Post on 08-Aug-2020
Intermolecular Forces
Opposites ♂♀ Attract
or
Viva la difference (ooh lah lah …)
1 © Prof. Zvi C. Koren 20.07.2010
Very restricted motion
Short- range order
Long-range order
Restricted motion
Short-range order
Long-range disorder
Nearly unrestricted
motion
Short-range disorder
Long-range disorder
Phases of Matter
Solid
(crystalline)
Intermolecular Forces & Density:
GasLiquid
> >>
2 © Prof. Zvi C. Koren 20.07.2010
BUT:
Water is Weird (see later):
ds < dl
For a given quantity:
Vgas >> Vliquid > Vsolid
(except for water)
Differences in Volume Between the States of Matter
(Phases)
For example for water:
Liquid:
d = 1 g/mL V = 18 mL/mole
Gas (assume ideal):
@STP V = 22.4 L/mole
Vgas ~ 103Vliquiddgas << dliquid < dsolid
Compressibility dV/dP:
gas >> liquid > solid
3 © Prof. Zvi C. Koren 20.07.2010
1. Why can a gas fill the entire container?
2. Why can a condensed phase (solid, liquid) exist?
3. Why can liquids (and powdered solids) be poured?
Answers1. Intermolecular forces (weak) between gas molecules
2. Intermolecular forces between solid molecules (and
between liquid molecules)
3. Intermolecular forces between liquid molecules
Questions
4 © Prof. Zvi C. Koren 20.07.2010
Between Neutral Molecules (van der Waals Forces ):Dipole – Dipole
Dipole – Induced Dipole
Self-induced (or instantaneous) Dipole – Induced Dipole
(“London force”)
Involving an Ion:Ion – Ion (in salts, crystal lattice)
Ion – Dipole
Note: These are INTERmolecular “forces”, not intramolecular “bonds”
Types of Intermolecular Forces
(Each type will now be discussed)
5 © Prof. Zvi C. Koren 20.07.2010
μ
moment dipole μ
van der Waals Forces – 1:
Dipole – Dipole Forces
H
H
O
-+
H
H
O
H
H
O
H Cl H Cl H Cl
μ
Other configurations are also possible
6 © Prof. Zvi C. Koren 20.07.2010
van der Waals Forces – 2:
Dipole – Induced-dipole Forces
O2(g) dissolves (though slightly) in water. Why?
μ
H
H
O OO
OO
H
H
O - -+
Permanent dipole Induced dipole
= Polarizability
= f(total # and locations of e’s)
7 © Prof. Zvi C. Koren 20.07.2010
::
::
Solubility (moles/m3) in seawater
00C 240C
He 0.36 0.31
Ne 0.42 0.36
Ar 1.7 1.0
Kr 3.2 1.9
Xe 6.1 3.1
N2 0.80 0.54
O2 1.9 1.1
CO2 65 32
Solubilities of Some Gases in Water
Solubility (at 20 oC), mg/100 g H2OGas
0.160H2
0.190N2
0.434O2
729Cl2
8 © Prof. Zvi C. Koren 20.07.2010
van der Waals Forces – 3:
Instantaneous-dipole – Induced-dipole Forces
(London forces)
OO
OO -+
Self-induced dipole
Instantaneous dip.
= Polarizability = f(total # and locations of e’s)
OO -+ OO -+
Induced dipole
Equilibrium: NO dipole
9 © Prof. Zvi C. Koren 20.07.2010
Boiling (or Vaporization):
Molecule–Molecule(l) Molecule(g) + Molecule(g)
10 © Prof. Zvi C. Koren 20.07.2010
Summary Questions
What forces exist between two polar molecules?
What forces exist between a polar and a nonpolar molecule?
What forces exist between two nonpolar molecules?
11 © Prof. Zvi C. Koren 20.07.2010
(N, O, F)-H (N, O, F)"נוף הנוף"
Hydrogen Bonding
(Super Dipole–Dipole Force)
- + -
H-bond
CH3CH2OH, ethanol
Boiling Point = 78 oC(CH3)2O, dimethyl ether
Boiling Point = –25 oC
--
+ -
12 © Prof. Zvi C. Koren 20.07.2010
+
Boiling Points of Nonmetal Hydrides
Tb
Period2 3 4 5
0
100H2O
H2Te
HF
HINH3
SbH3
SnH4
CH4
S
Se
Compare all 3 forces
for each sample
Note: of H2O = 1.9 D, D=Debye
13 © Prof. Zvi C. Koren 20.07.2010
Snow
flake
The Weird Properties of Water
14 © Prof. Zvi C. Koren 20.07.2010
15 © Prof. Zvi C. Koren 20.07.2010
Crystal structure of hexagonal ice (Wikipedia)
16 © Prof. Zvi C. Koren 20.07.2010
Ion-Ion Forces
Na+ Na+
Ionic Forces:Coulomb’s Law קולוןחוק Beween Two Charges
r
qqkU
21
dr
dUF
2
21
r
qqkF
Note: In the Force equation: r-2. In “U”: r-1.
TWO factors are important: “q1•q2” and “r”
Compare the melting points of:
LiF vs. KI and MgO vs. NaCl.
F = forcer = distanceq = charge = n·e
U = potential
energy
Melting (Fusion): M+X–(s)
Repulsion
Repulsion
Attraction
Na+Cl-
Cl- Cl-
M+(l) + X–(l)
Li
Na
K
F
Cl
Br
I
17 © Prof. Zvi C. Koren 20.07.2010
U
F- Cl- Br- I-
Li+
Na+
K+
Potential Energy (“Lattice Energy”) of Ionic Pairs
Recall Coulomb’s Law for U:
r
qqkU
21
Li
Na
K
F
Cl
Br
I
18 © Prof. Zvi C. Koren 20.07.2010
Also: H2O above and below the plane
Cl–
Hydration of Ions
Coordination Number for each ion = 6
Water is the Ligand
62O)Na(H , aquo complex of Na+, and
Ion-Dipole Forces
HH
OH
H
O
HH
OH
H
ONa+
- -
-
-
What is the meaning of “Na+(aq)” and “Cl–(aq)”?62O)Cl(H
H
H
O+
HH
O
HH
O
+
+
+
H
H
O
HH
O
-
+
Dipole
moment
19 © Prof. Zvi C. Koren 20.07.2010
Hydration reaction:
M+ + (H2O)n M(H2O)n+ + Heat
1 rH
Recall Coulomb’s Law,
(though here it’s not exactly the same equation):
Heat (or Enthalpy) of Hydration,
Hhyd (kJ/mol)
Ion Radius (pm)Cation
–51590Li+
–405116Na+
–312152K+
–296166Rb+
–263181Cs+
r
qqkU
21
20 © Prof. Zvi C. Koren 20.07.2010
Energies of Interaction (U)
Type of Interaction Factors Responsible Distance- U(kJ/mole)
for the Dependence
Interaction of U
Chemical
(intramolecular):
Covalent bond (single) orbital (wave) overlap 100 – 600
Ionic Forces:
Ion – Ion Charges r-1 400 – 4000
Ion – Dipole Charge & Dipole moment r-2 40 – 600
van der Waals Forces:
Dipole – Dipole (regular) Dipole moments r-3 5 – 25
H-bond 10 – 40
Dipole – Induced-dipole Dipole m. & Polarizability r-6 2 – 10
Self-induced dipole –
Induced-dipole
(“London”) Polarizabilities r-6 0.05 – 4021 © Prof. Zvi C. Koren 20.07.2010
Molar Mass is a simple indicator of the polarizability of a molecule
IMF’s as a Predictor of the State of Matter (at room temp.)
22 © Prof. Zvi C. Koren 20.07.2010
Vaporization Condensationl g g l
23 © Prof. Zvi C. Koren 20.07.2010
Hvap > 0, endothermic
Properties of Liquids:
Vapor Pressure & Boiling Point
Hcond < 0, exothermic
211
2 11
TTR
H
P
Pn
vap
(y = a •x + b)
CTR
HnP
vap
1 nP
1/T
••
••
slope = a
Volatility:
Which is the
more volatile
liquid?
Vapor Pressure vs. T
Clausius-Clapeyron Equation:RTHvapkeP
/
24 © Prof. Zvi C. Koren 20.07.2010
In general: As Tb , ΔHvap
Trouton’s Rule: ΔHvap / Tnb constant, T = [K]
25 © Prof. Zvi C. Koren 20.07.2010
At the Boiling Point, Tb (or b.p.): Pvap = pex + “dP”
“ex” = external
Normal Boiling Point, Tnb or n.b.p.: Pex = 1 atm
[Constant Pressure: P = f/A; f = w = mg]
s s l l l v g
vaporization
fusion
100
0
Tem
p.
(0C
)
Time (min)100 600
Heating/Cooling Curve for Water.
1 mol water is heated from –100C to 1100C.
A constant heating rate of 100 J/min is assumed.
Pistonm
26 © Prof. Zvi C. Koren 20.07.2010
Calculation of the Heats Involved With Each Step
in the Heating/Cooling Curve
vaporization
fusion
100
0
Tem
p.
(0C
)
Time (min)
100 600
Value for H2ONameSymbol
1.00 cal/g·deg
4.18 J/g ·deg
specific heat capacity
(of liquid)C(l)
18.00 cal/mol·deg
75.2 J/mol ·deg
molar heat capacity
(of liquid)
333 J/gheat of fusionΔHfus
2250 J/gheat of vaporizationΔHvap
)( lC“C-bar”
27 © Prof. Zvi C. Koren 20.07.2010
Find the values of C(s) and C(g) of H2O
critical point(374.10C, 218.3 atm)
@ T > Tc, “supercritical fluid”
Phase Diagram of Water
28 © Prof. Zvi C. Koren 20.07.2010
Gibbs Phase Rule:
F = C – P + 2F = Degrees of Freedom
2: surface (2-D)
1: curve (1-D)
1: point (0-D)
C= # of Components
P = Equilibrium Phases
Phase Diagram of Carbon Dioxide
-780C
1
(Tc,Pc)
“Dry Ice”
29 © Prof. Zvi C. Koren 20.07.2010
Energy required to break through the surface,
or
Energy required to disrupt a drop of liquid and spread the
material out as a film (to increase surface area).
@200C:
C2H5OH: 2.2 x 10 -2
H2O: 7.3 x 10-2
Hg: 46 x 10 -2
Surface molecules
(net inward force)
Interior
molecules
“skin”
More Liquid properties:
Surface Tension, (J/m2)
30 © Prof. Zvi C. Koren 20.07.2010
meniscus
Glass(silica: polar
Si-O bonds)
cohesive forces:
H2O – H2O
adhesive forces:
H2O – Glass
adhesive f. > cohesive f.: cohesive f. > adhesive f.
Hg
(concave)
(convex)
ad. f. = co. f. + gravitational f.
Glass is hydrophilic Mercury is hydrophobic
More Liquid properties:Capillary Action
31 © Prof. Zvi C. Koren 20.07.2010
Measures a liquid’s resistance to flow (time necessary)
Glycerol has 2 OH groups,
2 H-bonds possible
Other factors besides intermolecular forces need to be considered:
Length of molecular chain;
long chains become floppy and become entangled with one another.
More Liquid properties:Viscosity
32 © Prof. Zvi C. Koren 20.07.2010