condensed phases - liquids and solids

8/14/2019 Condensed Phases - Liquids and Solids

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Chemistry 8ed, Whitten etal, TB/C 2007

Solids and liquids areSolids and liquids are condensed statescondensed states..

their atoms, ions, or molecules are much closertheir atoms, ions, or molecules are much closerto one another than in gases.to one another than in gases.

are highly incompressible.are highly incompressible.

The intermolecular forces of attractionsintermolecular forces of attractions ininliquids and solids are strongliquids and solids are strong..

Chemistry 8ed, Whitten

etal, TB/C 2007

Schematic representation of the threecommon states of matter.


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If we compare the strengths ofstrengths of

interactionsinteractions among particles and thedegree of orderingdegree of ordering of particles, we seethat

Gases< Liquids < Solids


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forces between individual particles

(atoms, molecules, and ions) of asubstance

Ion-ion interaction

Dipole-dipole

H-bonding London forces or Dispersion forces

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the force of attraction between twooppositely charged ions

is governed by Coulombs law

Chemistry 8ed, Whittenetal, TB/C 2007

Ion-ion interaction


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Coulombs law determines:

the melting and boiling points of ionic compounds

the solubility of ionic compounds.

Ionic substances containing multiply charged ions, (egAl3+ , Mg2+ , O2- , S2- ) usually have higher melting/boiling

points than singly charged ions

For a series of ions of s imilar charge, the closer approachof smaller ions results in stronger interionic attractionsand consequently higher melting/boiling properties


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Ion-ion interaction

table of melting points

Melting Points of Some Ionic Compounds

Compound Mp (o C) Compound Mp (o C) Compound Mp (o C)

NaF 993 CaF2 1423 MgO 2800

NaCl 801 Na2 S 1180 CaO 2580

NaBr 747 K2 S 840 BaO 1923


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Ion-ion interaction

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the force of attraction between two

polar molecules

Example: BrF (a polar molecule)


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Dipole-dipole interactions

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also a force of attraction between two polar

molecules BUT ONLY APPEARS IN

MOLECULES HAVING A HYDROGEN ATOM

ATTACHED TO N, O, F (the three

electronegative atoms).


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Hydrogen-bonding

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- also called London forces and are very weak

forces

- this is the only attractive force in nonpolarmolecules

- occurs only in extremely short distances,i.e. 1/d7


Dispersion forces

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Consider He as an isolated atom. In a group of He atoms, the temporary dipole in one

atom induces other atomic dipoles.


Dispersion forces


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Similar effects occur in a group of I2

molecules.


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Dispersion forces

Induced dipole- induced dipole interaction

Interaction strictly between nonpolarmolecules

Dipole-induced dipole (originally, this is notconsidered as London forces) Interaction between a dipole (polar) molecule

and a nonpolar molecule


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Dispersion forces

are most important when POLARIZABILITY is

high:

high # of electrons (SQUISHY MOLECULE)

appearance of delocalized bonds (that actuallymakes the molecule FLAT due to sp2 and sp3

hybrid)

overall molecule is LARGE

molecule covers a larger area


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Dispersion forces

in general, the trend for IMFA strength is:

ion-ion> {ion-dipole}

> H-bonding> dipole-dipole

> London dispersion{dipole - induced dipole > induced dipole -

induced dipole}


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A molecule may have MORE THAN ONETYPE OF IMFA when it interacts with similarmolecules.

When comparing properties, it is best tolook at the OVERALL NET EFFECT OF THEEXISTING IMFA for the molecule as it acts inthe ENTIRE SUBSTANCE (not just betweentwo molecules).


AMORPHOUS & CRYSTALLINE

Amorphous solids do not have a well ordered molecularstructure.

Examples include waxes, glasses, asphalt.

Crystalline solids have well defined structures that consistofextended array of repeating units called unit cellsunit cells.

Crystalline solids display X-ray diffraction patternswhich reflect the molecular structure.

Properties at the Solid State



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AMORPHOUS & CRYSTALLINE


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UNIT CELLS

are the smallest repeating unit of a crystal



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There are seven basic crystal systems.


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There are four types of unit cell.- primitive - face centered- body centered - side centered

According to Bond between Particles

Molecular SolidsMolecular Solids Network CovalentNetwork Covalent

Ionic solidsIonic solids Metallic SolidsMetallic Solids


Properties at the Solid State Molecular Solids have molecules in each of the positions of the unit

cell

have low melting points, are volatile, and areelectrical insulators.

Examples: water, sugar, carbon dioxide, benzene



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Network Covalent Solids have atoms that are covalently bonded to one

another

Examples: Diamond, graphite, SiO2 (sand), SiC


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Ionic Solids have ions that occupy the positions in the unit cell

Examples: CsCl, NaCl, ZnS


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Metallic Solids

may be thought of as positively charged nucleisurrounded by a sea of electrons.

the positive ions occupy the crystal lattice positions.

Examples: Na, Li, Au, Ag


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Melting PointMelting Point

the temperature at which the solid form

of a substance is at equilibrium with its

liquid state

normal melting point is the

temperature at which the solid melts atexactly 1.00 atm of pressure



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SublimationSublimation

process where the solidtransforms directly to thevapor phase withoutpassing through theliquid phase

solid CO2 or dry icedoes this well


ViscosityViscosity

the resistance to flow

compare how waterpours out of a glasscompared to molasses,syrup or honey



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Surface TensionSurface Tension

is a measure of theunequal attractionsthat occur at thesurface of a liquid the molecules at the

surface are attractedunevenly


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Capillary ActionCapillary Action

is the ability of aliquid to rise (or fall)

in a glass tube orother container

affects meniscus ofliquids


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Cohesive forces are the forces that hold

liquids together.

Adhesive forces are the forces between aliquid and another surface.

Capillary rise implies that the:

Adhesive forces > cohesive forces Capillary fall implies that the:

Cohesive forces > adhesive forces


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Water exhibits a capillary rise. Mercury exhibits a capillary fall.


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EvaporationEvaporation

the process in which molecules escape fromthe surface of a liquid and become a gas; is

temperature dependent.


Boiling PointBoiling Point

is the temperature at which the liquids vaporpressure is equal to the applied pressure normal boiling point is the boiling point

when the pressure is exactly 1 atm

Distillation is a method we use to separatemixtures of liquids based on theirdifferences in boiling points.



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Vapor PressureVapor Pressure

is the pressure exerted by a liquids vaporon its surface at equilibrium.

Vapor Pressure (torr) and boiling point for threeliquids at different temperatures.

0oC 20oC 30oC normal boiling point

diethyl ether 185 442 647 36oCethanol 12 44 74 78oCwater 5 18 32 100oC


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Vapor Pressure as a function of temperature.


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IMFA strength leads to Higher melting point

Higher boiling point

Lower vapor pressure

Lower evaporation rate Stronger surface tension

Higher viscosity


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Comparing substances atcondensed state

40Chemistry 8ed, Whitten

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Comparing boiling point ofsubstances at condensed state

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condensed phases - liquids and solids

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