Chpt 10 - Condensed Phases

• Condensed phases• Intermolecular forces• Special bonding - molecular

solids, network solids, metallic• Phase diagrams & Heating

curves• HW: Chpt 10 - set #1 pg. 487-496, #s 12, 14-16, 19-

21, 24, 26, 31, 32, 34, 40, 44 Due Mon Dec. 7• HW: Chpt 10 - set #2 pg.487-496, #s 67, 68, 93, 94,

96, 101, 102, 117 - Due Tues Dec. 8

States of Matter

Differences?What do the phases look like?What makes the state of matter at a given temperature?

Intermolecular forces

Intermolecular Forces

• Intramolecular forces (chemical bonds) - forces that hold atoms together within a molecule

• Intermolecular forces - forces between molecules - aggregate or bulk material - Is it a solid, liquid or gas?– dipole-dipole force (~1% of strength of a bond) – Hydrogen bonding H and N,O,F bond– London dispersion forces

Dipole-Dipole Force

•Dipole moment – molecules with polar bonds often behave in an electric field as if they had a center of positive charge and a center of negative charge.•Molecules with dipole moments can attract each other electrostatically. They line up so that the positive and negative ends are close to each other.

Hydrogen Bonding

(a) Polar water molecule (b) hydrogen bonding between water molecules - blue dotted lines

Very strong dipole-dipole force between H and N,O,F(most electro-negative elements)

Hydrogen bonding graph of covalent hydrides

Why are these interaction forces happening?•Especially polar X-H bond•Small size of N,O, and F allow close approach of dipoles

London Dispersion Forces

• Weakest of the intermolecular forces

• Important for atoms & non-polar molecules

•As the motion of these atoms and molecules slows (low T) the interaction becomes apparent. •Halogens Trend!!!•Occurs in all molecules even polar ones

London Dispersion Forces - How?

Moving e- make a momentary nonsymmetric e- distribution, which produces a temporary dipole. This then can induce a similar dipole in a neighboring atom or molecule. Becomes significant for large atoms with large # of electrons.Termed polarizability of an electron cloud.

Characteristics Intermolecular Forces

• In general, the stronger the intermolecular forces, the higher the melting and boiling points.

• Decrease rapidly with increasing intermolecular distance especially for London dispersion– Nonpolar solids (I2 and CO2) sublimate

Dry Ice Sublimation at RTemp

Liquids characteristics• Low compressibility, lack of rigidity, and high density

compared with gases.• Surface tension – resistance of a liquid to an increase

in its surface area: Liquids with large intermolecular forces tend to

have high surface tensions. H2O dropletsPlaying with Hg video YouTube

http://www.youtube.com/watch?v=31CE2BYicyU&feature=fvw

• Capillary action – spontaneous rising of a liquid in a narrow tube: YouTube video water special

http://www.youtube.com/watch?v=CT4pURpXkbY&feature=related

Cohesive forces – intermolecular forces among the molecules of the liquid.

Adhesive forces – forces between the liquid molecules and their container.

Liquid - Cohesive or adhesive?

Which force dominates alongside the glass tube – cohesive or adhesive forces?

adhesive forces “Like attract like”

determines which will dominate

Surface molecule interactions

Cohesive vs. Adhesive meniscus graphic

Water (polar) interaction with glass surface (polar) and mercury (non-polar) with glass surface (polar)

Liquids characteristics - cont

• Viscosity – measure of a liquid’s resistance to flow: Liquids with large intermolecular

forces or molecular complexity tend to be highly viscous.

Solids

• Amorphous solids– Non-uniform structure

• glasses • waxes

• Crystalline solids– Uniform lattice structure (regular

arrangement of atoms) – Unit Cell - smallest repeating unit of the

lattice

Cubic Unit cell and lattices

X-ray diffraction (crystallography) used to determine arrangement of atoms

= 2 sin λ θn d

n = integer

lambda = wavelength of the X rays

d = distance between the atoms

theta = angle of incidence and reflection

Bragg Diffraction graphic

= 2 sin λ θn dBragg equation

Types of Crystalline solids

• Ionic Solids – ions at the points of the lattice that describes the structure of the solid.

• Molecular Solids – discrete covalently bonded molecules at each of its lattice points.

• Atomic Solids – atoms at the lattice points that describe the structure of the solid.

Lattice of crystalline solids

Structure and bonding in Metals• Closest Packing:

Assumes that metal atoms are uniform, hard spheres.

Spheres are packed in layers. Like oranges in grocery store display

abab packing - 3rd directly over 1st layer - called hexagonal closest pack (hcp)

Structure and bonding in Metals (con’t)

abca packing - 3rd layer not directly over 1st, 4th layer is over 1st - cubic closest pack (ccp) or face centered cubic (fcc) see next slide

Face Centered Cubic (FCC)

Metallic Bonding Nearest Neighbors

• The Indicated Sphere Has 12 Nearest Neighbors

Each sphere in closest packed (both fcp and hcp) has 12 equivalent nearest neighbors.

What about bcc ? simple cubic ?

Unit cell atoms

fcc and hcp8 x 1/8 spheres and 6 x 1/2 spheres = 4 total atoms in unit cellWhat about bcc? Or simple cubic?What does that say about density of metals?

Metallic Bonding• Sea of electrons - regular array of cations

surrounded by its valence electrons

Metallic bonding MO model

• Band Model (MO Model) - combinations of atomic orbitals.

Virtual continuum of levels, called bands. Many semiconductor applications

Metal alloys

• Metals melted together to make a solution (homogeneous solid!!) - 2 types– Substitutional Alloy – some of the host

metal atoms are replaced by other metal atoms of similar size.

– Interstitial Alloy – some of the holes in the closest packed metal structure are occupied by small atoms.

Metal alloys graphics

Which is a substitutional alloy?

Which is an interstitial alloy?

Network atomic solids

2 main allotropes of carbon (3rd is buckyballs). What is hybridization on each C atom in these two structures?

Graphite - sp2 hybridization

p-orbitals and Pi system in graphite for 1 layer (sheet). Graphite layers slide by each other because of e- repulsion. Large difference between diamond and graphite is type of bonding

Carbon Atoms in Graphite

Types and Properties of Solids - Table

Vapor pressure graphic

a) Not equilibrium (pressure increasing)

b) Equilibrium (pressure constant)

Not closed --> no Pvap just Patm

Vapor pressure rate diagram

Why does rate of condensation increase initially? While the rate of evaporation remain essentially constant ?

Vapor Pressure definition

• Pressure of the vapor present at equilibrium.

• The system is at equilibrium when no net change occurs in the amount of liquid or vapor because the two opposite processes exactly balance each other.

• The boiling point of the liquid is when the Pvap = Patm

• Normal boiling point of liquid is at 1 atm.

Vapor pressure trends

• Liquids in which the intermolecular forces are strong have relatively low vapor pressures.

• Vapor pressure increases significantly with temperature.

Vapor pressure of various liquids

Pvap rationale Temp vs. KE plot

T2 > T1, which means on average more molecules have sufficient energy to overcome liquid intermolecular forces (more evaporate --> rate faster)

Pvap - Clausius-Clapeyron equation

Plots of In(Pvap) vs. (b) 1/T

The vapor pressure increases dramatically with temperature.

The ratio (slope) is Hvap/R !!

Clausius–Clapeyron Equation

Pvap = vapor pressure

ΔHvap = enthalpy of vaporizationR = 8.3145 J/K·molT = temperature (in kelvin)

Allows us to calculate the ΔHvap of a substance from vapor pressure measurements!!

1

2

vap, vap

vap, 2 1

1 1ln = ⎛ ⎞ ⎛ ⎞

−⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠

T

T

P HP R T T

ln is natural logarithm For calculation: to undo ln use ex

Vapor pressure calc. problem

The vapor pressure of water at 25°C is 23.8 torr, and the heat of vaporization of water at 25°C is 43.9 kJ/mol. Calculate the vapor pressure of water at 65°C.

194 torr

Heating curve for water

Temp changing use Q = c x m x T

Temp not changing use H units are J/mol usually

Hvap liquid <--> gas Hfus solid <--> liquid

Why is Hvap > Hfus ?

Phase Diagrams (P,T)

•A convenient way of representing the phases of a substance as a function of temperature and pressure:

Triple pointCritical pointPhase equilibrium lines

Phase diagram for CO2

Phase Diagram definitions

• Triple point - point at which all 3 phases exist in equilibrium

• Critical Temperature, Tc - the temperature at which no amount of pressure will be able to condense the gas

• Phase equilibrium lines, points on the line have 2 phases in equilibrium, the melting/freezing line, the boiling/condensation line, and the sublimation/deposition line.

Phase Diagram for Water

What is different about phase diagram for water from most other substances?

The solid becomes a liquid at high pressures!!!

The liquid is more dense than the solid.

Concept check

As intermolecular forces increase, what happens to each of the following? Why? Boiling point Viscosity Surface tension Enthalpy of fusion Freezing point Vapor pressure Heat of vaporization-