CHM 501 Lecture



Weak Interactions

These types of forces are responsible for formation of liquids and solids in covalently bonded species.

Ion-dipole coulombic attraction; usually a cation with a dipole

E = energy Z = ion charge = dipole moment 0 = permittivity of free space

r = distance between ion and center of the dipole

Dipole-dipole requires either a molecular dipole or a bond dipole

directional, shorter range

Ion-Induced Dipole

is the polarizability of the neutral

Dipole-Induced Dipole



Instantaneous Dipole-Induced Dipole (van der Waals or London Dispersion)

E = –3IEα²⁄r6

IE = ionization potential

van der Waals interaction explain the condensation of nonpolars:
halogen
bp (oC, 1 atm)
n-alkane
bp (oC, 1 atm)
F2
–187
CH4
–161
Cl2
–35
C2H6
–88
Br2
59
C3H8
–42
I2
183 (extrapolated)
C4H10
–1
   
C8H18
125



Hydrogen Bonding

A special case of dipole-dipole (maybe), but is relatively common and has large energies
(approaching covalent bond energies)

Characteristics:

1. Directional

2. Linear or near linear bond angles

3. Covalent-like bond energies 10-15 kcal/mole, 40-60 kJ/mole

4. Occurs between H+ and elements more electronegative than H+ (O, N, F)

HF has very unusual properties because of strong H-bonding

1. high bp

hydrogen halide
bp(oC,1atm)
 
HF
+9
 
HCl
–87
 
HBr
–67
 
HI
–35

2. oligomeric in the gas phase (HF)6 cyclic hexamer

3. weak acid in water, Ka ~ 10–4

HF(aq) + H2O(l H3O+(aq) + F(aq)

2HF(aq H2F+(aq) + F(aq)

HF(aq) + F(aq H2F(aq)

multiple competing equilibria