Sparks CH301. WHY IS EVERYTHING SO DIFFERENT? Gas, Liquid or Solid? UNIT 3 Day 7

Transcription

1 Sparks CH301 WHY IS EVERYTHING SO DIFFERENT? Gas, Liquid or Solid? UNIT 3 Day 7

2 What are we going to do today? Discuss types of intermolecular forces. Compare intermolecular forces for different molecules. Discuss the effect of these forces on properties of substances.

3 INTERMOLECULAR FORCES: fact check Water and Ethanol attracted to charged rod. Hexane and Carbon Tetrachloride NOT attracted to charged rod. Explanation: presence of dipole moment How is it possible for Hexane and Carbon Tetrachloride to exist as liquids?

4 INTERMOLECULAR FORCES: fact check How is it possible for Hexane and Carbon Tetrachloride to exist as liquids? THERE MUST BE AN ELECTROSTATIC ATTRACTION! EXPLANTION: INDUCED DIPOLE INDUCED DIPOLE

5 INTERMOLECULAR FORCES: Induced dipole Induced dipole Exist in all condensed substances Caused by induced dipoles, which come and go in nonpolar molecules and atoms E 1 r 6

6 Look at the table on p. 73 of your course packet. 1. Comment on the correlation of molecular weight and boiling point for the first four substances (methane octane). What do you think this says about the strength of the induced dipole-induced dipole interactions as MW increases? 2. Now look at the last value on the table carbon tetrachloride. Does this fit with your MW:interaction strength correlation? Suggest another factor you think might be important in determining strength of interactions.

7 INTERMOLECULAR FORCES: Induced dipole Induced dipole Strength depends on polarizability

8 INTERMOLECULAR FORCES: fact check STRENGTH DEPENDENT ON POLARIZABILITY CCl 4, (154 g/mol) boiling point = 77 C C 8 H 18 (114 g/mol) boiling point = 126 C Strength also depends on SHAPE of molecule

9 Hard question.. Why can t the charged glass rod induce a dipole? Lots of common examples: oils versus fats plastic wrap vs milk jugs FAVORITE ANALOGY - VELCRO

10 INTERMOLECULAR FORCES Induced dipole induced dipole This type of intermolecular force goes by several different names: Induced dipole Induced dipole Dispersion Forces London Forces Van der Waal s Forces

11 Answer question #3 on page 74 using data from your chart. Why do you think this is true?

12 Hydrogen bonding* Subset of dipole-dipole interactions Molecules with H attached to F, O, or N * Hydrogen Bond a misnomer implies a chemical bond; just a very strong dipole-dipole interaction.

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14 Hydrogen bonding* Strength depends on distance and dipole moment.

15 Quiz: CLICKER QUESTION Draw dot structures for ethyl alcohol, CH 3 CH 2 OH and consider if hydrogen bonding would occur in a sample of this substance. A. Yes, this molecule would exhibit hydrogen bonding. B. No, this molecule would not exhibit hydrogen bonding.

16 Quiz: CLICKER QUESTION Would hydrogen bonding occur in a sample of CH 3 CH 2 OCH 2 CH 3? A.Yes. B.No.

17 WHAT HAVE WE LEARNED TODAY? PHYSICAL PROPERTIES DEPEND ON COMPOSITION & SHAPE OF COMPOUND CLASSIFY INTERMOLECULAR FORCES ION-ION vs. DIPOLE-DIPOLE vs. INDUCED DIPOLE INDUCED DIPOLE PREDICT WHAT TYPE OF IMFs EXIST FOR A PARTICULAR COMPOUND

18 Learning Outcomes Define the three major types of intermolecular forces (IMF) discussed in class: dipole-dipole, H-bonding, and dispersion (London, van der Waals, induced dipole-induced dipole) Explain how molecular size and shape affect the magnitude of the dispersion forces