1
Liquids
Molecular – vdW forces, H-bonds
Metallic – melted metals, ions + electrons, electrostatic
forces
Ionic – melted salts, FLINAK (LiF + NaF + KF), freely moving
anions and cations, ion electric conductivity,
EtNH3
+ NO3
- m.p. 12 °C
N N
N N
(CH2
)n
NN
N
N N
HO OH
Cl-, AlCl4
-, Al2Cl7
-,
Al3Cl10
-, PF6
-, SnCl3
-,
BCl3
-, BF4
-, NO3
-,
OSO2CF3
- (triflate),
CH3C6H4SO3
-,
N(SO2CF3)2
-, PO4
3-
2
Hole Theory of Liquids
Solids (molecular) – close packing in lattice, molecules touch,
vdW radii
Liquids – same nearest-neighbor distances as in solids, lower
density, coordination number decreases with increasing
temperature
Ar (s) c.n. 12
Ar (l) c.n. 10 – 11 at melting, density lower by 12%
Ar (l) c.n. 4 at critical temperature
Liquids – free space (voids) in nearly close packed structure,
molecules with a high Ekin move through the structure,
molecules with a low Ekin engage in vdW interactions
3
Hole Theory of Liquids
Two types of molecules in liquids:
1. molecules neighboring a vacation (hole) – similar to (g)
2. molecules surrounded by other molecules – similar to (s)
Structure of liquids is between regular structure of solids and
random motion of gases.
Ekin of liquid molecules is too high to keep them in lattice
positions, but too low to leave vdW interactions and
escape from a container
4
Surface Tension
A force in a surface of a
liquid, that keeps the surface
area at minimum – spherical
shape.
Surface Tension = Energy to
form 1 m2 of new surface
[N m–1 = J m–2]
Molecules on a surface of a
liquid interact with other
molecules inside liquid –
unequal forces
5
Surface Tension
Surface Tension = Energy to form new surface = to take
molecules from inside a liquid (strongly held) and bring
them to surface (weakly bound)
Free surface energy E
E = γ S
γ = surface tension [N m–1 = J m–2]
S = surface area
F = γ l [N m–1 = J m–2]
dS
dE
=γ
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Surface Tension
Phase boundary (T = 20 oC) γ, Surface tension [mJ m−2]
Water / Air 72.75
Hg / Air 472
Benzene / Air 28.88
Water / Air (100 oC) 58.0
Vodoměrka
Desinfection
Surfactants - soaps
7
Surface Tension of Water
Surface tension decreases
with increasing temperature.
8
Surface Tension Meaurements
Tensiometer
Plate - Wilhelmy
Tensiometer
Ring – DeNouy
2 π D γ = F
Hanging droplet
dS
dE
=γ
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Paper Chromatography
10
Viscosity
Internal friction, resistance to flow
Increases with intermolecular forces:
OH OH OH
OH OH
OH
Increases with chain length, entanglement
Decreases with increasing temp η = A exp (E / RT)
Stokes equation
F = 6 π η r v
η = viscosity [kg m–1 s–1]
r = ball radius
v = speed
11
Evaporation and Condensation
Molecules at surface with sufficient Ekin and correct movement
direction can overcome vdW forces and surface tension and leave
liquid to gas phase even below boiling point
Evaporation of liquid = Energetically rich
molecules leave – liquid cools
Condensation = collisions of vapor molecules (g) with surface (l),
loss of Ekin, molecules trapped by vdW forces into (l)
vaporization and condensation enthalpy
ΔHvap > 0 endo ΔHcondí < 0 exo
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Vapor Pressure
Vapor
Pressure
Dynamic
equilibrium
Closed space
13
Vapor Pressure
14
Vapor Pressure
1325470377Diethylether
92.523.817.5Water
50 ºC25 ºC20 ºCTemp.
Compound
Vapor pressure increases with temp (760 torr = 101.325 kPa)
[torr]
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Vapor Pressure and Kinetic Theory
At a given T, liquids with
weak vdW forces have
higher vapor pressure
Weak vdW forces
strong vdW forces
Only molecules with Ekin > Eimf
can leave a liquid
16
Vapor Pressure and Temperature
Only molecules with Ekin > Eimf
can leave a liquid
17
Vapor Pressure
Normal pressure
101.325 kPa
18
Boiling point = temp, at which vapor pressure equals
ambient pressure
Normal boiling point = temp, at which vapor pressure equals
ambient pressure of 101.325 kPa
Sublimation point = temp, at which vapor pressure of a solid
equals ambient pressure
Normal sublimation point = temp, at which vapor pressure of
a solid equals ambient pressure of 101.325 kPa
Boiling and sublimation can be induced by heating or
lowering ambient pressure
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Normal Boiling Points of Group 14– 17
Hydrides
17
Strong H-bonds keep
molecules in liquid =
low vapor pressure
= high boiling points
16
15
14
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p-T Phase Diagram
21
Critical Point of Benzene
300.7 ºC
307.4 ºC
309.2 ºC
Phase boundary (meniscus)
between l and g disappears
22
Water Density (g, l, s)
23
p-T Phase Diagram
Increasing pressure
decreases melting point
of water = anomaly
Increasing pressure
causes solidification of
liquid
24
Clausius-Clapeyron Equation
m
m
VT
H
dT
dp
Δ
Δ
=
Clapeyron eq. of phase transition
For l-g equil:
1) Vm(g) >> Vm(l), then ∆Vm = Vm(g)
2) Vm(g) from id. gas eq.
p
RT
gVm =)(Differential Clausius-Clapeyron eq.
2
ln
RT
H
dT
pd mΔ
=
Integrated Clausius-Clapeyron eq.
25
Antoine Equation
p = vapor pressure (bar)
T = temp (K)
-45.6221663.1255.08354344. - 373.
-45.8541659.7935.07680334. - 363.
-39.4851733.9265.20389304. - 333.
-31.7371838.6755.40221273. - 303.
-198.043643.7483.55959379. - 573.
CBATemp. interval (K)
Water
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Trouton Rule
11,
0
0
, 90 −−
=
Δ
=Δ molJK
T
H
S
b
vapm
vapm
ΔG = 0 in equlibrium, at phase transitions
ΔG = ΔH - TΔS = 0
For different liquids (nonpolar) at normal
boiling point, vaporization molar entropy is
roughly the same:
Not true for water at 100 ºC – very strong H
bonds = ordered structure = small entropy of
liquid water
ΔS0
vap = 109 J K-1 mol-1
liquid
vapor
ΔS
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Diffusion
In liquids and gases, in solids at increased
temperature
Spontaneous mixing of compounds
Mass transfer
Equalization of concentrations
Result of random movements of molecules
dx
dc
D
Adt
dn
J −==
First Fick’s law
for diffusion flux J
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First Fick’s Law
J = diffusion flux
[mol s−1 m−2]
n = molar amount [mol]
D = diffusion coeficient
[m2 s−1]
dc/dx = gradient of
concentration
A = area [m2]
x
diffusion flux