1. When Intermolecular Forces Rule in Chemistry and Biology PGCC CHM 101 Sinex

2. PropertySolidLiquidGas Volumedefinite indefinite Shapedefiniteindefinite Relative densityhigh~1 g/mLvery low Compressibilitylow-none very high Fluidity/mobilitynoyes Particle packingclose far apart Diffusion ratevery slowmediumfast Motion of particles vibrations only translation rotation Forces between particles IMF’s or bonding IMF’s only none if ideal gas Comparison of properties for the states of matter

3. The liquid state of matter Molecules are closely packed but free to move about randomly. There must be a force of attraction between the molecules.

4. As molecules start to vaporize Molecules are great distances apart in gaseous state but free to move about randomly. What about in solids? Click hereClick here for a simulation of the states of matter

5. Force-distance relationship molecules in contact with each other For the F attraction proportional to 1/distance

6. Force-distance relationship 1

7. 2

8. 3

9. 4

10. 5 Less than 20% the strength at 5 molecular distances

11. The strength of IMF’s H-bonding (fixed distance ~200 pm) Ion-ion (1/r) Ion-dipole (1/r 2 ) Dipole-dipole (1/r 3 ) Ion-induced dipole (1/r 4 ) Dipole-induced dipole (1/r 6 ) Induced dipole-induced dipole (1/r 6 ) Increasing strengths of IMF Inverse functions of distance to various powers How do the other functions behave compared to 1/r? Click hereClick here for an interactive Excel spreadsheet to explore

12. Hydrogen bonding in water O H 186 pm … O-H 96 pm covalent bond Click hereClick here for Chime activity

13. AT pair GC pair Base Pairs in DNA: H-bonding Why not here? Click hereClick here for Chime activityClick hereClick here for Chime structure of DNA

14. Lipid Bilayer: Induced Dipole – Induced Dipole Interaction for Hydrocarbon Chains phospholipid Click here for Chime activity

15. What holds the iodine molecules together in solid iodine? 272 pm for I 2 covalent bond 427 pm between two molecules in crystalline structure About half an iodine atom space between molecules

16. Why do low molar mass substances that are non-polar molecules, such as H 2, N 2 O 2, or CO 2 have very low boiling points? Since they are non-polar, the IMF will be induced dipole- induced dipole, which is the weakest IMF. To get the molecules to interact with one another, the temperature needs to be decreased considerably (lower kinetic energy).

17. For an ideal gas we assume there are no interactions between the atoms or molecules. Why is this a good assumption? Atoms or molecules in the gaseous state at room conditions are too far apart for IMF’s to operate. IMF’s are short range attractive forces. Attractive forces become important at low temperatures or high pressures. WHY?

18. Some review questions 1.If a gas is compressed to high pressures, it will liquefy. Why? Click hereClick here for Chime structures to address the next two questions. 2.What holds a protein in the alpha- helix form? 3.How about a beta-sheet protein?

19. LESSON LEARNED All of biology is held together by chemistry! Go tell a biologist!!!!