Intermolecular Forces and Liquids and Solids

1. Intermolecular Forces and Liquids and Solids

2. Questions • Why do some solids dissolve in water but others do not? • Why are some substances are gaseous 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

3. Intramolecular forces occur between atoms Intermolecular forces occur between molecules Why mixtures mix • Consider a glass of wine. Why do alcohol, water, & pigment mix together? • There must be attractive forces. • The factors that determine solubility are the strength of IMFs and speed of molecules.

4. Intermolecular Forces Generally, intermolecular forces are much weaker than intramolecular forces. Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. • Intermolecular vs Intramolecular • 41 kJ to vaporize 1 mole of water (inter) • 930 kJ to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force boiling point melting point DHvap DHfus DHsub 11.2

5. The strengths of intermolecular forces are generally weaker than either ionic or covalent bonds. 16 kJ/mol (to separate molecules) + - + - 431 kJ/mol (to break bond)

6. Excess lone pairs

7. Hydrogen Excess H atoms

8. Each lone pair and H atoms has a partner on another water molecule.

9. H bond (Mr=34) b.p =431K (Mr=38) b.p=85K (Mr=36.5) b.p=188K

10. H bond (Mr=44) b.p =231K (Mr=44) b.p =294K, CH3COH, ethanal (Mr=46) b.p =352K, CH3CH2OH, ethanol

11. Types of Intermolecular Forces Orientation of Polar Molecules in a Solid 3. Dipole-Dipole Forces Attractive forces between polar molecules

12. Types of intermolecular forces (between neutral molecules): Dipole-dipole forces: (polar molecules) .. + S .. dipole-dipole attraction : : O O : .. - - .. + S .. : : O O : .. - - What effect does this attraction have on the boiling point?

13. Polar molecules have dipole-dipole attractions for one another. +HCl----- +HCl- dipole-dipole attraction

14. Types of Intermolecular Forces 4. Dispersion Forces – van der Walls forces/London forces (weakest) Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules

15. Dispersion Forces Occur between every compound and arise from the net attractive forces amount molecules which is produced from induced charge imbalances The larger the molecule the greater it’s Dispersion Forces are.

16. 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

17. London Dispersion Forces • Non - polar molecules also exert forces on each other. • Otherwise, no solids or liquids. • Electrons are not evenly distributed at every instant in time. • Have an instantaneous dipole. • Induces a dipole in the atom next to it. • Induced dipole- induced dipole interaction.

18. London Dispersion Forces London dispersion forces: (instantaneous dipole moment) ( also referred to as van der Waal’s forces) attraction - + - + “electrons are shifted to overload one side of an atom or molecule”.

19. London forces Induced dipole: Instantaneous dipole: Eventually electrons are situated so that tiny dipoles form A dipole forms in one atom or molecule, inducing a dipole in the other

20. polarizability: the ease with which an atom or molecule can be distorted to have an instantaneous dipole. In general big molecules are more easily polarized than little ones.

21. Which one(s) of the above are most polarizable? Hint: look at the relative sizes.

22. Intermolecular Forces Dispersion forces usually increase with molar mass. 4. Dispersion Forces Continued Polarizabilityis the ease with which the electron distribution in the atom or molecule can be distorted. • Polarizability increases with: • greater number of electrons • more diffuse electron cloud

23. O O S What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

25. Solubility and Intermolecular Forces • Like dissolves like • Polar solutes dissolve in polar solvents • Nonpolar solutes dissolve in nonpolar solvents • Molecules with similar intermolecular forces will mix freely • Molecules that can form H bondings will have greater solubility in water.

26. Ionic Solute with Polar Solvent The hydration enthalpy is a measure of the strength of the interaction of the water molecule w/ the ion.

27. Hydration enthalpy is the energy change when one mole of gaseous ions is converted to one mole of hydrated ions. • It increases w/ the charge on the ion and decreases w/ the size of the ions. Therefore, it’s greater for cations than the anions since they’re smaller in size and the water molecules can give a better packing because of its bent geometry.e.g hydration enthalpy is great for Al3+ .

28. Ionic Solute withNonpolar Solvent

29. Nonpolar Solute withNonpolar Solvent

30. Nonpolar Solute with Polar Solvent

33. Example: • Compare the solubilities of • ethanoic acid, ethanol and propane. • Propanone and HI d) ethanoic acid and pentanoic acid

34. Hydrocarbon chain disrupts the H bonding in water.

35. Take a look at the solubility rules on p.128.

36. H bonding 4-nitrophenol b.p=279°C Forms H bond mainly w/ other 4-nitrophenol molecules 2-nitrophenol b.p=216°C Forms H bond mainly intra-molecularly

37. Intramolecular H bonding in α-helix

38. A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions. An amorphoussolid does not possess a well-defined arrangement and long-range molecular order. A unit cell is the basic repeating structural unit of a crystalline solid.

39. Properties of Ionic Compounds 1. Crystalline solids - a regular repeating arrangement of ions in the solid: • Ions are strongly bonded together. • Structure is rigid. 2.High melting points • Coordination number- number of ions of opposite charge surrounding it

40. - Page 198 Coordination Numbers: Both the sodium and chlorine have 6 NaCl Both the cesium and chlorine have 8 CsCl Each titanium has 6, and each oxygen has 3 TiO2

41. Do they Conduct Electricity? • Conducting electricity means allowing charges to move. • In a solid, the ions are locked in place. • Ionic solids are insulators. • When melted, the ions can move around. 3.Melted ionic compounds conduct. • NaCl: must get to about 800 ºC. • Dissolved in water, they also conduct (free to move in aqueous solutions)

42. - Page 198 The ions are free to move when they are molten (or in aqueous solution), and thus they are able to conduct the electric current.

43. + - + - - + - + + - + - - + - + Ionic solids are brittle Force

44. - + - + - + - + + - + - - + - + Ionic solids are brittle • Strong Repulsion breaks a crystal apart, due to similar ions being next to each other. Force

46. carbon atoms Types of Crystals • Covalent (Network or macromolecular) Crystals – e.g. Silica(SiO2),SiC, BN , Si. • Stronger than IM forces. • Held together by covalent bonds • Hard, high melting point • Poor conductor of heat and electricity graphite diamond

49. This p bond overlap forms a huge p bonding network. • Electrons are free to move through out these delocalized orbitals. • The layers slide by each other.