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TOPIC: Intermolecular Forces Part 1: Dispersion Forces Do Now: How do particle diagrams of liquids & solids compare to those of gases?

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Presentation on theme: "TOPIC: Intermolecular Forces Part 1: Dispersion Forces Do Now: How do particle diagrams of liquids & solids compare to those of gases?"— Presentation transcript:

1 TOPIC: Intermolecular Forces Part 1: Dispersion Forces Do Now: How do particle diagrams of liquids & solids compare to those of gases?

2 SOLID LIQUIDGAS

3 Why do some substances exist as gases, some as liquids, and some as solids at room temp?

4 Part of answer has to do with forces between separate molecules (called intermolecular forces)

5 Intermolecular forces between molecules. They are weaker. Intramolecular forces are between individual atoms (we will learn this later) Intramolecular forces Intermolecular forces

6 Intermolecular Forces=IMF Inter means “between” or “among” Intermolecular forces = forces between neighbouring compounds

7 ***Separation of charge is responsible for the forces between the Molecules*** … Most atoms don’t have a charge, unless they are ions, so we often refer to them as having partial charges and write them like this

8 1. Dispersion Forces (van der waals) 1. Dispersion Forces (van der waals): ● weakest IMF ● occur between nonpolar (symmetrical) molecules ● Click here for animation (slide 4 of 13) Click here for animation NonnoNonpolar = no poles (no + or -) Can’t tell one end of molecule from other end electrons are evenly distributed

9 instantaneous and momentary fluctuate results from motion of electrons induce if charge cloud not symmetrical will induce asymmetry in neighbor’s charge cloud!

10 4 categories of Nonpolar Molecules - all these have DISPERSION FORCES (you need to memorize) Noble Gas –group 18:  He, Ne, Ar, Kr, Xe, Rn 7 diatomic elements  H 2, N 2, O 2, Cl 2, F 2, I 2, Br 2 Pure Hydrocarbons – molecules with only C and H  General formula C x H y : examples = CH 4, C 2 H 6, C 3 H 8 these 3 small symmetrical molecule  CO 2, CF 4, CCl 4

11 Dispersion Forces and Size The larger the molecule, the great the Dispersion forces = stronger IMF B/c, the larger the electron cloud, the greater the fluctuations in charge  Noble Gases: Rn has greater dispersion forces = strongest IMF  Diatomic Elements: I 2 is larger then F 2, so I 2 is larger (way more electrons) so greater dispersion forces, I 2 is a solid at room temp. F 2 is much smaller (less electrons) weaker dispersion forces, F 2 is a gas at room temp.

12 You try… Which has the greatest dispersion forces between it’s molecules? C 3 H 8 C 8 H 18 CH 4 C 5 H 12 Which is most likely a liquid/solid (not a gas) at room temp? C 3 H 8 C 8 H 18 CH 4 C 5 H 12 Which is most likely a gas at room temp? C 3 H 8 C 8 H 18 CH 4 C 5 H 12

13 The weaker the IMF, the lower the boiling point (BP) Br 2 = boils at 58.8°C, 137.8°F Compared to Water = boils at 100°C, 212°F So water must have stronger IMF

14 TOPIC: Intermolecular Forces Part 2: Dipole- Dipole and Hydrogen Bonding Do Now: List the 4 categories of Nonpolar Molecules – all of these have DISPERSION FORCES Noble Gas –group 18:  He, Ne, Ar, Kr, Xe, Rn 7 diatomic elements  H 2, N 2, O 2, Cl 2, F 2, I 2, Br 2 Pure Hydrocarbons – molecules with only C and H  General formula C x H y : examples = CH 4, C 2 H 6, C 3 H 8 these 3 small symmetrical molecule  CO 2, CF 4, CCl 4

15 All molecules have Dispersion forces (the regents calls these Van der Waals) 2 other types of forces (IMF): 1. Dipole-Dipole forces 2. Hydrogen bonds -if one of these are present, they are more important.

16 2. Dipole-dipole forces 2. Dipole-dipole forces: Stronger then dispersion forces occur between polar (asymmetrical) molecules (they have a partial charge at each pole – one is typically much larger than the other) Click here for animation (slide 3 of 13) Click here for animation

17 Dipole-dipole Forces & Polar Molecules Polar Molecule shows permanent separation of charge; has poles: one end partially (-) & one end partially (+); Asymmetrical

18

19 3. Hydrogen bonds 3. Hydrogen bonds: strongest IMF occur between molecules that have an : H-F H-O or H-N bonds ONLY Strongest Intermolecular Force Hydrogen Bonding Dipole-Dipole Dispersion

20 Hydrogen Bonding H-O N-H Occurs between molecules with H-F, H-O, or H-N bonds

21 Hydrogen Bonding Hydrogen bonding is extreme case of dipole-dipole bonding F, O, and N are all small and electronegative  strong electrons attraction  H has only 1 electron, so if being pulled away H proton is almost “naked” H end is always positive & F, O, or N end is always negative

22 Strength of Hydrogen Bonding Fluorine most electronegative element, so  H-F bonds are most polar and exhibit strongest hydrogen bonding, so strongest IMF  H-F is stronger than H-O which is stronger than H-N (H-bonding…sound like FON to me!!!)

23 O H H O H H H-Bonding = strongest IMF much harder to “pull” molecules apart

24 C Dispersion Forces= weakest IMF much easier to “pull” molecules apart C H H H H H H H H

25 Hydrogen bonding: strongest IMF influences physical props a great deal H-F > H-O > H-N IMF vs Physical Properties If IMF  then:  Boiling point   Melting point   Heat of Fusion   Heat of Vaporization  while:  Evaporation Rate  Change from solid to liquid w/o changing temp Change from liquid to gas w/o changing temp Rate at which conc. will go from liquid to gas

26 Why do some substances exist as gases, some as liquids, and some as solids at room temp? #1 reason = IMF

27  If IMF are strong, substance will be solid or liquid at room temp  Particles want to clump together  If IMF are weak, substance will be gas at room temp  Particles free to spread apart

28 Why do some substances exist as gases, some as liquids, and some as solids at room temp? #1 reason = IMF #2 reason = temperature (avg. KE)

29 Temp = average KE If we change T we change KE Increase KE will help “pull” molecules apart (overcome IMF)

30 Indicate type of IMF for each molecule: NH 3 Ar N 2 HCl HF Ne O 2 HBr CH 3 NH 2 Hydrogen bonding Dispersion forces Dipole-dipole forces Hydrogen bonding Dispersion Dipole-dipole Hydrogen bonding


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