Slide 30 / 136 8 Intermolecular forces are strongest in A solids B liquids C gases
Slide 30 (Answer) / 136 8 Intermolecular forces are strongest in A solids B liquids C gases Answer A [This object is a pull tab]
Slide 31 / 136 9 A substance boils when the kinetic energy of its molecules A overcomes the intermolecular forces bonding them together B overcomes the intramolecular forces bonding them together C reaches 100 Celsius D none of the above
Slide 31 (Answer) / 136 9 A substance boils when the kinetic energy of its molecules A overcomes the intermolecular forces bonding them together B overcomes the intramolecular forces bonding them together C reaches 100 Celsius Answer D none of the above A [This object is a pull tab]
Slide 32 / 136 Types of Intermolecular Forces Return to Table of Contents
Slide 33 / 136 Intermolecular Forces There are three types of Intermolecular Forces (also known as van der Waals Forces) that bond molecules together: Dipole-dipole interactions London dispersion forces (LDF's) Hydrogen bonding
Slide 34 / 136 Dipole-Dipole Interactions A dipole is a polar molecule. Remember what makes a molecule polar? Bond Type Electronegativity Difference very small or zero Non-Polar Covalent about 0.2 to 1.6 Polar Covalent above 1.7 Ionic (between metal & non-metal)
Slide 35 / 136 Dipoles HF is an example of a polar molecule or dipole. The fluorine end of the molecule has higher electron density than the hydrogen end. H F We use the symbol to designate a dipole (2 poles). The "+" end is on the more positive end of the molecule and the arrow points towards the more negative end.
Slide 36 / 136 Dipole-Dipole Interactions The interaction between any two like charges is Molecules that have permanent repulsive (black) dipoles are attracted to each other. The positive end of one is attracted to the + negative end of the other and vice-versa. - - + These forces are only important when the - molecules are close to each other. - + + Only polar molecules will have this type - + - of Intermolecular Force. + The interaction between any two opposite charges is attractive ( red)
Slide 37 / 136 Dipole-Dipole Interactions The polarity of a molecule is measured by its dipole moment, m . The more polar the molecule, the greater its dipole moment. The more polar the molecule, the stronger the attraction between molecules, the higher the boiling point. Dipole Molecular Boiling Substance Moment Weight Point u(D) (k) (amu) Acetonitrile, CH 3 CN 41 3.9 355 Acetaldehyde, CH 3 CHO 44 2.7 294 Methyl chloride, CH 3 Cl 50 1.9 249 Dimethyl ether, CH 3 OCH 3 46 1.3 248 Propane, CH 3 CH 2 CH 3 44 0.1 231
Slide 38 / 136 10 Which of the molecules below will have the highest boiling point? Substance Molecular Dipole A CH 3 CH 2 CH 3 Wt. Moment 44 0.1 CH 3 CH 2 CH 3 B CH 3 OCH 3 46 1.3 CH 3 OCH 3 50 1.9 CH 3 Cl Answer C CH 3 Cl 44 2.7 CH 3 CHO D CH 3 CHO 41 3.9 CH 3 CN E CH 3 CN
Slide 39 / 136 11 Which of the following will have the lowest boiling point? Substance Molecular Dipole A CH 3 CH 2 CH 3 Wt. Moment B CH 3 OCH 3 44 0.1 CH 3 CH 2 CH 3 46 1.3 CH 3 OCH C CH 3 Cl 3 50 1.9 CH 3 Cl D CH 3 CHO 44 2.7 CH 3 CHO E CH 3 CN 41 3.9 CH 3 CN Answer
Slide 40 / 136 London Dispersion Forces London Dispersion Forces occur between all molecules. They result from the fact that electrons are in constant motion and sometimes are the same side of the molecule. When they are on one side, the molecule is polarized: one side is negative and the other is positive; the molecule acts like a dipole. δ - δ + - -
Slide 41 / 136 London Dispersion Forces That polarization creates an electric field that oppositely polarizes nearby molecules...leading to an attraction. δ - δ - δ + δ + - - - -
Slide 42 / 136 London Dispersion Forces While the electrons in helium atoms repel each other, they occasionally wind up on the same side of an atom. At that instant, the helium atom is polar, with an excess of electrons on one side and a shortage on the other. e- 2+ e- δ- δ+ Helium atom
Slide 43 / 136 London Dispersion Forces Another helium atom nearby becomes polarized as the electrons on the left side of the first atom repel the electrons in the second atom. electrostatic attraction e- e- 2+ 2+ e- e- Helium atom 1 Helium atom 2 London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole. δ- δ+ δ- δ+
Slide 44 / 136 Polarizability These forces are present in all molecules, whether they are polar or nonpolar. The tendency of an electron cloud to distort in this way is called polarizability . Because larger molecules have more electrons, they are more polarizable. Molecules with more electrons experience stronger London dispersion forces.
Slide 45 / 136 London Dispersion Forces Examine the trends among the Halogens and the Noble Gases: Number Boiling Number Noble Boiling Halogen of Point of gas point (K) electrons electrons (K) 2 F 2 18 85.1 He 4.6 238.6 Ne 34 10 Cl 2 27.3 332.0 Ar Br 2 70 18 87.5 457.6 Kr I 2 106 36 120.9 Xe 54 166.1 the greater the number of electrons, the more polarizable the particles are, resulting in stronger London dispersion forces.
Slide 46 / 136 12 Only polar molecules are bonded together by London dispersion forces. True False
Slide 46 (Answer) / 136 12 Only polar molecules are bonded together by London dispersion forces. True False Answer False: because electrons are always moving, nonpolar molecules can be temporarily polarized. LDF's occure between all types of molecules. [This object is a pull tab]
Slide 47 / 136 13 Molecules with more electrons experience stronger London dispersion forces. True False
Slide 47 (Answer) / 136 13 Molecules with more electrons experience stronger London dispersion forces. True False Answer True [This object is a pull tab]
Slide 48 / 136 14 Which of the following molecules will have the highest boiling point? A F 2 B Cl 2 C Br 2 D I 2
Slide 48 (Answer) / 136 14 Which of the following molecules will have the highest boiling point? A F 2 B Cl 2 Answer D C Br 2 D I 2 [This object is a pull tab]
Slide 49 / 136 15 Which of the following molecules will have the lowest boiling point? A F 2 B Cl 2 C Br 2 D I 2
Slide 49 (Answer) / 136 15 Which of the following molecules will have the lowest boiling point? A F 2 B Cl 2 Answer C Br 2 A D I 2 [This object is a pull tab]
Slide 50 / 136 16 Which of the following gases will have the highest boiling point? A He B Ne C Ar D Kr E Xe
Slide 50 (Answer) / 136 16 Which of the following gases will have the highest boiling point? A He B Ne Answer C Ar E D Kr E Xe [This object is a pull tab]
Slide 51 / 136 17 Which of the following gases will have the lowest boiling point? A He B Ne C Ar D Kr E Xe
Slide 51 (Answer) / 136 17 Which of the following gases will have the lowest boiling point? A He B Ne Answer A C Ar D Kr E Xe [This object is a pull tab]
Slide 52 / 136 Which Have a Greater Effect? Dipole-Dipole Interactions or Lond Dispersion Forces Dipole-Dipole London Dispersion Forces If two polar molecules If one molecule is much are of comparable larger than another , size , dipole-dipole dispersion forces will likely interactions are the determine its physical dominating force. properties. If molecules are nonpolar , dispersion forces will dominate, since all molecules experience dispersion forces.
Slide 53 / 136 Hydrogen Bonding The graph shows the boiling points for four polar and four non-polar compounds. For the non-polar series (CH 4 to SnH 4 ) boiling points increase with higher number of electrons. There are stronger dispersion forces due to greater polarizability.
Slide 54 / 136 Hydrogen Bonding Examine the boiling points for the four polar compounds (4,2,2 = bent) called Group 16 hydrides. First look at the trend from H 2 S to H 2 Te. The boiling points are higher than the non-polar series, and the boiling points increase with greater molecular weight/ greater numbers of electrons as expected. What is going on with water? Based on molecular weight/electron number, it should have the lowest boiling point among the polar compounds, but instead its boiling point is extremely high.
Slide 55 / 136 Hydrogen Bonding The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds .
Slide 56 / 136 Hydrogen Bonding Hydrogen bonding arises in part from the high electronegativity and small radius of nitrogen, oxygen, and fluorine. When hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed. F F Click here to watch an animation about Hydrogen Bonding
Slide 57 / 136 Hydrogen Bonding Water is the only substance that is less dense in the solid state than in the liquid state; therefore, solid water, or ice, floats on liquid water. If it didn't, life on Earth would be very different. For instance, lakes would freeze from the bottom and fish couldn't survive winters. Hydrogen bonding creates the space in ice that explains its low density. Click here to watch an animation of the Water - Ice transition
Slide 58 / 136 18 Which of the following molecules has hydrogen bonding as one of its intermolecular forces? A HF B HCl C HBr D HI E All of the above
Slide 58 (Answer) / 136 18 Which of the following molecules has hydrogen bonding as one of its intermolecular forces? A HF B HCl Answer C HBr A D HI E All of the above [This object is a pull tab]
Slide 59 / 136 19 Which of the following molecules has hydrogen bonding as one of its IMF's? A CH 3 F B CH 3 Cl C HBr D NO 2 E None of the above
Slide 59 (Answer) / 136 19 Which of the following molecules has hydrogen bonding as one of its IMF's? A CH 3 F B CH 3 Cl Answer A C HBr D NO 2 E None of the above [This object is a pull tab]
Slide 60 / 136 Ion-Dipole Interactions There is a fourth intermolecular force between ions and molecules that will be important as we explore solutions later this year. Ion-dipole interactions are not considered a van der Waals force. The ion-dipole forces cause ionic substances to dissolve in polar solvents. + - - + + - + - + _ - + - + - + + - Cation-dipole attractions Anion-dipole attractions
Slide 61 / 136 Summary of Are ions involved? No Yes Interactions Are polar molecules Are polar molecules involved? and ions both present? Yes No No Yes Are hydrogen atoms bonded to N, O, or F atoms? Yes No Hydrogen Dispersion Dipole-dipole Ion-dipole *Ionic bonding: forces only: forces: H 2 S, forces: Bonding: H 2 O, NH 3 Ar, I 2 CH 3 , Cl NaCl in H 2 O NaCl, KI Van der Waals Forces Click here to watch a summary of IMF
Slide 62 / 136 IMF Summary London Hydrogen- Dispersion Dipole-Dipole Bonding Forces Strongest IMF Stronger IMF Strength Weakest IMF Only polar All nonpolar Types of Only polar molecules with molecules; molecules molecules H bonded to All molecules N, O, or F Look at Look at given Look for H-N, When in number of Dipole H-O, or H-F doubt... electrons moment bonds
Slide 63 / 136 20 Which of the following has London dispersion forces as its only IMF? A PH 3 B H 2 S C HCl D SiH 4 E None of the above
Slide 63 (Answer) / 136 20 Which of the following has London dispersion forces as its only IMF? A PH 3 B H 2 S Answer C HCl D D SiH 4 E None of the above [This object is a pull tab]
Slide 64 / 136 21 How many of these substances would have dipole-dipole interactions? H 2 O CO 2 CH 4 NH 3 A 0 B 1 C 3 D 3 E 4
Slide 64 (Answer) / 136 21 How many of these substances would have dipole-dipole interactions? H 2 O CO 2 CH 4 NH 3 A 0 B 1 Answer C 3 C D 3 E 4 [This object is a pull tab]
Slide 65 / 136 22 Which of the following molecules will have the highest boiling point? A H 2 O B CO 2 C CH 4 D NH 3
Slide 65 (Answer) / 136 22 Which of the following molecules will have the highest boiling point? A H 2 O B CO 2 Answer C CH 4 A D NH 3 [This object is a pull tab]
Slide 66 / 136 23 Which of the following diatomic molecules has the highest boiling point? A N 2 B Br 2 C H 2 D Cl 2 E O 2
Slide 66 (Answer) / 136 23 Which of the following diatomic molecules has the highest boiling point? A N 2 B Br 2 C H 2 Answer B D Cl 2 E O 2 [This object is a pull tab]
Slide 67 / 136 24 Of the following diatomic molecules, which as the lowest boiling point? A N 2 B Br 2 C H 2 D Cl 2 E O 2
Slide 67 (Answer) / 136 24 Of the following diatomic molecules, which as the lowest boiling point? A N 2 B Br 2 C H 2 Answer C D Cl 2 E O 2 [This object is a pull tab]
Slide 68 / 136 25 Which one of the following derivatives of methane (CH 4 ) has the lowest boiling point? A CBr 4 B CF 4 C CCl 4 D CI 4
Slide 68 (Answer) / 136 25 Which one of the following derivatives of methane (CH 4 ) has the lowest boiling point? A CBr 4 B CF 4 Answer B C CCl 4 D CI 4 [This object is a pull tab]
Slide 69 / 136 26 Which one of the following derivatives of methane (CH 4 ) has the highest boiling point? A CBr 4 B CF 4 C CCl 4 D CI 4
Slide 69 (Answer) / 136 26 Which one of the following derivatives of methane (CH 4 ) has the highest boiling point? A CBr 4 B CF 4 Answer C CCl 4 D D CI 4 [This object is a pull tab]
Slide 70 / 136 IMF's and Physical Properties Return to Table of Contents
Slide 71 / 136 Intermolecular Forces Affect Many Physical Properties The strength of the attractions between particles can greatly affect the properties of a substance or solution.
Slide 72 / 136 Properties of Liquids: Viscosity Resistance of a liquid to flow is called viscosity . It is related to the ease with which molecules can move past each other. Viscosity increases with stronger intermolecular forces and decreases with higher temperature. Which liquid to the right is more viscous? Substance Formula Viscosity ( kg/m-s) Hexane CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 3.26 x 10 -4 Heptane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 4.09 x 10 -4 Octane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 5.42 x 10 -4 Nonane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 7.11 x 10 -4 Decane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 1.42 x 10 -3
Slide 73 / 136 Properties of Liquids: Surface Tension Surface tension results from the net inward force experienced by the molecules on the surface of a liquid.
Slide 74 / 136 Properties of Liquids: Surface Tension The surface tension of a liquid is directly related to the attractive forces between its molecules. The stronger the attractive forces the more surface tension is needed to increase the surface area of the liquid. Water has a relatively high surface tension 7.29 x 10 -2 J/m 2 at 20 ℃ . However, mercury has an even higher surface tension: 4.6 x 10 -1 J/m 2. What do you think could cause mercury to have such a high surface tension relative to water?
Slide 74 (Answer) / 136 Properties of Liquids: Surface Tension The surface tension of a liquid is directly related to the attractive forces between its molecules. The stronger the attractive forces the more surface tension is needed to increase the surface area of Answer the liquid. Mercury has metallic bonds which are stronger than hydrogen bonds. Water has a relatively high surface tension 7.29 x 10 -2 J/m 2 at 20 ℃ . [This object is a pull tab] However, mercury has an even higher surface tension: 4.6 x 10 -1 J/m 2. What do you think could cause mercury to have such a high surface tension relative to water?
Slide 75 / 136 27 A substance's viscosity is directly proportional to the strength of its intermolecular forces? True False
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