January 14, 2013

farrowbrainUrban and Civil

Nov 15, 2013 (3 years and 8 months ago)

59 views

Week 2 Monday January 14, 2013 page 1

solid
-
liquid equilibrium

Clausius
-
Clapeyron doesn’t apply


























































































Assume ΔH
fus

and ΔS
fus

don’t change with T:















(





)








(





)

That’s a good equation for really high pressures, like underneath glaciers.

sold
-
solid phase transition

polymorphism: when a
compound

forms several dif
ferent crystal structures

example: ZnS has two forms:

1. zinc blende, diamond
-
like structure, tetrahedral network

2. wurzite, hexagonal

polymorphism in
elements

is called
allotropy

example: carbon has 3 forms: 1. graphite 2. diamond 3. Buckyball

Sulfur has several allotropes: 1. rhombic = orthorhombic 2. monoclinic

Sulfur has 3 triple points:

1.
rhombic, monoclinic, gas in equilibrium at 95°C

2. monoclinic, liquid, gas in equilibrium at 119°C

3. rhombic, monoclinic, liquid in equili
brium at 151°C

metastable: phase α is said to be metastable with respect to β if G
m
α

> G
m
β

or µ
α

> µ
β

two boxes: one down flat and one up on end. The one up on end is metastable.

examples of metastable: diamond, superheated H
2
O(liq), supercooled li
quid, supersaturated solution,
superheated water coming out of undersea geysers

higher order transitions

-

everything we’ve done so far is first
-
order

liquid
-
gas or solid
-
liquid: Δ = q
p

≠ 0 Δ
V

≠ 0 1
st

order




(


)


C
P

at mp (melting point) is not defined

higher order transitions

For second order, C
P

increases by a finite amount.




(


)




ΔV = 0 ΔH = q
P

= 0

Example of second order transition: normal conductivity to superc
onductivity

For Hg: T
tran

= 4.2K at 1atm

Clausius
-
Clapeyron is meaningless, since ΔV = 0

For lambda (λ) transitions, C
P

approaches ∞ from both sides

examples of lambda transitions: paramagnetism to ferromagnetism

liquid He(I) to liquid He(II)