Dipole-Dipole Interactions London Dispersion Forces

Transcription

1 CATALYST

2 Lesson Plan GLE Physical Science 22. Predict the kind of bond that will form between two elements based on electronic structure and electronegativity of the elements (e.g., ionic, polar, nonpolar) (PS-H-C5). 24. Describe the influence of intermolecular forces on the physical and chemical properties of covalent compounds (PS-H-C5).

3 Essential Questions 1. How is the bonding in molecular compounds different from the bonding in ionic compounds? 2. What factors affect molecular properties?

4 Objectives 1. How do electronegativity values determine the charge distribution in a polar bond? 2. How do the strengths of intermolecular attractions compare with the strengths of ionic and covalent bonds? 3. Why are properties of some covalent compounds so diverse?

5 Vocabulary u van der Waals forces u dipole interaction u dispersion force u hydrogen bond u network solid

6 Do Now Review Electronegativity

7 8.4B: INTERMOLECULAR FORCES

8 Intra vs Intermolecular Forces What happens when a +v and a ve charge are near each other? + -

9 Intra vs Intermolecular Forces What happens when a +v and a ve charge are near each other? ATTRACTION + -

10 Intra v. Inter molecular forces 1 Ionic and covalent bonds are intramolecular forces; they are STRONG and hold a compound or molecule together. Intra: inside Intermolecular forces WEAKLY hold together two DIFFERENT molecules. Inter: in-between

11 Van der Waals Forces 2 Two types 1. Dipole-Dipole, and 2. Dispersion Forces (London Force)

12 Dipole-Dipole Interaction 3 Dipole-Dipole interactions occur between two polar molecules. Ex: N-O, O-C, and C-N bonds! Higher EN will be -. Weaker than H-Bond

13 Dipole-Dipole Interaction N and C electronegativity: = H-C N

14 Dipole-Dipole Interaction 4 When C becomes slightly positive, it pulls the electron pair in the H-C bond closer to itself. This in turn makes neutralizes C but ends up making H slightly more positive. Thus the molecule becomes a dipolar molecule. + - H-C N

15 Dipole-Dipole Molecular Interactions: HCN - + N C-H - + N C-H - + N C-H + - H-C N + - H-C N + - H-C N - + N C-H - + N C-H - + N C-H

16 Dispersion Forces 5 The weakest of all forces. When molecules come together, the electrons from one molecule force the electrons on the other molecule to move to the opposite side. This shift causes a momentary attraction between the two molecules.

17 Dispersion Forces

18 Hydrogen Bonding 6 Hydrogen bonding occurs when H is bonded to N, O or F. These atoms are strongly electronegative. H will be slightly + and the other atom will be slightly -. O H H H N H H

19 Hydrogen Bonding 7 Hydrogen bonding is the strongest intermolecular force. O H H

20 Notes v INTRAmolecular forces (stronger): Strongest: Ionic bonds Polar covalent bonds Weakest: Nonpolar covalent bonds v INTERmolecular forces (weaker): Strongest: Hydrogen bonding Dipole-Dipole Weakest: London Forces v Stronger intermolecular forces means Higher boiling point Higher melting point Less volatile (evaporates less easily) Higher freezing point

21 Guided Practice: Example What is the strongest intermolecular forces in these molecules SO CH3 OH Check in this order: Hydrogen (N, O, F) Dipole (electronegativity) London (always has this, but it s the weakest)

22 Your Practice Example What is the strongest intermolecular forces in these molecules O H C CH3 H 3 C CH 2 OH Check in this order: Hydrogen (N, O, F) Dipole (electronegativity) London (always has this, but it s the weakest)

23 Your Practice Example What type of forces? Label any + or charges. Hydrogen, Dipole, London O H C NH 2

24 Your Practice Example What is the strongest intermolecular forces in these molecules Hydrogen, Dipole, London H H C H H OH C H H H

25 Exit Ticket 1. What is the strongest intermolecular force in ethanol? H C C OH H H H H 2. Why does propane have stronger IMFs than methane? Propane = CH 3 -CH 2 -CH 3 Methane = CH 4