2006 Brooks/Cole - Thomson Inter- Inter- molecular molecular Forces Forces Have studied Have studied INTRA INTRA molecular molecular forces—the forces holding forces—the forces holding atoms together to form atoms together to form molecules. molecules. Now turn to forces between Now turn to forces between molecules — molecules — INTER INTER molecular molecular forces. forces. Forces between molecules, Forces between molecules, between ions, or between between ions, or between molecules and ions. molecules and ions.
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Have studied Have studied INTRAINTRAmolecular molecular forces—the forces holding forces—the forces holding atoms together to form atoms together to form molecules.molecules.
Now turn to forces between Now turn to forces between molecules —molecules — INTERINTERmolecular forces. molecular forces.
Forces between molecules, Forces between molecules, between ions, or between between ions, or between molecules and ions.molecules and ions.
Water is highly polar Water is highly polar and can interact and can interact with positive ions to with positive ions to give give hydratedhydrated ions in water.ions in water.
Water is highly polar Water is highly polar and can interact and can interact with positive ions to with positive ions to give give hydratedhydrated ions in water.ions in water.
Many metal ions are hydrated. This is Many metal ions are hydrated. This is the reason metal salts dissolve in the reason metal salts dissolve in waterwater..
Attraction between ions and dipole depends on Attraction between ions and dipole depends on ion chargeion charge and and ion-dipole distanceion-dipole distance..
Measured by ∆H for MMeasured by ∆H for Mn+n+ + H + H22O --> [M(HO --> [M(H22O)O)xx]]n+n+
Influence of dipole-dipole forces is seen in Influence of dipole-dipole forces is seen in the boiling points of simple molecules.the boiling points of simple molecules.
A special form of dipole-dipole attraction, A special form of dipole-dipole attraction, which enhances dipole-dipole attractions.which enhances dipole-dipole attractions.
H-bonding is strongest when X and Y are H-bonding is strongest when X and Y are N, O, N, O, or For F
Hydrogen Bonding in H2OH bonds ---> abnormally high specific heat capacity of H bonds ---> abnormally high specific heat capacity of
water (4.184 J/g•K)water (4.184 J/g•K)
This is the reason water is used to put out fires, it is the This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.thunderstorms release huge energy.
H-bonding is especially strong in biological systems — such as H-bonding is especially strong in biological systems — such as DNA. DNA. DNA — helical chains of phosphate groups and sugar molecules. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O.Chains are helical because of tetrahedral geometry of P, C, and O.Chains bind to one another by specific hydrogen bonding between Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases.pairs of Lewis bases. ——adenine with thymineadenine with thymine ——guanine with cytosineguanine with cytosine
•Process of Process of inducing a dipole inducing a dipole is is polarizationpolarization
•Degree to which Degree to which electron cloud of electron cloud of an atom or an atom or molecule can be molecule can be distorted in its distorted in its polarizabilitypolarizability..
Formation of a dipole in two nonpolar IFormation of a dipole in two nonpolar I22 molecules.molecules.
Induced dipole-induced dipole
The induced forces between IThe induced forces between I22 molecules are very weak, so molecules are very weak, so solid Isolid I22 sublimes (goes from a solid to gaseous molecules). sublimes (goes from a solid to gaseous molecules).
The magnitude of the induced dipole The magnitude of the induced dipole depends on the tendency to be distorted. depends on the tendency to be distorted.
FIGURE 13.18:FIGURE 13.18: VP as a function of T.VP as a function of T.
1. The curves show all conditions of P and T where LIQ 1. The curves show all conditions of P and T where LIQ and VAP are in and VAP are in EQUILIBRIUMEQUILIBRIUM
2. The VP rises with T.2. The VP rises with T.
3. When VP = external P, the liquid boils.3. When VP = external P, the liquid boils.
This means that BP’s of liquids change with altitude.This means that BP’s of liquids change with altitude.4. If external P = 760 mm Hg, T of boiling is the 4. If external P = 760 mm Hg, T of boiling is the
NORMAL BOILING POINTNORMAL BOILING POINT
5. VP of a given molecule at a given T depends on IM 5. VP of a given molecule at a given T depends on IM
forces. Here the VP’s are in the orderforces. Here the VP’s are in the order
CC 22HH 55HH 55CC 22 HHHH 55CC 22 HHHH
waterwateralcoholalcoholetherether
increasing strength of IM interactionsincreasing strength of IM interactions
HEAT OF VAPORIZATIONHEAT OF VAPORIZATION is the heat is the heat req’d (at constant P) to vaporize the liquid.req’d (at constant P) to vaporize the liquid.
LIQ + heat ---> VAPLIQ + heat ---> VAP
Compd.Compd. ∆H∆Hvapvap (kJ/mol) (kJ/mol) IM ForceIM Force
Lines connect all conditions of T and P where EQUILIBRIUM exists Lines connect all conditions of T and P where EQUILIBRIUM exists between the phases on either side of the line. (At equilibrium between the phases on either side of the line. (At equilibrium particles move from liquid to gas as fast as they move from gas to particles move from liquid to gas as fast as they move from gas to liquid, for example.)liquid, for example.)
Above Above critical T no critical T no liquid exists liquid exists no matter no matter how high the how high the pressure.pressure.
As P and T increase, you finally reach the CRITICAL T and PAs P and T increase, you finally reach the CRITICAL T and P
At P < 4.58 mmHg and T < 0.0098 ˚C solid HAt P < 4.58 mmHg and T < 0.0098 ˚C solid H22O O can go directly to vapor. This process is called can go directly to vapor. This process is called SUBLIMATIONSUBLIMATION
This is how a frost-free refrigerator works.This is how a frost-free refrigerator works.
Raising the pressure at constant Raising the pressure at constant T causes water to melt.T causes water to melt.
The NEGATIVE SLOPE of the S/L The NEGATIVE SLOPE of the S/L line is unique to Hline is unique to H22O. Almost O. Almost everything else has positive everything else has positive slope.slope.
LIQUID H2OICEfavored atlow P
favored athigh P
SolidH2O
LiquidH2O
P
T
760mmHg
0 °C
Normalfreezingpoint
The behavior of water under pressure is an example of The behavior of water under pressure is an example of LE LE CHATELIER’S PRINCIPLE CHATELIER’S PRINCIPLE At Solid/Liquid equilibrium, At Solid/Liquid equilibrium, raising P squeezes the solid. raising P squeezes the solid. It responds by going to phase with greater density, i.e., the It responds by going to phase with greater density, i.e., the liquid phase.liquid phase.
In any system, if you increase P the In any system, if you increase P the DENSITYDENSITY will go up. will go up. Therefore — as P goes up, equilibrium favors phase with Therefore — as P goes up, equilibrium favors phase with the larger density (or the larger density (or SMALLERSMALLER volume/gram). volume/gram).