Chapter 1 Dipole Moments, Molecular Polarity and...
Transcript of Chapter 1 Dipole Moments, Molecular Polarity and...
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Chapter 1
Dipole Moments,Molecular Polarity
and Intermolecular Forces
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Polarity of Molecules
For a molecule to be polar, it must
have polar bonds, and
have an unsymmetrical shape
Polarity affects the intermolecular forces of attraction
and therefore affects boiling points and solubilities
Nonbonding pairs affect molecular polarity.
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CH4 NH3 H2O
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Predicting Polarity of Molecules
1. Draw the Lewis structure and determine the molecular geometry.
2. Determine whether the bonds in the molecule are polar.
3. Determine whether the polar bonds add together to give a net dipole moment.
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Molecular Polarity
μ = 0.234 D
FN
FF
:
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Attractive Forces
Particles are attracted to each other by electrostatic forces.
The strength of the attractive forces depends on the kind(s) of particles.
The stronger the attractive forces between the particles, the more they resist moving.
The strength of the attractions between particles of a substance determines its physical state.
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Kinds of Attractive Forces
Hydrogen Bonds between Molecules An especially strong dipole–dipole attraction resulting from the attachment of H to an extremely electronegative
atom
Dispersion Forces between Molecules Temporary polarity in molecules due to unequal
electron distribution
Dipole–Dipole Attractions between Molecules Permanent polarity in molecules due to their structure
Ion–Dipole Attractions - Not Intermolecular Between mixtures of ionic compounds and polar compounds
(esp. aqueous solutions)
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Some molecules are considered nonpolar because of the atoms which they contain and the
arrangement of these atoms in space.
CH4 BH3 C2H2 CO2
Nonpolarizedelectronclouds
But these molecules can all be “condensed.”
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Origin of Instantaneous Dipoles
δδ-δδ+
The δδ- charge repels electrons.
The δδ+ charge attracts electrons.
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Size of the Induced DipoleThe magnitude of the induced dipole depends on several factors:
Polarizability of the electrons
Volume of the electron cloud
larger molar mass ⇒ more electrons ⇒ larger electron cloud ⇒ increased polarizability ⇒ stronger attractions
Larger molecules have more
electrons, leading to increased
polarizability.
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Size of the Induced DipoleShape of the molecule
more surface-to-surface contact ⇒ larger induced dipole
⇒ stronger attraction
Molecules that are flat have more surface
interaction than spherical ones.
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Gas Radius Molar Mass B.P.(K)
He 31 4 4.2
Ne 38 20 27
Ar 71 40 87
Kr 88 84 120
Xe 108 131 165
Rn 120 222 211
Effect of Molecular Sizeon Magnitude of Dispersion Force
As the molar mass increases, the number of
electrons increases. Therefore, the strength of
the dispersion forces increases.
The stronger the attractive forces
between the molecules, the
higher the boiling point.
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Boiling Points of Straight Chain Alkanes NonPolar Molecules
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Effect of Molecular Shapeon Size of Dispersion Force
n-pentane molar mass=72.15
b.p = 36.1 ºC
2-methylbutane molar mass=72.15
b.p = 27.9 ºC
2,2-dimethylpropane molar mass=72.15
b.p = 9.5 ºC
A larger surface-to-surface contact between molecules results in stronger dispersion force attractions and a
higher boiling point.
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Practice – Choose the Substance in Each Pair with the Higher Boiling Point
a) CH4 C4H10
b) C6H12 C6H12
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Some molecules are inherently polar because of the atoms which they contain and the
arrangement of these atoms in space.
H2O NH3 CH2O HCl
δ− δ+ A crude representation of a polar molecule
Dipole–Dipole Attractions
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Dipole–Dipole Attractions
Polar molecules have a permanent dipole because of bond polarity and shape
1) dipole moment 2) as well as the always present induced dipole
The permanent dipole adds to the attractive forces between the molecules
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Name Formula Molar mass Structure Structure b.p. m.p.
formaldehyde CH2O 30.03 -19.5º -92º
ethane C2H6 30.07 -88º -172º
H
H
H
H
HH
C
C
Effect of Dipole–Dipole Attraction on Boiling and Melting Points
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Determine if dipole–dipole attractions occur between CH2Cl2 molecules
Lewis
Structure
Bond
Polarity
Molecule
Polarity Formula
Cl—C 3.0−2.5 = 0.5
polar
C—H 2.5−2.1 = 0.4
nonpolar
4 bonding areas
no lone pairs tetrahedral
shape
polar molecule; therefore dipole–
dipole attractions do exist
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Hydrogen Bonding
When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it:
O─H, N─H, F─H
Because hydrogen has no other electrons, when its electron is pulled away, the nucleus becomes deshielded, exposing the H proton.
The exposed proton acts as a very strong center of positive charge.
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H-Bonding in Water
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2 3 4 5
100
0
-100
-200
Group IVAGroup VAGroup VIAGroup VIIA
SnH4
GeH4
SiH4
CH4
Boili
ng P
oint
(ºC)
Period
H2Te
H2Se
H2S
H2O
NH3
HF
HI
HBr
HCl
SbH3
AsH3
PH3
Hydrogen Bonding and Boiling Points
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Name Formula Molar mass Structure Structure b.p. m.p.
ethanol C2H6O 46.07 78.2º -114.1º
dimethyl ether C2H6O 46.07 -22º -138.5º
Effect of Hydrogen-Bonding on Boiling and Melting Points
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One of these compounds is a liquid at room temperature (the others are gases). Which one and why?
MM = 30.03PolarNo H-Bonds
MM = 34.03PolarNo H-Bonds
MM = 34.02PolarH-Bonds
Because only hydrogen peroxide has the additional very strong H-bond additional attractions, its intermolecular attractions will be the strongest. We therefore expect hydrogen peroxide to be the liquid.
-19ºC -78ºC +150ºCb.p.
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All Molecules
Polar Molecules
Molecules containing O-H, N-H, or F-H
Bonds
Dispersion forces
Dipole forces
H-bonding
Hierarchy of Intermolecular Forces
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Melting Points of n-Alkanes
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Alkanes Ethers Alcohols Amines
CH3CH2CH3
-42.1ºCH3OCH3
-23.7ºCH3CH2OH
+78ºCH3CH2NH2
+16.6º
CH3CH2CH2CH3
-0.5ºCH3OCH2CH3
+10.8ºCH3CH2CH2OH
+97.4ºCH3CH2CH2NH2
+47.8º
CH3CH2CH2CH2CH3
+36.1ºCH3CH2OCH2CH3
+34.5ºCH3CH2CH2CH2OH
+117.3CH3CH2CH2CH2NH2
+77.8º
Boiling Points of Other Organic “Families”
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Solubility
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SolubilityWhen one substance (solute) dissolves in another (solvent) it is said to be
When one substance does not dissolve in another it is said to be
The solubility of one substance in another depends on two factors - nature’s tendency toward mixing, and the solute and solvent.
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Attractive Forces Acting Between Ions & Molecules
δ−δ+ δ−δ+ δ−δ+
δ−δ+ δ−δ+ δ−δ+ δ−δ+
δ−δ+
Dispersion Forces Very weak, due to a temporary shift in electron distribution dependent on the size of the molecule
Dipole-dipole attractions - chiefly between molecules
Pure Electrostatic Attractions - chiefly ionic compounds
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What happens when you dissolve an ionic compound in water??
What happens when you dissolve a polar molecule in water??
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What Happens When an Ionic Compound Dissolves in Water?
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dipole-dipole attractions
What Happens When a Polar Covalent Compound Dissolves in Water?
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What happens when you try to dissolve a nonpolar molecule in water??
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What happens when you try to dissolve a nonpolar molecule in water??
Non polar solvents, such as ethanol, carbon tetrachloride, ether, and hexane, are also commonly used to dissolve nonpolar solutes, such as grease and oils.
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General Solubility Rule: “Like Dissolves Like”
Polar solutes form solutions with polar solvents.
Nonpolar solutes form solutions with nonpolar solvents.
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Selected Polar and Nonpolar Solvents POLAR SOLVENTS NONPOLAR SOLVENTS water, H2O hexane, C6H14
methanol, CH3OH heptane, C7H16
ethanol, C2H5OH toluene, C7H8
acetone, C3H6O carbon tetrachloride, CCl4
methyl ethyl ketone, CH3CH2C(O)CH3 chloroform, CHCl3
formic acid, HCOOH methylene chloride, CH2Cl2
acetic acid, CH3COOH ethyl ether, CH3CH2OCH2CH3
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Solubility* of a Series of Alcohols in Water and Hexane