Intermolecular forces 12/10/99 Questions Why do some solids dissolve in water but others do not? Why are some substances gases at room temperature, but others are liquid or solid? The answers have to do with … Intermolecular forces IEMF

Intermolecular forces 12/10/99 Intermolecular forces Overview There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces We have already looked at intramolecular bonds (IAMF)(ionic, covalent- the actual bond) Intermolecular forces (IEMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule) IEMFs come in three flavors: 1) dipole - dipole, 2) H-bonding, 3) London forces

Dipole - Dipole attractions 12/10/99 We have seen that molecules can have a separation of charge This happens in both ionic and polar bonds (the greater the END, the greater the dipoles) H Cl + – Molecules are attracted to each other in a compound by these +ve and -ve forces + – + – + – + –

H - bonding 12/10/99 H-bonding is a special type of dipole - dipole attraction that is very strong It occurs when N, O, or F are bonded to H Q- Calculate the EN for HCl and H2O A- HCl = 2.9-2.1 = 0.8, H2O = 3.5-2.1 = 1.4 The high END of N-H, O-H, and H-F bonds cause these to be strong forces (about 5x stronger than normal dipole-dipole forces) They are given a special name (H-bonding) because compounds containing these bonds are important in biological systems Remember: H-bonding is FON

London forces 12/10/99 Non-polar molecules do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids? London forces are named after Fritz London (also called van der Waal forces) (some textbooks lump London and Dipole Dipole together as Van der Waal) London forces are due to small temporary dipoles that exist in non-polar molecules Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical The resulting tiny dipoles cause attractions between atoms/molecules momentarily

Evidence of London forces London forces are the weakest of all the IEMFs, London forces are found in all molecules London forces become more significant as the molecule gets larger since a larger molecule has more electrons and greater possibility of distortion of the electron cloud CH4 (g), C8H18 (l), C10H22 (s) All non polar molecules only have London forces as IEMFs

London forces Instantaneous dipole: Induced dipole: + - + - - + 12/10/99 Instantaneous dipole: Induced dipole: + - + - - + Eventually electrons are situated so that tiny dipoles form A dipole forms in one atom or molecule, inducing a dipole in the other

Testing concepts 12/10/99 Which attractions are stronger: intermolecular or intramolecular? How many times stronger is a covalent bond compared to a dipole-dipole attraction? What evidence is there that nonpolar molecules attract each other? Which chemical on page 111 has the weakest intermolecular forces? Which has the strongest? How can you tell? Suggest some ways that the dipoles in London forces are different from the dipoles in dipole-dipole attractions. A) Which would have a lower boiling point: O2 or F2? Explain. B) Which would have a lower boiling point: NO or O2? Explain.

a) NH3, b) SF6, c) PCl3, d) LiCl, e) HBr, f) CO2 Which would you expect to have the higher melting point (or boiling point): C8H18 or C4H10? Explain. What two factors causes hydrogen bonds to be so much stronger than typical dipole-dipole bonds? So far we have discussed 4 kinds of intermolecular forces: ionic, dipole-dipole, hydrogen bonding, and London forces. What kind(s) of intermolecular forces are present in the following substances: a) NH3, b) SF6, c) PCl3, d) LiCl, e) HBr, f) CO2 (hint: consider EN and molecular shape/polarity) Challenge: Ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3) have the same formula (C2H6O). Ethanol boils at 78 C, whereas dimethyl ether boils at -24 C. Explain why the boiling point of the ether is so much lower than the boiling point of ethanol. 12/10/99

Testing concepts Intramolecular are stronger. 12/10/99 Testing concepts Intramolecular are stronger. A covalent bond is 100x stronger. The molecules gather together as liquids or solids at low temperatures. Based on boiling points, He (-268.9) has the weakest forces, S8 has the strongest (444.6). London forces Are present in all compounds Can occur between atoms or molecules Are due to electron movement not to EN Are transient in nature (dipole-dipole are more permanent). London forces are weaker

12/10/99 Testing concepts A) F2 would be lower because it is smaller. Larger atoms/molecules can have their electron clouds more easily deformed and thus have stronger London attractions and higher melting/boiling points. B) O2 because it has only London forces. NO has a small EN, giving it small dipoles. C8H18 would have the higher melting/boiling point. This is a result of the many more sites available for London forces to form. 1) a large EN, 2) the small sizes of atoms.

Testing concepts a) NH3: Hydrogen bonding (H + N), DipDip, London. 12/10/99 Testing concepts a) NH3: Hydrogen bonding (H + N), DipDip, London. b) SF6: London only (it is symmetrical). c) PCl3: EN=2.9-2.1. Dipole-dipole, London. d) LiCl: EN=2.9-1.0. Ionic, (London). e) HBr: EN=2.8-2.1. Dipole-dipole, London. f) CO2: London only (it is symmetrical) Challenge: In ethanol, H and O are bonded (the large EN results in H-bonding). In dimethyl ether the O is bonded to C (a smaller EN results in a dipole-dipole attraction rather than hydrogen bonding).

H – bonding and boiling point of compounds of hydrogen 12/10/99 10. Why does BP as period , 11. Why are some BP high at period 2?

Testing concepts 12/10/99 10. Boiling points increase down a period b/c size (more electrons) increases. A larger size (more electrons present) means greater London forces. 11. Boiling points are very high for H2O, HF, and NH3 because these are hydrogen bonds (high EN), creating large intermolecular forces

12. What IEMF are present in the following? 1) CH4 2) C2H6 3) C3H8 13. Rank the above compounds in terms of boiling points with the highest boiling point last.

12. 1) London (6 + 4 = 10 electrons) 2) London (12 + 6 = 18 electrons) 13. CH4 ˂ C2H6 ˂ C3H8

14. What IEMF are present in the following? 1) CH4 2) CHCl3 3) CHF3 15. Rank the above compounds in terms of boiling points with the highest boiling point last.

14. 1) London 2) London, dipole-dipole 3) London, dipole-dipole, H bonding 15. CH4 ˂ CHCl3 ˂ CHF3