intermolecular forces, liquids, and solids

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Intermolecular Forces, Liquids, and SolidsIntermolecular Forces, Liquids, and Solids

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Some Characteristic Properties of the States of Matter

Gas: Assumes both the volume and shape of the containerIs CompressibleDiffusion within a gas occurs rapidlyFlows readily

Liquid Assumes the shape of the portion of the container it occupiesDoes not expand to fill the containerIs virtually incompressibleDiffusion within a gas occurs slowlyFlows readily

Solid Retains its own shape and volumeIs virtually incompressibleDiffusion within a solid occurs extremely slowlyDoes not flow

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The strength of intermolecular forces vary over a wide range, but are generally weaker than covalent or ionic bonds

16 kJ will overcome the intermolecular attraction leading to vaporization

431 kJ of energy is required to break the covalent bond between HCl

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Dipole Dipole forcesDipole Dipole forces exist between neutral polar molecules, are effective only when polar molecules are very close together, and are generally weaker then ion-dipole interactions

Molecules that are attracting one another spend more time near each other than those molecules which repel. Thus the overall effect is a net attraction

For two particles of equal mass and size, the strengths of the intermolecular attractions increase with increasing polarity

Substance Dipole moment Bp

Propane 0.1 231Dimethyl ether 1.3 249Methyl chloride 2.0 249Acetaldehyde 2.7 293

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Ion-Dipole ForcesIon-Dipole Forces exist between an ion and the partial charge on the end of a polar molecule

The magnitude of the interaction depends upon the charge of the ion, the dipole moment of the polar molecule and the distance from the center of the ion to the midpoint of the dipole

Na+

Cl-

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London Dispersion ForcesLondon Dispersion Forces can exist between non-polar atoms and molecules where the movement of electrons create an instantaneous

dipole moment

The instantaneous distribution at any given moment can be The instantaneous distribution at any given moment can be different from the average distribution producing an different from the average distribution producing an

instantaneous dipole momentinstantaneous dipole moment

Because electrons repel, the motions of electrons Because electrons repel, the motions of electrons in one atom influencein one atom influence

the motions of electrons on its near neighborsthe motions of electrons on its near neighbors

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dipole momentThe ease with which the charge distribution in a molecule can be distorted by an external force is called its polarizability. The greater the polarizability of a molecule, the more easily its electron cloud can be distorted to give a momentary dipole.In general, larger molecules tend to have greater polarizability because their electrons are farther from the nucleusBecause molecular size and molecular mass tend to parallel one another, dispersion forces tend to increase in strength with increasing molecular weight.

Halogen Boiling pt (K) Noble gas Boiling point (K)F2 85.1 He 4.6Cl2 238.6 Ne 27.3Br2 332.0 Ar 87.5I2 457.6 Kr 120.9

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dipole moment

The shapes of molecules can play a role in the magnitude of dispersion forces.

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dipole momentLondon dispersion forces may also operate within polar molecules

and contribute to the overall attractive forces between the molecules; some times more than the dipole-dipole forces

HCl: bp = 189.5 KDipole Moment = 1.03 D

HBr: bp = 206.2 KDipole Moment = 0.79 D

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Hydrogen bondingHydrogen bonding is a special type of intermoleculer attraction that exists between the hydrogen atom in a polar bond (e.g., H F, H O, or H N) and an unshared electron pair on a nearby electronegative

atom (usually an F, O, or N atom on another molecule)

Because F, N, and O are so electronegative, a bond between hydrogen andany of these elements is quite polar, with hydrogen at the positive end.

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Metallic solids consist of metal atoms. Bonding is due to valence electrons that are delocalized throughout the entire solid. We can visualize these electrons as an array of positive ions immersed in a sea of delocalized electrons.

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Metallic bond

examples: Cu, Fe, or alloy

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Intermolecular Forces, Liquids, and SolidsIntermolecular Forces, Liquids, and SolidsProperties of Liquids: Viscosity and Surface Tension

The resistance of liquids to flow is called their viscosity. The greater the viscosity, the more slowly the liquid flows. Viscosity decreases with increasing temperatureViscosity decreases with increasing temperature.

Viscosity is related to the ease with which individual molecules of a liquid can movewith respect to one another. It thus depends on the attractive forces between moleculesand and their structural character.

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Intermolecular Forces, Liquids, and SolidsIntermolecular Forces, Liquids, and SolidsProperties of Liquids: Viscosity and Surface Tension

Molecules in the interior are attracted equally in all directions, whereas those at the surface experience a net inward force

This net inward force pulls molecules from the surface into interior, thereby reducing the surface area

Surface tension is therefore defined as the energy required to increase the surface of a liquid by a unit amount

Forces that bind like molecules to one another are called cohesive forces.

Forces that bind molecules to a surface are calledadhesive forces

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Changes of StateChanges of StateWhenever a change of state involves going to a less ordered state, energy

must be supplied in order to overcome intermolecular forces

Melting is also called fusion. The enthalpy of change associated withmelting is called heat of fusion. (e.g. 6.01 kJ/mol for water)

The heat needed for vaporization is called the heat of vaporization (e.g. 40.67 kJ/mol for water)

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Changes of StateChanges of StateEnthalpy and Temperature Changes Accompanying HeatingEnthalpy and Temperature Changes Accompanying Heating

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Calculate the enthalpy change associated with converting 1.00 mol of ice at -25°C to water vapor at 125 °C and 1 atm. The heat capacities per gram (specific heats) of ice, water, and steam are 2.09 J/g-°C , 4.18 J/g-°C , and 1.84 J/g-°C, respectively. The heat of fusion of ice is 6.01 kJ/mol, and the heat of vaporization of water is 40.67 kJ/mol

•Heat required to bring the ice from -25 °C to 0 °C :

1.00 mol H2O18.0 g.

1.00 mol H2O

2.09 Jg -°C

25 °C = 940 J

•Heat of fusion: 1.00 mol H2O6.01 kJ

mol H2O= 6.01 kJ

•Heat required to bring liquid• water from 0 °C to 100 °C :

1.00 mol H2O18.0 g.

1.00 mol H2O

4.18 J

g -°C100 °C =7520 J

Heat of vaporization 1.00 mol H2O40.67 kJ

mol H2O= 40.67 kJ

Heat required to raise thetemperature of the vapor to 125 °C

18.0 g.

1.00 mol H2O

1.84 Jg -°C

25°C =830 J

Ht = .940 kJ + 6.01 kJ + 7.52 kJ + 40.67 kJ + .830 kJ = 55.97 kJ

1.00 mol H2O

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Changes of StateChanges of StateVapor PressureVapor Pressure: pressure exerted by evaporating liquid in the space: pressure exerted by evaporating liquid in the space

above that liquid once dynamic equilibrium has been establishedabove that liquid once dynamic equilibrium has been established

The vapor pressure of a substance increases as the temperature increases

Substances with a high vapor pressure evaporate more rapidly than thosewith low vapor pressures. Theses substances are more volatile

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Changes of StateChanges of StateVapor Pressure and Boiling Point: Vapor Pressure and Boiling Point: A liquid boils when its vaporA liquid boils when its vapor

pressure equals the external pressure acting on the surface of the liquidpressure equals the external pressure acting on the surface of the liquid

The higher the outside pressure, the higher the boiling point

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A phase diagram is a graphical way to summarize the conditions under which equilibria exist between the difference states of matter.

Line A-B is the vaporcurve of the liquid

The line A-C represents the variation invapor pressure during sublimation.

Line A-D represents thechange in melting pointof the solid underincreased pressure

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A phase diagram is a graphical way to summarize the conditions under which equilibria exist between the difference states of matter.

Note that the gas phase is the stable phase under condition of low pressure and high temperature

Note that the solid phase is the stable phase under condition of high pressure and low temperature

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Why are the fusion-freezing lines different for H2O and CO2?Why does CO2 sublime under normal conditions?

How is freeze-dry food prepared?

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AS THE TEMPERATURE IS INCREASED AS THE TEMPERATURE IS INCREASED : At point 6-A, the H2O exists entirely as a solid. When the temperature reaches point 4-B, the solid begins to melt, and an equilibrium condition occurs between the solid and the liquid. At a yet high temperature, point 7-C, the solid has been converted entirely to a liquid. When point 8-D is encountered, vapor forms, and a liquid-vapor equilibrium is achieved. Upon further heating, to point 9-E, the H2O is converted entirely to the vapor phase.

6 4 7 987

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A crystalline solid is a solid whose atoms, ions, or molecules are ordered in well-defined arrangements. Amorphous solids have no

structure.

A unit cell is the repeating unit that makes up the crystalline solid. The array of repeating points is called the crystal lattice

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The lattices of crystalline solids can be described in terms of seven basic types. The following three types are the simplest of the seven.

Coordination number = # of nearest neighbors # of atoms in unit cell

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The Crystalline (Body-Centered) Lattice of Sodium ChlorideThe Crystalline (Body-Centered) Lattice of Sodium Chloride

Na+ Cl-

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The total cation-anion ratio in each unit cell must be the same as that for the entire crystal. Therefore a unit cell of sodium chloride

must have equal number of Na+ and Cl-

Na+ Cl-

(¼ Na+ per edge ) (12 edges) = 3 Na+

(1 Na+ per center) (1 center ) = 1 Na+

Na+

(1/8 Cl- per corner ) (8 corners) = 1 Cl-

(1/2 Cl- per face) (6 faces ) = 3 Cl-

Cl-

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Aluminum crystallizes in a face-centered arrangement

a.how many atoms are in the unit cell?

b.what is the coordination number?

c.each atom has diameter of 2.86 Å, what is the length of a side?

d. what is the density?

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The Close Packing of Spheres: A Study of Metallic SolidsThe Close Packing of Spheres: A Study of Metallic Solids

Lets make an assumption using Methane as a example that many molecules may be approximated as being roughly spherical

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The coordination number of close packing structures = 12

The coordination number of body-centered cubic structures = 8

The coordination number of primitive cubic structures = 6

The Close Packing of Spheres: A Study of Metallic SolidsThe Close Packing of Spheres: A Study of Metallic Solids

hexagonal close pack cubic close pack = face-centered cubic

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Using X-Ray Diffraction to Determine the Structure of Crystalline SolidsUsing X-Ray Diffraction to Determine the Structure of Crystalline Solids

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Covalent Network Solids are Held together in large networks or Covalent Network Solids are Held together in large networks or chains by covalent bonds. These solids are much harder and have chains by covalent bonds. These solids are much harder and have higher melting points than molecular solidshigher melting points than molecular solids

Molecular solids consist of atoms or molecules held together by dipole-dipole or London dispersion forces or hydrogen bonds

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Ionic solids consist of ions held together by ionic bonds. The strength of the bonds depend upon the charges of the ions. Melting point increases as bond strength increases

The structure of the solid is dependent upon the charges andThe structure of the solid is dependent upon the charges andrelative sizes of the ions.relative sizes of the ions.The coordination number increases as the ratio of the cation radius to the anion radius increases.

NaCl

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• Atomic solids – (london) eg. He, Ar etc.atoms exist at lattice pts

• molecular solids (London, dipole, H-Bond) CH4, NH3

molecules exist at lattice pts• Ionic solids (electrostatic attractions) NaCl

cations & anions exist at lattice pts

• Metallic solids- (Cations held by a sea of electrons) Cu, Fe

cations exist at lattice pts• Network solids – (3 dimensional covalent bonds)

diamond (C), quartz (SiO2)atoms exist at lattice pts

intermolecular forces, liquids, and solids

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