Liquids and SolidsChapter 11

INTERMOLECULAR FORCES

Liquids vs. SolidsPhysical properties are due to

intermolecular forcesUnderstood in terms of kinetic-molecular

theoryGases are highly compressible and assume

the shape and volume of their containerLiquids are almost incompressible, assume

the shape but not the volume of the containerSolids are incompressible and have a definite

shape and volume

Liquids vs. SolidsSolids and liquids are condensed phases

Converting a gas into a liquid or solid requires the molecules to get closer to each other

Forces holding solids and liquids together are called intermolecular forces

INTERMOLECULAR FORCES

Intermolecular ForcesAttraction between molecules

Weaker than ionic or covalent bonds (16 kJ/mol vs. 431 kJ/mol for HCl)

Melting or boiling breaks intermolecular forces

Condensing forms intermolecular forcesMelting points / Boiling points reflect

strength of intermolecular forcesHigh melting/boiling points indicates strong

attractive forces

Intermolecular ForcesVan der Waals forces exist between

neutral moleculesIncludes London-dispersion forces, dipole-

dipole forces, and hydrogen-bonding forcesIon-dipole interactions are important in

solutionsALL are WEAK electrostatic interactions

(~15% as strong as a covalent or ionic bond)

Van der Waals ForcesIon-Dipole

Interaction between an ion and the partial charge on the end of a polar molecule (dipole)

Important in formation of solution between ionic substances in polar liquids (ex. NaCl in water)

Dipole-DipoleExist between neutral polar moleculesPolar molecules attract each otherNeed to be close together to form strong

attractionsWeaker than ion-dipole forces

Van der Waals ForcesLondon Dispersion ForcesWeakest of all intermolecular forcesPossible for two adjacent neutral

molecules to affect each otherNucleus of one molecule (atom) attracts the

electrons in an adjacent molecule (atom)Electron “clouds” become distorted –

temporaryTemporary distortion creates an

instantaneous dipoleOne instantaneous dipole can create an

instantaneous dipole in a nearby molecule (atom)

Temporary dipoles attract each other

Van der Waals ForcesLondon Dispersion ForcesMolecules must be very close together for

these attractive forces to occurPolarizability is the ease with which an

electron cloud can be deformedThe larger the molecule- the more polarizable

it isForces increase as molecular weight

increasesForces depend on the shape of the

molecule

Van der Waals ForcesHydrogen BondsBoiling points of compounds with hydrogen

bonded to an electronegative atom are abnormally high

Special case of dipole-dipole interactionsRequires:

H bonded to a small electronegative elementAn unshared pair of electrons on a nearby

small electronegative atom/ionHydrogen only has one electron, so in an

electronegative bond it is “electron bare”

PROPERTIES IN LIQUIDS

Properties in LiquidsViscosity

Viscosity is the resistance of a liquid to flow.A liquid flows by sliding molecules over each

other.The stronger the intermolecular forces, the

higher the viscosity.Surface Tension

Bulk molecules (those in the liquid) are equally attracted to their neighbors.

Surface molecules are only attracted inwards towards the bulk molecules

Surface Tension

Surface Tension Surface tension is the amount of energy required

to increase the surface area of a liquid.

Cohesive forces bind molecules to each other. Adhesive forces bind molecules to a surface

Meniscus is the shape of the liquid surface. Adhesive > Cohesive : U-shaped meniscus (water)

Capillary Action: When a narrow glass tube is placed in water, the meniscus pulls the water up the tube

PHASE CHANGES

Phase Changes

Enthalpy of Phase ChangesSublimation: Hsub > 0 (endothermic). Vaporization: Hvap > 0 (endothermic).Melting or Fusion: Hfus > 0

(endothermic).

Deposition: Hdep < 0 (exothermic). Condensation: Hcon < 0 (exothermic).Freezing: Hfre < 0 (exothermic).

Heating Curves Plot of temperature change versus heat added is a

heating curve.

During a phase change, adding heat causes no temperature change.These points are used to calculate Hfus and Hvap.

Supercooling: When a liquid is cooled below its melting point and it still remains a liquid. Achieved by keeping the temperature low and

increasing kinetic energy to break intermolecular forces.

Critical Temperature and PressureGases liquefied by increasing pressure at

some temperature.

Critical temperature: the minimum temperature for liquefaction of a gas using pressure.

Critical pressure: pressure required for liquefaction.

Vapor Pressure on a Molecular Level

Vapor PressureDynamic Equilibrium: the point when as many

molecules escape the surface as strike the surface. Vapor pressure is the pressure exerted when the

liquid and vapor are in dynamic equilibrium.

Volatility, Vapor Pressure, and Temperature If equilibrium is never established then the liquid

evaporates. Volatile substances evaporate rapidly. The higher the temperature, the higher the average

kinetic energy, the faster the liquid evaporates.

Vapor Pressure and Boiling PointLiquids boil when

the external pressure equals the vapor pressure.

Temperature of boiling point increases as pressure increases.

PHASE DIAGRAMS

Phase Diagrams

Phase DiagramsWater vs. Carbon Dioxide

SOLIDS

Unit Cells Crystalline solid: well-ordered, definite

arrangements of molecules, atoms or ions. Crystals have an ordered, repeated structure.

The smallest repeating unit in a crystal is a unit cell. Unit cell is the smallest unit with all the symmetry

of the entire crystal.

Three-dimensional stacking of unit cells is the crystal lattice.

Unit Cell vs. Lattice

Three common types of unit cell.Primitive cubic, atoms at the corners of a simple

cube, each atom shared by 8 unit cells;

Body-centered cubic (bcc), atoms at the corners of a cube plus one in the center of the body of the cube, corner atoms shared by 8 unit cells, center atom

completely enclosed in one unit cell;Face-centered cubic (fcc), atoms at the corners of a

cube plus one atom in the center of each face of the cube, corner atoms shared by 8 unit cells, face atoms shared by 2

unit cells.

Unit Cells

Solids: Four Types Molecular (formed from molecules) - usually soft

with low melting points and poor conductivity.

Covalent network (formed from atoms) - very hard with very high melting points and poor conductivity.

Ions (formed from ions) - hard, brittle, high melting points and poor conductivity.

Metallic (formed from metal atoms) - soft or hard, high melting points, good conductivity, malleable and ductile.

Covalent Network Solid

Ionic Lattice CsCl Structure

Cs+ has a coordination number of 8.

Cation to anion ratio is 1:1. Zinc Blende Structure (ZnS).

S2- ions adopt a fcc arrangement. Zn2+ ions have a coordination

number of 4. The S2- ions are placed in a

tetrahedron around the Zn2+ ions. Fluorite Structure (CaF2).

Ca2+ ions in a fcc arrangement. There are twice as many F- per

Ca2+ ions in each unit cell.