Learning Check Determine the VSEPRT geometry, bond angle, and hybridization of each indicated atom in the following molecule. 1
Types of Covalent Bonds 1. A sigma (σ) bond is formed by endto-end overlap of orbitals. All single bonds are σ bonds. 2. A pi (p) bond is formed by sideways overlap of orbitals. A p bond is weaker than a σ bond because sideways overlap is less effective than end-to-end overlap. A double bond consists of one σ bond and one p bond. 2
Double Bond (ie ethylene) unhybridized 2p orbitals 3
Triple Bond (ie acetylene) Each C is sp hybridized and has two unhybridized p orbitals. 4
[11.43] Isoniazid is an antibacterial agent that is very useful against may common strains of tuberculosis. A valid Lewis structure is: How many bonds are in the molecule? What is the hybridization of each C and N atom? 5
Chapter 12 Intermolecular Forces: Liquids, Solids, and Phase Changes
7 Outline 1. An Overview of Physical States and Phase Changes 2. Quantitative Aspects of Phase Changes 3. Types of Intermolecular Forces 4. Properties of the Liquid State 5. The Uniqueness of Water Sections 12.3, 12.4 only
8 Molecules are held together by intramolecular forces (bonds within a molecule). Also, there are attractive intermolecular forces in solids, liquids, and gases. Influence physical properties Influence chemical properties
9 Intramolecular forces are bonding forces that exist within a molecule or ionic compound holding it together (i.e. internal chemical bonds). Ionic substances have intramolecular bonds that result from electron transfer. Covalent molecules have intramolecular bonds that result from sharing one or more electrons pairs between non-metals.
10 Intermolecular forces are attractive forces between molecules that explain many physical properties of compounds on the planet. Viscosity Freezing and Boiling Point Wetting or Not Wetting Surface Tension capillary action
Intermolecular forces play a critical role in life. For example, they hold together the double helix of DNA. 11
12 When washing clothes, an oily portion of soap interacts with grease and dirt, solubilizes it and removes it from clothing (or hands). CH3(CH2)16COO - Na +
13 Hydrophobic paint Video: https://www.youtube.com/watch?v=uon5etewch8
14 Vapor pressure is the pressure exerted by the vapor at liquid-gas equilibrium. Equilibrium Time Molecules in liquid begin to vaporize Molecules vaporizing and condensing at such a rate that no net change in vapor pressure occurs.
15 Factors affecting Vapor Pressure temperature As temperature increases, the fraction of molecules with enough energy to enter the vapor phase increases, and the vapor pressure increases. higher T higher P intermolecular forces The weaker the intermolecular forces, the more easily particles enter the vapor phase, and the higher the vapor pressure. weaker forces higher P
16 The temperature at which the vapor pressure equals the external pressure over the liquid is the boiling point. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm. Evaporation Boiling
17 The following diagram shows a close-up view of part of the vapor pressure curves for two liquids: the green upper curve for a pure solvent, and the red lower curve for a solution. Which of the two is more volatile? The more volatile solvent will have a higher vapor pressure (more gas molecules in the gas phase above its liquid). The green curve has a higher vapor pressure at all temperatures. It has weaker IMF s.
18 Intermolecular Forces 1. Ion-Dipole: occurs between an ion and a polar molecule 2. Dipole-Dipole: occurs between neutral polar molecules 3. Induced Dipoles: occurs when an ion or a dipole induces a spontaneous dipole in a neutral polarizable molecule 4. London Dispersion Forces: occurs in all molecules.
19 Ion-Dipole Attractive forces between an ion in solution and a neighboring polar molecule (+) end of dipole attracted to anions (-) end of dipole attracted to cations an anion in solution a cation in solution
20 Dipole-Dipole Attractive forces between polar molecules (same or not the same) Boiling point differences can be explained by dipole-dipole interactions.
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22 Dipole-Dipole (H-bonding) Hydrogen bonding is the attraction between the H atom of one molecule and a lone pair of the N, O, or F atom of another molecule
23 Learning Check Which of the following substances exhibits H bonding? For any that do, draw the H bonds between two of its molecules.
24 Induced Dipole Attraction between an ion or dipole charge and a polarizable non-polar molecule Non-polar molecule
25 London Dispersion Forces Attraction that occur when temporary dipoles are formed due to random electron motions in all polarizable molecules. Smaller particles are less polarizable than larger ones because their electrons are held more tightly.
26 Molecular Shape and Boiling Point There are more points at which dispersion forces act. There are fewer points at which dispersion forces act.
Determining IMF in a Sample 27
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29 Practice Exercises For each pair of substances, identify the key bonding and/or intermolecular force(s), and predict which one of the pair has the higher boiling point: (a) CH 3 NH 2 or CH 3 F (b) CH 3 OH or CH 3 CH 2 OH
30 Announcement March 19, Thursday LT#3: 120 points: 40 items MC x 2.5 pts each + 2 PS items x 10 pts each 6-7:30 PM Schmitt Hall C-114 Coverage: Chapters 7-11 March 20, Friday Quiz # 10, 11, 12