-bond like the O is doing here and here. CHE 170 Packet 11 - 19

  20. Application! • The more molecules are attracted to each other the likely they want to “leave” each other. Thus, boiling point . So does the freezing point. • Conversely, the less molecules are attracted to each other the likely they want to “leave” each other. Thus boiling point . So does the freezing point. CHE 170 Packet 11 - 20

  21. Intermolecular forces can affect physical properties! Because water is bent, it is a polar molecule. Thus, water molecules stick together (due to H-bonding) making its boiling point higher than predicted. CHE 170 Packet 11 - 21 similar image and chart on Tro page 397 (and 416)

  22. Intermolecular Forces (IMF) Handout Quick Quiz Review! • What is the ranking from weakest to strongest imf again? • What is the only imf that a nonpolar compound can have? • What are the strongest imf’s that polar compounds can have? • What imf can only be found in solutions? CHE 170 Packet 11 - 22

  23. Little Group Quiz! Rank each of the following in order of increasing boiling point. Explain your reasoning. • CH3(CH2)3CH3 CH3(CH2)3OH CH3CH2OCH2CH3 • CH3CH3 CH3F CH3OH • H2O NH3 CH4 CH3CH3 CHE 170 Packet 11 - 23

  24. What about ions?How can we explain their intermolecular forces? Within an ion (intramolecular) we have ion-ion forces CHE 170 Packet 11 - 24

  25. The strongest intermolecular force! What about ions?How can we explain their intermolecular forces? Within an ion (intramolecular) we have ion-ion forces For ions in solution we have ion-dipole forces So, what happens if we put an ionic compound into a solvent that has no dipole moment? CHE 170 Packet 11 - 25 Tro page 398

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  27. Dissolving an ionic compound in a polar solvent: • Ions in solution we have ion-dipole forces • The dipoles on our polar molecules spread out the opposite charges and surround them. CHE 170 Packet 11 - 27

  28. Okay, so we now know the intermolecular forces if we only have one compound present and we know which imf’s are only found in solutions. • But, how do we know if a solution will form? Let’s apply intermolecular forces to solutions (we’ll do more formally in CHE 180 in chapter 12)! • If we mix two different chemical species, what determines whether they are soluble in each other or not? • Sometimes dissimilar compounds can dissolve: dipole-induced dipole and ion-dipole. But, compounds always prefer to dissolve in “like” compounds. CHE 170 Packet 11 - 28

  29. Water is a molecule that can with itself. I2 is a molecule that has with itself. CCl4 is a molecule that has what type of imf with itself?. Now, I2 can dissolve in water using , but if given the choice between polar water and nonpolar CCl4, I2 will choose CCl4 as these pictures show. Understand?

  30. The Shape of a Molecule can affect physical properties! • Because water is bent, it is a molecule. • So, water molecules tend to stick together (like little magnets). • Vegetable oil is made up of mostly C–H bonds which makes it a . molecule. • So when we mix, these two separate and can’t mix just like a mixture of marbles and spherical magnets! Let’s try it! CHE 170 Packet 10 - 30 Tro page 356

  31. Like Dissolves Like Quiz Determine if the following will form solutions or not. Why? • O2 in H2O • CH3(CH2)6CH3 in H2O • NaCl in H2O • CH2Cl2 in NH3 • CH2Cl2 in CH3(CH2)6CH3 • CH3COOH in H2O CHE 170 Packet 11 - 31 Whole slide (with answers) shown on notes page.

  32. An IMF application:Why do we need soap, won’t water alone work? • Well, most dirt consists of nonpolar molecules like oil. • Water is a molecule. Thus, it dissolve or wash away oil. • What do we do?!?! CHE 170 Packet 11 - 32

  33. How do soaps work? • Soaps are both polar and nonpolar. They are nonpolar so they’ll dissolve into . and polar so that they’ll dissolve in . Notice the nonpolar tail has all C’s and H’s. The polar head contains C & O’s. CHE 170 Packet 11 - 33

  34. Problem with Soap • Soap works great, except in hard water. Why? • Well, unfortunately soap forms insoluble salts with calcium ions, iron ions and magnesium ions. This leads to soap scum. Yuck! • Guess what is found in hard water? • So, what do we do?!?!?! • Synthetic soaps, called detergents, work just like soap (nonpolar tail for dirt, polar head for water), but their calcium, iron, and magnesium salts tend to be soluble! Detergent Solution CHE 170 Packet 11 - 34

  35. IMF application: Capillary Action • Capillary action is the interaction between a solvent with itself and a surface – both are IMF. • With water and glass: • It’s a dipole-dipole interaction between the water & glass as we can see here in the top picture. • The bottom picture shows the two forces at work: adhesive is water attracted to the glass and cohesive is water attracted to itself. CHE 170 Packet 11 - 35

  36. More Aspects of Capillary Action • The smaller the glass tube, the higher water can climb. CHE 170 Packet 11 - 36

  37. stronger cohesive forces adhesive forces water mercury Water forms a concave meniscus, whereas mercury forms a convex meniscus. The different shapes are determined by the adhesive forces of the molecules of the liquid and the adhesive forces of the molecules of the liquid with the walls of the tube and the cohesive forces between the molecules of the liquid. CHE 170 Packet 11 - 37 see similar pictures on Tro page 401

  38. “Real World” Situations Situation 1: Antifreeze which is ethylene glycol (HOCH2CH2OH) dissolves in water; whereas, motor oil (large hydrocarbons) does not dissolve, it floats on top of water. • Why does one dissolve and the other not? • Why does the oil float and not sink? CHE 170 Packet 11 - 38

  39. “Real World” Situations Situation 2: An insect or paperclip can “float” on water but not hexane. Why? CHE 170 Packet 11 - 39 similar image on Tro page 399

  40. More on Surface Tension hydrogen bonding occurs across the surface and below the surface the net vector for attractive forces is downward hydrogen bonding occurs in three dimensions CHE 170 Packet 11 - 40

  41. “Real World” Situations Situation 3: Cooking oil contains structures like this: Why is cooking oil more viscous than water? CHE 170 Packet 11 - 41

  42. “Real World” Situations Situation 4: Salting the french-fry oil doesn’t make the potatoes salty. Why? CHE 170 Packet 11 - 42

  43. Quick Quiz Review, again! • What is the ranking from weakest to strongest imf again? • Is that last force really an imf? If not, what is it? • What is the only imf that a nonpolar compound can have? • What are the strongest imf(s) that polar compounds can have? • What imf(s) can only be found in solutions? CHE 170 Packet 11 - 43

  44. Concept Area III: State Changes • You should be able to sketch or interpret a phase diagram and define the physical processes represented on the diagram. • You should be able to characterize the important processes that occur with liquids: vaporization, vapor pressure, boiling point, critical point, temperature, pressure. • You should be able to characterize the important processes that occur with solids: melting (fusion), freezing, sublimation. CHE 170 Packet 11 - 44

  45. States of Matter Solid Liquid Gas We have learned that the speed of the particles determines what state the compound will have. So, what determines the particles’ speed? and to a lesser extent. CHE 170 Packet 11 - 45

  46. Exothermic and Endothermic • We haven’t covered chapter 6 yet. So, we need to stop and learn some terms. • Enthalpy, H, is the amount of energy in a system. We usually measure ΔH, the change in energy during a reaction or process. • Exothermic and endothermic are the terms applied to indicate which way the heat went during a process. • Some reactions/processes require heat to go and are called: . Their ΔH values are . • Some reactions/processes give off heat when they go and are called: . Their ΔH values are . CHE 170 Packet 11 - 46

  47. Properties of Liquids • Molar Enthalpy or Heat of Vaporization, DHvap • The heat needed to convert 1 mol of liquid to gas. This process is called or . • Does it take heat or give off heat? • So, should DHvap be positive or negative? • Therefore is it exothermic or endothermic? • What if we take a gas and convert to liquid? This process is called . • In , what happens with… • Heat? • DHcond? • Process? condensation CHE 170 Packet 11 - 47

  48. Why? • Why do we sweat when we’re hot? • Why do cats and dogs pant when hot? • Can we cool a hot apartment by opening the door on the refrigeration? • Why does a glass get wet on the outside when it has a cold drink. Why not for a hot drink? Why doesn’t a cold drink get wet in the winter? • Why does rubbing alcohol feel cooler on skin than water even when both are the same temperature? CHE 170 Packet 11 - 48

  49. Vapor Pressure • At any given temperature, a few liquid particles will escape into the gas phase. A few gas particles will also condense into liquid. • Why is that? (Need a hint? Maxwell-Boltzmann.) • So, all particles are not moving at the same speed! • The pressure exerted by the evaporating molecules of a liquid is called vapor pressure. • So, since rubbing alcohol evaporates quicker than water, its vapor pressure is than water’s. CHE 170 Packet 11 - 49

  50. What happens to the vapor pressure as we heat a liquid? • Why? • What happens when the vapor pressure equals the external pressure (usually atmospheric)? • Finally, what is in the bubbles of a boiling liquid after it has been boiling for several minutes? CHE 170 Packet 11 - 50

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