Everything You Wanted To Know About IFA

1. Everything You Wanted To Know About IFA And Were Afraid I Would Make You Learn.

2. + + + + H H C H H H H C H H H H C H H H H C H H + + + + + + + + + + + + London Dispersion Forces nonpolar molecules often repel each other.

3. + + + H H C H H H H C H H H H C H H + + + + + + + + + + H H C H H + + + LDF At Work + + H H C H H H H C H H + – + + momentary attraction (LDF) + – Loss of LDF: Repulsive forces take over momentary attraction (LDF)

4. I sometimes use the metaphor of skaters on an ice rink, to describe how LDF build. Sometimes the skaters (the electrons) get bunched up creating a temporarily induced (instantaneous) dipole. Wait a second for this video from: http://www.youtube.com/watch?v=3t1Jn_jrsQkto load and see if this helps you to understand the origin or development of LDF. If it does not load, then click on the url and take a look.

5. As the number of electrons increases, the possibility of these induced dipoles between molecules increases… Thus LDF can be considered to be cumulative and can be cited for the vast number physical properties of a substance. Each LDF is weak, but when there are multiple LDF forces generated, the cumulative result can be impressive. Think about propane (C3H8) and paraffin wax (C35H72) or the phases of the group 17 elements: F2(g), Cl2(g) Br2(l), I2(s)

6. Dipole Attractions - + Moderate in strength – but they are present between dipoles … as well as LDF The + and - are more permanent and thus exert a more long term effect of attraction. H S H - + + H S H - + H S H + +

7. The Wonder Of Hydrogen Bonds (Please remember that they ain’t bonds! They are a form of IFA.)

8. Some Facts • H-bonding is strongest between one molecule with hydrogen bonded to F, O, N (via a polar covalent bond) & a second molecule with atoms of F, O, N • H-Bonds are found between “super-dipole” molecules • These super-dipole molecules have unusually large partial ± charges.

9. – + – Te +     H O H  + H   + H Partial Charges Compared Oxygen is so small, the extra electrons take on extra significance. This is true for fluorine and nitrogen as well (F,O,N). Look at the size of the partial charges. Compare the effect of extra electrons on a larger atom, like tellurium. Look at the difference in the size of the partial charges.

10. The Water Molecule I am all that and a bag of chips 2.1 3.5   O H       2.1 H

11. Electronegativity Drag Wish You Were Here My Lovley e- Ah Yes, My Little Chickadees! Come to me!   O H       Write! H

12. Is it cold in here – or is it just me? The Effects Absolute Power Corrupts Absolutely! +   O – H     I know I had an e- when I came into this bond + H

13. The Effects +   O – H     The isolation of the electrons on such a small radius (such as atoms of F,O,N) coupled to a very high electronegativity creates a dipole that is extreme. This extremely large partial charge creates very strong IFA. + H

14. •• •• •• •• •• •• •• •• •• •• •• •• H O H H O H H O H H O H H O H H O H Interactions Between Water Molecules HYDROGEN BOND

15. •• •• H O H •• •• •• •• •• H O H •• •• •• H O H H O H H O H Hydrogen bonds are very strong – they hold the molecules together to create the liquid phase. It takes energy to separate the molecules from each other. •• •• H O H 500 kJ

16. •• •• •• •• •• •• •• •• •• •• •• •• H O H H O H H O H H O H H O H H O H As heat is added, the H-bonds weaken – and average kinetic energy increases- but a phase change (e.g. boiling) doesn’t come easily. 1,500 kJ

17. H O H •• •• •• •• •• •• •• •• •• •• H O H H O H H O H H O H H O H •• •• Finally! Boiling Occurs 2,259 kJ Heat of Vaporization

18. Other Molecules and Hydrogen Bonding Remember, atoms of N,O,F are so small, yet so electronegative that the strongest level of H-bonds occur. These strong IFA account for unusually high boiling points and lower vapor pressures

19. Normal Boiling Points for Group 16 Compounds of Hydrogen

20. Normal Boiling Points for Group 16 Compounds of Hydrogen 100 Normal Boiling Point (ºC) 0 -2 -42 -62 H2O H2Te H2Se H2S 18 34 81 130 Molecular Mass (amu)

22. H Bonds & Nucleic Acids

23. Nucleic Acids • Every somatic (body) cell of a human being possesses 1.5 meters of DNA. • That is the equivalent of 4.9 feet per cell! • Now, how strong are Hydrogen Bonds?

26. CONCLUSIONS • There are many classifications of IFA • The classification depends upon the strength of the  charges on the atoms of a molecule and the extent of the attraction between molecules • Some IFA are stronger than other types • LDF are found between all molecules (due to the existence of electrons) and are the weakest, but cumulative. • H-bonds are a specialized form and are the strongest.