chm14 - Intermolecular Forces and Liquids and Solids A...

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Intermolecular Forces and Liquids and Solids
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A phase is a physically distinct homogenous part of a system…. .a physical “state” of matter (gas, liquid, solid). 2 Phases Solid phase - ice Liquid phase - water
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What determines whether, under atmospheric pressure, a substance is a gas, liquid, or solid? Temperature……. If ↓ T of a gas, then ↓ average KE, which ↓ particle velocity, which ↓ collision frequency, which ↓ volume of gas, which ↑ particle density, which ↓ the space between particles, which ↑ interparticular attraction. If ↓ T of a gas enough, enough KE of gas particle motion converts to PE of interparticular attraction so that condensation begins and liquid forms. If ↓ T of the liquid enough, more interparticular attraction results in freezing and solid forms. And vice-versa to go from solid to liquid to gas…. .
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Competition between the kinetic energy (KE) of particle motion and the potential energy (PE) of electrostatic particle attraction/repulsion determines whether a substance is gas, liquid, or solid at a certain temperature (remember that average KE is proportional to T in Kelvin). Gases: KE > PE Liquids: PE > KE (less KE) Solids: PE > KE (much less KE) The PE between particles results from attractive/repulsive forces (re: Coulomb’s Law) called intermolecular forces (IMF) . Need to know and understand the various types of IMF so that we can understand the physical properties (BP, FP, MP, solubilities, etc.) of different substances that are influenced by the IMF.
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Intermolecular 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 ) Generally, inter molecular forces are much weaker than intra molecular forces. Measure” of intermolecular force boiling point melting point H vap H fus H sub
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Types of IMF in order of strength of attraction: IMF Energy (kJ/mol) Examples Ion-Dipole 40-600 Na + Cl (aq) H-Bond 10-40 H 2 O, CH 3 OH Dipole-Dipole 5-25 CH 2 O, HBr Ion-Induced Dipole 3-15 Fe 2+ and O 2 Dipole-Induced Dipole 2-10 O 2 in H 2 O Dispersion/London 0.05-40 CCl 4 , Br 2
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Intermolecular Forces Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction
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Coulomb’s Law: Electrostatic E α [(q+)(q-)] / d
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Intermolecular Forces Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid
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Stronger net dipole moments have stronger dipole-dipole IMF. Recall that molecular shape influences net dipole moment (polarity). Need to understand molecular polarity to be able to understand
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chm14 - Intermolecular Forces and Liquids and Solids A...

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