Why does more NaCl dissolve in 100 g of water than in 100 g of gasoline? Chapter 10

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1 I sometimes wonder (because I m a nerd). Why does more NaCl dissolve in 100 g of water than in 100 g of gasoline? Chapter 10 Why does 2O have a higher boiling point than hexane (C3C2C2C2C2C3)? Liquids, Solids, and Phase Changes ow do strands of DNA stay together without being covalently or ionically bonded to one another? Why do water and oil not mix (much)? 1 Bond polarity is determined by differences in electronegativity. ow do we know if a molecule is polar overall? Bonds are polar if bonding electrons are shared unequally due to differences in electronegativity Partial negative and partial positive Geometry! Cl BF3 Non-polar Polar Cl δ δdipole points toward more electronegative atom 3 Overall Polarity When bond dipoles are the same magnitude, certain geometries can allow them to cancel, resulting in non-polar molecules PF5 CF4 BF3 δ δ This is true as long as all of the atoms attached to central atom are the same. Trigonal Bipyramidal Tetrahedral Trigonal Planar

2 Since C 3 Cl has a carbon attached to two different types of atoms, the dipole will not cancel (they have different magnitudes) Just when we thought we have it figured out Are there any molecules where all atoms have the same electronegativity, but the molecule is still polar? In C 3 Cl bonding electrons pulled toward more electronegative chlorine Overall dipole moment = polar molecule igher electron density All oxygens have the same electronegativity, so it shouldn t be polar but O O O Ozone is polar, but why? Shape! Intermolecular Forces (IMF) Intermolecular Forces (IMF): attractive forces between molecules Intermolecular forces are weaker than bonds (intramolecular forces), but have profound effects on the properties of liquids 431 kj/mol Intermolecular Forces Ion-dipole (10-50 kj/mol) ydrogen bonds (10-40 kj/mol) Dipole-dipole (3-4 kj/mol) (Dipole-induced dipole) Increasing strength London Dispersion (1-10 kj/mol) 16 kj/mol Dipole-Dipole Forces Dipole-dipole forces are between polar molecules. ydrogen Bonds (bridges) A hydrogen bond is NOT a covalent bond!!! ydrogen bond (bridge): special type of dipoledipole force; attractive force between a hydrogen atom bonded to a very small, electronegative atom (F, O, N) and lone e- pair Molecule 1 X X Molecule 2 X = F, O, N The partial positive () of one molecule is attracted to the partial negative () of another molecule 11 Covalent bond. This is NOT an -bond This is the -bond 12

3 ydrogen Bridges ydrogen Bonding in 2 O Molecules hydrogen bond to themselves or to other molecules -bonds O O 13 Double-stranded DNA What keeps the strands together? ydrogen Bridging Which of the following pure substances will experience hydrogen bonding? -bonding between T,A & G,C 2 O 2 Se Br F N 3 PF 3 16 Ion-Dipole Forces Ions have full charges that are attracted to the partial charge on polar molecules (dipoles) Ion-Dipole Forces Ion-dipole forces explain why many ionic compounds are able to dissolve in water

4 London Dispersion Forces London dispersion forces: attractive forces that result from temporary shift of electrons in atoms or molecules; present in all molecules Electron clouds can be shifted around a molecule or atom through interactions with other molecules and atoms. This shift creates an imbalance in the electron distribution. This is an instantaneous dipole. Boiling Points and Dispersion forces Boiling point increases with the size of molecules because of increases in London forces with larger electron clouds solid liquid gas As the mass of an atom or a nonpolar molecule increases, so does the size of the electron cloud, which gives more area with which to interact with other electron clouds Dispersion Forces Which member of each pair has the stronger London Dispersion forces? Ne or Kr Cl 2 or F 2 SiCl 4 or C 4 C 4 or CCl 3 Intermolecular Forces For each substance below, indicate the strongest type of intermolecular force observed. 2 O F Br N 3 PF 3 C 3 O CO F 2 21 CO 2 N 2 22 Trends in Intermolecular Forces Which member of each pair has stronger intermolecular forces (and higher boiling point)? C 3 O or C 3 S C 4 or C 3 C 2 C 3 CO or F 2 CO or F CO 2 or N 3 N 3 or N 2 Like dissolves like Like dissolves like is a good qualitative rule of thumb to determine if one substance will dissolve in another. Basically, it states that two substances with similar intermolecular forces should be able to dissolve in each other Polar substances Nonpolar substances In general dissolve dissolve Polar substances Nonpolar substances Polar substances dissolve Ionic substances 23

5 Structure and Solubility Alcohol C 3 C 2 O C 3 C 2 C 2 O Solubility (ml/100ml 2 O) Structure and Solubility The larger the carbon tail, the less soluble the alcohol in 2 O C 3 C 2 C 2 C 2 O 0.11 C 3 C 2 C 2 C 2 C 2 O C 3 C 2 C 2 C 2 C 2 C 2 O carbons 6 carbons ml/100 ml 2 O What is the trend? Fats/oils and water Why don t oil and water mix very well? Corn Oil Soaps / Detergents and Intermolecular Forces Soaps and detergents have both a polar (and ionic) and a non-polar area within the molecule Notice the C/ tails Nonpolar (hydrophobic) Polar and Ionic (hydrophilic) Water What are the intermolecular forces between: Oil/oil Water/water Soaps / Detergents and Intermolecular Forces Small spheres of soap are called micelles Cholesterol Ion Dipole Dipole - Dipole Grease / Oil Particle Water surrounding the micelle London Dispersion

6 eating Curve of Water Phase Diagram Adding energy to water usually increases the temperature. Except during melting and boiling. Critical Pressure solid liquid gas B: Critical point Supercritical fluid A B: Vaporization curve Vaporization, condensation A C: Melting curve Melting (fusion), freezing A D: Sublimation curve Why? To boil or melt, water must absorb a certain amount of energy Critical Temperature Sublimation, deposition A: Triple point Phase Diagrams: CO 2 and Water Adding pressure will melt the solid Adding pressure will freeze liquid Phase Diagram Features of a phase diagram: Triple point: temperature and pressure at which all three phases (s, l, and g) are in equilibrium Vaporization curve: generally, as pressure increases, temperature increases Melting curve: as pressure increases, the solid phase is favored if the solid is more dense than the liquid Normal melting point: melting point at 1 atm Normal boiling point: boiling point at 1 atm 34 Critical Temperature and Pressure Critical point: liquid and gas phases are indistinguishable (point marking T c and T p ) Critical temperature, T c : highest temperature at which a substance can exist as a liquid (cannot be liquefied) no matter how much pressure is applied Critical pressure, T p : minimum pressure that must be applied to bring about liquefaction at the critical temperature I sometimes wonder (because I m a nerd). Why does more NaCl dissolve in water than in gasoline? Why does 2 O have a higher boiling point than hexane? ow do strands of DNA stay together without being covalently or ionically bonded to one another? Why do water and oil not mix (much)?