1. Chapter 11 2 Ch 11 Page 467 Intermolecular forces?
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Transcript of 1. Chapter 11 2 Ch 11 Page 467 Intermolecular forces?
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Chapter 11
Ch 11Page 467
Intermolecular forces?
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STATES OF MATTER
Plasma- Electrically charged particles in a gaseous state.
We will focus on solids liquids and gasses.
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States of Matter
Ch 11.1Page 468
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Comparison ofSolids, Liquids, and Gases
• The density of gases is much less than that of solids or liquids.
Densities (g/mL)
Solid Liquid Gas
H2O 0.917 0.998 0.000588
CCl4 1.70 1.59 0.00503
Gas molecules must be very far apart compared to liquids and solids.
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States of Matter
• A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a “well-defined” boundary.
• A phase is a part of a system with a distinct set of physical properties. Crossing
Solid - ice
Liquid - water
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We call the transitions between different states of matter phase changes
States of Matter
What dictates the current state of the system?or
Why is there a change between states?
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Phase Changes
gas liquid solidcool cool
heatheat
meltingevaporation
condensation freezing
increasing temperature
heat is absorbed
heat is released
Energy needed to break intermolecular interactions.
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Ideal Gas Equation
PV = nRT
P = pressureV = volumen = number of molesR = gas constantT = temperature
Assumptions:• Gas atoms/molecules are perfect spheres (spherical cow!)• Size of atoms/molecules are negligibly small• There are no intermolecular/atom forces
Ok assumptions for gasses, does not work for solids and liquids.
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Liquids and Solids
Variables:Size and shape of the molecules/atomsThe intramolecular forces
Not all solids and liquids are equal:Viscosity, conductivity, surface tension, volume,
density, melting point, boiling point, hardness, color, flexibility, cost
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Intramoelcular vs Intermolecular• Intramolecular forces
– exist within each molecule– influence the chemical properties of the substance– ionic, covalent, and metallic bonds
• Intermolecular forces– exist between the molecules– influence the physical properties of the substance– Responsible for the state of the system:
gas, liquid, solid.Ch 11.2Page 469
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Intramolecular vs. Intermolecular Forces• Intramolecular forces:
– Ionic bonds (400-4000 kJ/mol)– Covalent bonds (150-1100 kJ/mol)– Metallic bonds (75-1000 kJ/mol)
• Intermolecular forces:– 0.05-100 kJ/mol
– much weaker
Example
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water (intra)
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Liquids and Solid
• Why do some solids dissolve in water but others do not?
• Why are some substances gases at room temperature, but others are liquid or solid?
• What gives metals the ability to conduct electricity, what makes non-metals brittle?
Intermolecular forces
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1) dipole-dipole
2) ion-dipole
3) Induced/instantaneous dipole:ion-induced dipoledipole-induced dipoleinstantaneous dipole-induced dipole
4) hydrogen bonding
Flavors of Intermolecular Forces
Ch 11.2Page 470
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Dipole-Dipole Forces
A B
Bond DipoleMolecular Dipole
A B
Molecules that have permanent dipoles are attracted to each other via dipole-dipole interactions.
The positive end of one is attracted to the negative end of the other and vice-versa. Larger dipole = stronger attraction.
Ch 11.2Page 469
Molecular dipole-neutral molecule with a positive and negative electron distribution.
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Dipole-Dipole Forces
• Both attractive and repulsive forces.
• Typical energy range: 5-25 kJ/mol.
In solution:
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In solids:
Dipole-Dipole Forces
NHO
Cl
All dipoles cancel.
p-chloroacetanilide-
Dipoles sum.
PyroelectricityPiezoelectricity
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1) dipole-dipole
2) ion-dipole
3) Induced/instantaneous dipole:ion-induced dipoledipole-induced dipoleinstantaneous dipole-induced dipole
4) hydrogen bonding
Flavors of Intermolecular Forces
Ch 11.2Page 470
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Ion-Dipole ForcesIon Dipole (polar)
Ch 11.2Page 470
The ion is attracted to the opposite charge on the polar molecule.
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1) dipole-dipole
2) ion-dipole
3) Induced/instantaneous dipole:ion-induced dipoledipole-induced dipoleinstantaneous dipole-induced dipole
4) hydrogen bonding
Flavors of Intermolecular Forces
Ch 11.2Page 470
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Dipole-Dipole Forces
• Both attractive and repulsive forces.
• Typical energy range: 5-25 kJ/mol.
In solution:
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Ion-Dipole Forces
• Plays a major role when an ionic compound dissolves in water.
• Typical energy range: 40-600 kJ/mol.
• Dependent on: ion size/charge and dipole strength
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1) dipole-dipole
2) ion-dipole
3) Induced/instantaneous dipole:ion-induced dipoledipole-induced dipoleinstantaneous dipole-induced dipole
4) hydrogen bonding
Flavors of Intermolecular Forces
Ch 11.2Page 470
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Instantaneous or Induced Dipole• Fluctuations of electron density create an instantaneous
dipole (or induced dipole) at an arbitrary moment in time.
non-polarmolecule/atom
instantaneous dipoleor
induced dipole
This property of the electron cloud to be distorted is called polarizability
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Polarizability Trends• Polarizability increases down the table:
bigger electron cloud = easier to distort
• Polarizability decreases from left to right across the table more positive nuclear charge = less flexible electron cloud
• Cations are less polarizable than their neutral parent atoms because they are smaller; anions are more polarizable because they are larger.
• “Longer” molecules are more easily polarized than more compact molecules.
C
CH3
CH3
H3CH3C
H2C
H2C
CH2
H2C
CH3
more easily polarized less easily polarized
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Non-polar Intermolecular Interactions:
ion-induced dipole
dipole-induced dipole
instantaneous dipole-induced dipole
non-polarmolecule/atom
instantaneous dipoleor
induced dipole
Non-polar Molecules
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Induced Dipole Forces
• Ion-induced dipole: 3-15 kJ/mol
• Dipole-induced dipole: 2-10 kJ/mol
+ cation
+ anionNeutral atom
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Dispersion forces usually increase with molar mass.
A.K.A. London Dispersion ForcesInstantaneous dipole-induced dipole
A.K.A. Dispersion Forces
instantaneous dipole
induced dipole
Interaction between non-polar atoms and/or molecules.
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Gecko Wall Walking
van der Waals force:
dipole-dipole
dipole-induced dipole
London dispersion
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What type(s) of intermolecular forces exist between the following pairs?
(a) HBr and H2S
(b) Cl2 and CBr4
(c) I2 and NO3-
(d) NH3 and C6H6
Hint: First classify the species into three categories: ionic, polar (possessing a dipole moment), and nonpolar.
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Solutions (a) Both HBr and H2S are polar molecules. Therefore, the
intermolecular forces present are dipole-dipole forces.
(b) Both Cl2 and CBr4 are nonpolar, so there are only dispersion forces between these molecules.
(c) I2 is nonpolar, so the forces between it and NO3- (ion) are
ion-induced dipole forces.
(d) NH3 is polar, and C6H6 is nonpolar. The forces are dipole-induced dipole forces.
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1) dipole-dipole
2) ion-dipole
3) Induced/instantaneous dipole:ion-induced dipoledipole-induced dipoleinstantaneous dipole-induced dipole
4) hydrogen bonding
Flavors of Intermolecular Forces
Ch 11.2Page 470
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Hydrogen Bonding
Decreasing molar massDecreasing boiling point
Anomalous trends in boiling points and other properties.
Ch 11.2Page 473
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A hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and a lone pair on O, N, or F atom.
A H… B A H… AorWhere A & B are N, O, or F
Hydrogen bonding molecules have:
• A strong permanent dipole (electronegative elements)
• A hydrogen atom
• An atom with lone pair electrons (N, F, O)
Hydrogen Bonding
Typical energy:10-40 kJ/mol.
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Hydrogen Bonding These are forces between polar molecules that contain an H atom bonded to
a small, highly electronegative atom with lone electron pair(s): N, O, F
NCH3H3C
CH3
N(CH3)3 N(CH3)4+
Can Hydrogen bond Cannot Hydrogen bond
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N CH3H3C
CH3
CH3
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Hydrogen Bonding in H2O
Liquid(water)
Solid(ice)
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Note: that some molecules like HCOOH (formic acid) and others can form hydrogen bonds in multiple ways.
CH
OO
H
H OH
HO
H
Hydrogen Bonding
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Which of the following can form hydrogen bonds with water?
CH3OCH3
CH4
F2
HCOOH
Na+
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Strategy A species can form hydrogen bonds with water if it contains one of the three electronegative elements (F, O, or N) and has a lone pair or it has a H atom bonded to one of these three elements.
Solution There are no electronegative elements (F, O, or N) in either CH4 or Na+. Therefore, only CH3OCH3, F2, and HCOOH can form hydrogen bonds with water.
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Why does hydrogen bonding matter?
Strong enough to hold a helix!
Weak enough for replication!
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Why does hydrogen bonding matter?
Protein Structure
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1) dipole-dipole
2) ion-dipole
3) Induced/instantaneous dipole:ion-induced dipoledipole-induced dipoleinstantaneous dipole-induced dipole
4) hydrogen bonding
Flavors of Intermolecular Forces
Ch 11.2Page 470
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Intermolecular Forces
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1) dipole-dipole (5-25 kJ/mol)
2) ion-dipole (40-600 kJ/mol)
3) Induced/instantaneous dipole:ion-induced dipole (3-15 kJ/mol)dipole-induced dipole (2-10 kJ/mol)instantaneous dipole-induced dipole (<2 kJ/mol)
4) hydrogen bonding (10-40 kJ/mol)
Strength of Intermolecular Forces
Intermolecular Forces general correlate with boiling point.
greater force = higher boiling point
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Strength of Intermolecular Forces
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Properties of liquids
• Surface Tension– intermolecular forces at liquid-air interface
• Capillary Action– competition between intermolecular forces
• Viscosity– strength of intermolecular forces
Ch 11.3Page 470
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Surface Tension
• The surface molecules have higher energy than the interior molecules.
• That’s why liquids tend to minimize their surface and form droplets.
• Surface tension is a measure of attractive forces acting at the surface of a liquid.
• The molecules at the surface are attracted unevenly.
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
Ch 11.3Page 475
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Surface TensionWater Mercury
Not really surface tension but still cool.
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Liquid Surface Tension (J/m2) at 20°C
Boiling Point, °C
Ethanol 2.3×10–2 78.4°C
Butanol 2.5×10–2 118°C
Water 7.3×10–2 100°C
Mercury 48×10–2 630°C
Stronger intermolecular forces usually result in the greater surface tension.
Surface Tension
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Capillary ActionCapillary action (sometimes capillarity, capillary motion, or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity.
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Forces Involved in the Capillary Action
• Cohesive forces are the forces that hold liquids together (intermolecular forces).
• Adhesive forces are the forces between a liquid and another surface (intermolecular forces).
– Capillary rise implies that:Adhesive forces > cohesive forces
– Capillary fall implies that the:Cohesive forces > adhesive forces
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Capillary Action
Water
Capillary fall.
Mercury
Capillary action is the ability of a liquid to rise (or fall) in a glass tube or other container
Capillary rise.
Adhesive forces > cohesive forces Adhesive forces < cohesive forces
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Capillary Action
WaterConcave meniscus
Mercury:Convex meniscus
adhesive forces > cohesive forcesadhesive forces < cohesive forces
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Capillary Action
Smaller diameter, higher the climb.Volume (gravity) vs. Surface area (adhesion)
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Viscosity• Viscosity is the resistance of a liquid to flow.
– For example, water is less viscous than syrup or honey.
• It is related to the ease with which molecules can move past each other.
• Decreases with increasing temperature.
• Viscosity depends on intermolecular forces and on the shape of molecules.
– Liquids with small spherical molecules are less viscous than liquids with long molecules that can become entangled and pour slowly.
Ch 11.3Page 470
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Viscosity
Strong intermolecular forces = higher viscosity
Low Viscosity
High Viscosity
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ViscosityPitch Drop “Experiment”
Started in 1927 by Professor Thomas Parnell
University of Queensland in Australia
Worlds longest experiment9th drop fell April 17th, 2014
http://theninthwatch.com/
https://www.youtube.com/watch?v=vZ5Vm4vABH4
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Chapter 11
Ch 11Page 467
Intermolecular forces according to Google Images: