lecture 34 chapter 11 sections 1 – 4 · lecture 34 chapter 11 sections 1 – 4 • intermolecular...
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Lecture 34 Chapter 11 Sections 1 – 4
• Intermolecular interactions
• Dispersive
• Dipolar
• Hydrogen-bonding
• Phases: gas, liquid solid
• Solids
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Ideal Gas Law vs. van der Waals Eq.
• Remember Ideal Gas Law
• Empirically explained by the van der Waals equation
1or ==nRTpVnRTpV
( ) nRTnbVV
anp =−
+ 2
2
attraction molecular size
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Melting and Boiling Points
• Both are indicators of the strengths of intermolecular forces:– Normal freezing point (fp): the temperature at which a solid
and liquid coexist at equilibrium under a pressure of 1 atm– Normal boiling point (bp): the temperature at which a liquid
and vapor coexist at equilibrium under a pressure of 1atm• Vaporization: l g• Condensation: g l
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Types Intermolecular Interactions
1. Dispersion – attraction between the negatively charged electron cloud of one molecule and the positively charged nuclei of a neighbor molecule.
2. Dipolar – attraction between negatively charged portions of one molecule with the positively charged portions of another molecule.
3. Hydrogen bonding – attraction between lone pair electrons on an O, N or F atom with a partially positive hydrogen atom.
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Dispersion Forces• Dispersion forces are found in all molecular substances• Such forces are electrostatic in nature and arise from
attractions between neighboring nuclei and electrons. Sometimes called induced dipoles.
• The magnitude of dispersion forces depends on how easy it is to polarize the electron cloud of a molecule.
• A larger molecule (more electrons) has a larger polarizability.
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Boiling Point vs Molecular Size
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Dipolar Forces
• Occur when one polar molecule encounters another polar molecule.
• The positive parts are attracted to negative parts.• Dipolar forces are usually stronger than dispersion forces.• Dipolar forces increase with an increase in the polarity of
the molecule.
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Models of acetone and 2-methylpropane:
What is the big difference?
Acetone is a polar molecule, 2-methylpropane is not.
Therefore, acetone will experience dipolar forces, but 2-methylpropane will not.
2-methylpropane acetone
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Order three molecules from lowest to highest boiling point.
54321
A B C
25%25%25%25% 1. lowest A B C highest
2. lowest C B A highest3. lowest A C B highest4. lowest B C A highest
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Hydrogen Bonding Forces
1. One molecule has a hydrogen atom attached by a covalent bond to an atom of oxygen, nitrogen, or fluorine, i.e. a polar H-X bond
2. The other molecule has a slightly negative oxygen, nitrogen, or fluorine atom.
This is mostly just the attraction between slightly positive H and slightly negative O, N, or F
NOT REALLY A BOND
This is the strongest of all intermolecular forces
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Many of water’s unusual properties are due to H-bonding
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Solid vs. Liquid vs. Gas
• Gas– little intermolecular interaction– take on any shape– expand to fill any volume
• Liquid– strong intermolecular interaction– take on any shape– very resistant to compression (fixed volume)– exhibit surface tension, capillary action, viscosity, vapor pressure
• Solid– very strong intermolecular interaction– fixed volume and shape– extensive variety of molecular packing
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Surface Tension
• The resistance of a liquid to an increase in its surface area. ORA liquid tends to minimize its surface area.
• The surface molecules of a liquid have a net inward force of attraction, forming a “skin”.
• The toughness of the skin is called surface tension.
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Capillary Action– The upward movement of water up a capillary against
the force of gravity.– Due to attractive forces between the glass (polar) and
the water (also polar)
Viscosity– Resistance to flow– For example, water vs. oil
• Dependent on intermolecular forces• Dependent on size & shape of molecules
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Vapor Pressure
Vapor pressure: the partial pressure of a vapor in dynamic equilibrium with its liquid (in a closed container).
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Forces in solids
• In liquids and gases, molecules are free to move continually and randomly.
• In solids, particles are constrained to fixed positions.• Particles can only vibrate and occasionally rotate.• There are four major types:
– molecular solids– network solids– metallic solids– ionic solids
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Forces in Solids
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Network Solids of Carbon
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Today• Finish reading Chapt 11• Finish CAPA• Take last year’s exam
Wednesday• Problems, problems, problems
Remember: You are done with the homework when you understand it!