2. 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? What gives metals the ability to conduct electricity, what makes non-metals brittle? The answers have to do with … Intermolecular forces Questions

3. Overview There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces We have already looked at intramolecular bonds (ionic, polar, non-polar) Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule) IMFs come in six flavours: 1) ionic, 2) dipole - dipole, 3) H-bonding, 4) London forces, 5) covalent (network solids), 6) metallic Intermolecular forces

4. Intermolecular Forces The forces with which molecules attract each other. Intermolecular forces are weaker than ionic or covalent bonds. Intermolecular forces are responsible for the physical state of a compound (solid, liquid or gas).

5. Intermolecular Forces Van der Waals Forces – Dipole Interactions – Dispersion Forces Hydrogen Bonds

6. Van der Waals Forces They are the weakest attractions between molecules.

7. Dipole A polar molecule that has two poles.

8. Van der Waals-Dipole Interactions Electrostatic interaction between the oppositely charged regions of polar molecules (dipoles).

9. Ionic, Dipole - Dipole attractions We have seen that molecules can have a separation of charge This happens in both ionic and polar bonds (the greater the EN, the greater the dipoles ) HCl ++ –– Molecules are attracted to each other in a compound by these +ve and -ve forces ++ –– ++ –– ++ –– ++ ––

10. 9 Dipole-Dipole Interactions Dipole—dipole interactions are the attractive forces between the permanent dipoles of two polar molecules. Consider acetone. The dipoles in adjacent molecules align so that the partial positive and partial negative charges are in close proximity. These attractive forces caused by permanent dipoles are much stronger than weak van der Waals forces.

11. London forces 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) London forces are due to small 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

12. London forces 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

13. Hydrogen Bonding H-bonding is a special type of permanent dipole - dipole attraction that is very strong Hydrogen bonding is the attraction between a hydrogen atom of a molecule to an unshared pair of electrons in another molecule. Hydrogen bonding occurs in molecules where hydrogen is covalently bonded to a very electronegative element with a lone pair of electrons on the electronegative atom. Hydrogen bonding occurs in molecules containing N, O, F when N, O, or F are bonded to H. The high  EN of NH, OH, and HF bonds cause these to be strong forces (about 5x stronger than normal dipole-dipole forces)

14. An atom or ion’s electronegativity is its ability to pull electrons towards itself in a covalent bond. The most electronegative elements are found towards the top right corner of the periodic table.

15. Hydrogen Bonding, Continued Hydrogen bonds are the strongest of all intermolecular forces. Hydrogen bonds are possible because in hydrogen atoms there is no shielding of the nucleus. Hydrogen bonds are responsible for the physical properties of many biological substances and, more importantly, water. Molecules with hydrogen bonds have higher boiling points than molecules that don’t.

16. Hydrogen bonds Give me an example! This does not have any hydrogen bonds. Carbon is not very electronegative, and it has no lone pairs of electrons in methane. methane, CH 4 …

17. Hydrogen bonds Give me a real example! This does have hydrogen bonds. Nitrogen is very electronegative, and it has one lone pair of electrons in ammonia. ammonia, NH 3 …

18. Hydrogen bonds Give me another example! This has not one, but two hydrogen bonds. Oxygen is very electronegative, and it has two lone pairs of electrons in water. water, H 2 O …

19. 18 Hydrogen bonding typically occurs when a hydrogen atom bonded to O, N, or F, is electrostatically attracted to a lone pair of electrons on an O, N, or F atom in another molecule. Hydrogen Bonding

20. 19 Hydrogen bonding

21. The origin of hydrogen bonds Hydrogen: attached directly to one of the most electronegative elements Electronegative elements: not only significantly negative charged, but also has at least one "active" lone pair. Electrons: contained in a relatively small volume of space which therefore has a high density of negative charge.

22. 21 Water Chemistry The polarity of water causes it to be cohesive and adhesive. cohesion: water molecules stick to other water molecules by hydrogen bonding adhesion: water molecules stick to other polar molecules by hydrogen bonding

23. 22 Properties of Water 1.Water has a high specific heat - A large amount of energy is required to change the temperature of water. 2. Water has a high heat of vaporization. - The evaporation of water from a surface causes cooling of that surface. 3. Solid water is less dense than liquid water. - Bodies of water freeze from the top down. 4. Water is a good solvent. - Water dissolves polar molecules and ions.

24. 23 Properties of Water 5. Water organizes nonpolar molecules. - hydrophilic: “water-loving” -hydrophobic: “water-fearing” - Water causes hydrophobic molecules to aggregate or assume specific shapes. 6. Water can form ions. H 2 O  OH - + H + hydroxide ionhydrogen ion

25. Most properties that water displays is a result of the attraction that different molecules of water have for each other. Water is cohesive, or attracted to itself. The high cohesion of water is the reason for it’s high surface tension. Water has a high boiling point (relative to other molecules its size). The large network of hydrogen bonds found in water causes the water molecules to “like” to be near one another in the liquid phase. It takes a lot of energy to force the molecules apart from each other into the gas phase.

26. Hydrogen bonding in water results in some unusual properties; o Higher than expected boiling point o High specific heat capacity (absorbs a lot of heat energy with only a small change in temperature) o Ice is less dense than water

27. H bond in complex molecules Question: What holds the two DNA strands together?

28. What hydrogen bonds help to do? Multiple hydrogen bonds hold the two strands of the DNA double helix together hold polypeptides together in such secondary structures as the alpha helix and the beta conformation help enzymes bind to their substrates help antibodies bind to their antigen help transcription factors bind to each other and DNA ……

29. Which of the following molecules can have hydrogen bonding? A) F 2 B) CH 4 C) H 2 O D) CH 3 Cl E) NH 3

30. 29 Note: as the polarity of an organic molecule increases, so does the strength of its intermolecular forces. Summary of all the intermolecular forces

31. 30 Physical Properties—Boiling Point The boiling point of a compound is the temperature at which liquid molecules are converted into gas. In boiling, energy is needed to overcome the attractive forces in the more ordered liquid state. The stronger the intermolecular forces, the higher the boiling point. For compounds with approximately the same molecular weight:

32. 31 Because ionic compounds are held together by extremely strong interactions, they have very high melting points. With covalent molecules, the melting point depends upon the identity of the functional group. For compounds of approximately the same molecular weight:

33. Solubility Solubility is the extent to which a compound, called a solute, dissolves in a liquid, called a solvent. Compounds dissolve in solvents having similar kinds of intermolecular forces -“Like dissolves like.” Polar compounds dissolve in polar solvents. Non-polar or weakly polar compounds dissolve in non-polar or weakly polar solvents. Water and organic solvents are two different kinds of solvents. Water is very polar and is capable of hydrogen bonding with a solute. Many organic solvents are either non-polar, like carbon tetrachloride (CCl 4 ) and hexane [CH 3 (CH 2 ) 4 CH 3 ], or weakly polar, like diethyl ether (CH 3 CH 2 OCH 2 CH 3 ). Most ionic compounds are soluble in water, but insoluble in organic solvents.

34. Metallic Bond Bond that exists between metal atoms Alloy – two or more different metal atoms bonded together

35. Cations packed in “a sea of electrons”

36. Electron Sea Model Metals consist of closely packed cations floating in a “sea of electrons”. Metal atoms give up valence electrons and form + ions The released electrons move freely around the + metal ions All of the atoms are able to share the electrons. The electrons are not bound to individual atoms.

37. Properties of Metallic Bonds Good conductors of electricity – free electrons Malleable and ductile – not in rigid position so ions can be shaped and drawn into wires Conduct heat because of free electrons Lusterous – absorb and emit light in regular pattern due to free electrons Electrons act as a lubricant, allowing cations to move past each other

38. Alloys Mixtures of two or more elements, at least one of which is a metal. Made by melting, mixing, then cooling the metals. May contain non-metals like carbon. Properties are superior to their components. Stainless Steel –Fe 80.6%, Cr 18%, C 0.4%, Ni 1% Cast Iron - Fe 96%, C 4% Sterling silver –92.5% Ag, 7.5% Cu –Harder than silver Bronze –7:1 Cu to Sn (tin)