Transcript INTERMOLECULAR FORCES OF ATTRACTION

INTERMOLECULAR
FORCES OF
ATTRACTION
Attraction existing between molecules
in a given substance
- Responsible for the phase of the
substance at a given temperature; for
example, strong IMFA – solid at room
temperature; weak IMFA – gas at room
temperature
-
COMPARISON OF MOLECULAR
MASS, BOILING POINT
SUBSTANCE
MOLECULAR
WEIGHT (amu)
BOILING
POINT (K)
PROPANE CH3CH2CH3
44
231
DIMETHYL ETHER
CH3OCH3
46
248
METHYL CHLORIDE
CH3Cl
50
249
ACETALDEHYDE
CH3CHO
44
294
ACETONITRILE CH3CN
41
355
Relative Magnitudes of Forces
The types of bonding forces vary in their
strength as measured by average bond energy.
Strongest
Covalent bonds (400 kcal)
Hydrogen bonding (12-16 kcal )
Weakest
Dipole-dipole interactions (2-0.5 kcal)
London forces (less than 1 kcal)
TYPES OF IMFA
(weakest to strongest)
1. VAN DER WAAL’S FORCES
a. London dispersion force
– caused by the movement of the
electrons around the nucleus
- effect is very temporary
- present in all substances and only
force present in non-polar molecules
LONDON DISPERSION FORCE
Induced dipole – temporary charges
due to presence of electrons from
neighboring atoms
BOILING POINTS OF HALOGENS
substance
F2
Cl2
Br2
I2
Boiling point ( oC)
-188
-35
60
185
The higher the number of electrons, the
stronger is the London dispersion force.
SHAPE OF THE MOLECULE
AFFECTS BOILING POINT
b. dipole-dipole interaction
– present only in polar covalent
molecules
- caused by the dipole moment
(differences in the electronegativy
of the elements present)
Polarity
A molecule, such as HF, that has a center
of positive charge and a center of negative
charge is said to be polar, or to have a
dipole moment.
H F
+
-
Ethanol is Polar
c. Hydrogen bonding
- present in polar covalent
compounds where hydrogen is
directly attached to Fluorine,
Oxygen and Nitrogen ( the three
most electronegative elements)
FON
Hydrogen bonding only occurs if H
is attached to F, O and N.
Hydrogen Bonding
Bonding between hydrogen and more
electronegative neighboring atoms such as
oxygen and nitrogen
Base pairing in DNA by hydrogen bonding
Hydrogen bonding in water
H bonding in ice
2. IONIC BONDING –
- present only in ionic compounds.
- Electrostatic force of attraction.
An ionic compound dissolved in
water.
FORMULA TO DETERMINE THE
NUMBER OF RINGS AND/OR
DOUBLE AND TRIPLE BONDS
ρ=
2 + 2C + N – H – X
2
where
C = number of carbon atoms
N = number of nitrogen atoms
H = number of hydrogen atoms
X = number of halogen (Group 17) atoms