Chapter 10 Chemical Reactions

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 H2O have a higher boiling point than hexane (CH3CH2CH2CH2CH2CH3)? Liquids, Solids, and Phase Changes • How do strands of DNA stay together without being covalently or ionically bonded to one another? • Why do water and oil not mix (much)? 1 Polar Covalent Bonds and Dipole Moments Polar Covalent Bonds and Dipole Moments Bond polarity is determined by differences in electronegativity. How 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 δ+ δ- Geometry! HCl BF3 Non-polar Polar δ- δ+ H δ- δ+ Cl δ+ δδ- δDipole points toward more electronegative atom 3 Overall Polarity No Overall Polarity Polar Covalent Bonds and Dipole Moments Polar Covalent Bonds and Dipole Moments When bond dipoles are the same magnitude, certain geometries can allow them to cancel, resulting in non-polar molecules PF5 CF4 δ- BF3 δδ- δδ- δ+ δ- δ- δ+ δ- δ- δ+ δ- This is true as long as all of the atoms attached to central atom are the same. δ- δ- No Overall Polarity Trigonal Bipyramidal No Overall Polarity Tetrahedral No Overall Polarity Trigonal Planar Polar Covalent Bonds and Dipole Moments 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? Since CH3Cl has a carbon attached to two different types of atoms, the dipole will not cancel (they have different magnitudes) In CH3Cl bonding electrons pulled toward more electronegative chlorine Higher electron density All oxygens have the same electronegativity, so it shouldn’t be polar…but… Overall dipole moment = polar molecule O O O δ+ Ozone is polar, but why? δ- δ- Shape! Intermolecular Forces (IMF) Intermolecular Forces Intermolecular Forces (IMF): attractive forces between molecules Ion-dipole (10-50 kJ/mol) Hydrogen bonds (10-40 kJ/mol) Intermolecular forces are weaker than bonds (intramolecular forces), but have profound effects on the properties of liquids Increasing strength Dipole-dipole (3-4 kJ/mol) (Dipole-induced dipole) 431 kJ/mol London Dispersion (1-10 kJ/mol) 16 kJ/mol Dipole-Dipole Forces Hydrogen Bonds (bridges) A hydrogen bond is NOT a covalent bond!!! Dipole-dipole forces are between polar molecules. δ+ δ- δ+ δ- δ+ Hydrogen 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 δ- Molecule 1 X H X Molecule 2 X = F, O, N The partial positive (δ+) of one molecule is attracted to the partial negative (δ-) of another molecule 11 Covalent bond. This is NOT an H-bond This is the H-bond 12 Hydrogen Bridges Hydrogen Bonding in H2O Molecules hydrogen bond to themselves or to other molecules H O H H-bonds H O H 13 Double-stranded DNA Hydrogen Bridging • Which of the following pure substances will experience hydrogen bonding? What keeps the strands together? H2O H2Se HBr HF NH3 PF3 H-bonding between T,A & G,C 16 Ion-Dipole Forces Ion-Dipole Forces Ion-dipole forces explain why many ionic compounds are able to dissolve in water Ions have full charges that are attracted to the partial charge on polar molecules (dipoles) + 17 London Dispersion Forces Boiling Points and Dispersion forces Boiling point increases with the size of molecules because of increases in London forces with larger electron clouds London dispersion forces: attractive forces that result from temporary shift of electrons in atoms or molecules; present in all molecules 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. solid 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. liquid This is an instantaneous dipole. gas 19 Dispersion Forces Intermolecular Forces Which member of each pair has the stronger London Dispersion forces? Ne Cl2 SiCl4 CH4 20 or or or or • For each substance below, indicate the strongest type of intermolecular force observed. H2O HBr PF3 F2 CO2 N2 Kr F2 CH4 CHCl3 HF NH3 CH3OH CO 21 Trends in Intermolecular Forces 22 Like dissolves like “Like dissolves like” is a good qualitative rule of thumb to determine if one substance will dissolve in another. Which member of each pair has stronger intermolecular forces (and higher boiling point)? Basically, it states that two substances with similar intermolecular forces should be able to dissolve in each other CH3OH CH4 CO CO CO2 NH3 or or or or or or In general… CH3SH CH3CH2CH3 F2 HF NH3 N2 Polar substances 23 dissolve Polar substances Nonpolar substances dissolve Nonpolar substances Polar substances dissolve Ionic substances Structure and Solubility Structure and Solubility Solubility Alcohol (mL/100mL H2O) CH3CH2OH ∞ CH3CH2CH2OH ∞ The larger the carbon tail, the less soluble the alcohol in H2O CH3CH2CH2CH2OH 0.11 CH3CH2CH2CH2CH2OH 0.030 2 carbons 0.0058 CH3CH2CH2CH2CH2CH2OH ∞ 6 carbons 0.0058 ml/100 mLH2O What is the trend? Fats/oils and water Soaps / Detergents and Intermolecular Forces Why don’t oil and water mix very well? Soaps and detergents have both a polar (and ionic) and a non-polar area within the molecule Corn Oil Nonpolar (hydrophobic) Notice the C/H tails Water What are the intermolecular forces between: •Oil/oil •Water/water Soaps / Detergents and Intermolecular Forces Small spheres of soap are called micelles Ion – Dipole Dipole - Dipole Grease / Oil Particle Water surrounding the micelle London Dispersion Cholesterol Polar and Ionic (hydrophilic) Heating Curve of Water Phase Diagram Critical Pressure Adding energy to water usually increases the temperature. Except during melting and boiling. B: Critical point liquid Supercritical fluid A – B: Vaporization curve Vaporization, condensation solid A – C: Melting curve gas Melting (fusion), freezing A – D: Sublimation curve Sublimation, deposition Why? To boil or melt, water must absorb a certain amount of energy Phase Diagrams: CO2 and Water Critical Temperature A: Triple point Phase Diagram Features of a phase diagram: Adding pressure will freeze liquid 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 Adding pressure will melt the solid Normal melting point: melting point at 1 atm Normal boiling point: boiling point at 1 atm 34 Critical Temperature and Pressure I sometimes wonder (because I’m a nerd)…. Critical point: liquid and gas phases are indistinguishable (point marking Tc and Tp) • Why does more NaCl dissolve in water than in gasoline? Critical temperature, Tc : highest temperature at which a substance can exist as a liquid (cannot be liquefied) no matter how much pressure is applied • Why does H2O have a higher boiling point than hexane? Critical pressure, Tp : minimum pressure that must be applied to bring about liquefaction at the critical temperature • How do strands of DNA stay together without being covalently or ionically bonded to one another? • Why do water and oil not mix (much)?