Why mixtures mix Consider a glass of wine. Why do alcohol,
water, & pigment mix together? There must be attractive forces.
Intramolecular forces is the force of attraction between atoms in a
molecule Intermolecular forces is the attraction between molecules
Intermolecular forces are responsible for many properties of
molecular compounds, including crystal structures (e. g. the shapes
of snowflakes), melting points, boiling points,
Slide 4
Types of Intermolecular Forces of attraction 1) Ionic (Ionic
bonds form if the EN is 1.7 or greater) 2) dipole dipole ( EN is
around 0.5-1.7) 3) H-bonding( is a type of dipole-dipole) 4) London
forces or Van der Waals Forces (exist in all molecules, but are
especially important in non- polar covalent molecules, where EN is
less than 0.5).
Slide 5
Ionic intermolecular forces of attraction are strongest
Dipole-dipole are not as strong as ionic Hydrogen bonds are about
five times stronger than regular dipole-dipole bonds. London forces
are weakest force of attraction.
Slide 6
London forces Non-polar molecules do not have dipoles like
polar molecules. How, then, can non-polar compounds form solids or
liquids? London forces are named after Fritz London (also called
van der Waal forces) London forces are due to small dipoles that
exist in non-polar molecules. These forces exist between all
molecules Because electrons are moving around in atoms there will
be instants when the charge around an atom is not symmetrical The
resulting tiny dipoles cause attractions between
atoms/molecules
Slide 7
Dipole-dipole interaction This happens in all molecules having
polar bonds. But Polar molecules are held to each other more
strongly than the non-polar molecules of comparable molecular
weight. occur when polar molecules are attracted to one another.
Electrostatic attraction between (+) and (-) ends of molecules This
accounts for the attraction one water molecule has for another
water molecule.
Slide 8
H-bonding H-bonding is a special type of dipole - dipole
attraction that is very strong It occurs when hydrogen is
covalently bonded to a very electronegative atom like N, O, or F
because of the small size of hydrogen, its positive charge can get
very close to the negative dipole of another molecule. H is so
strongly positive that it will sometimes exert a pull on a lone
pair in a non-polar compound The high EN (electronegativity
difference) between two atoms like of NH, OH, and HF bonds cause
these to be strong forces (about 5x stronger than normal
dipole-dipole forces). Calculate the EN for HCl and H 2 O Soln:HCl
= 2.9-2.1 = 0.8 H 2 O = 3.5-2.1 = 1.4 They are given a special name
(H-bonding) because compounds containing these bonds are important
in biological systems O H HO H H
Slide 9
Molecules with 3 Atoms HCN Even though the C-O bond is polar,
the bonds cancel each other out because the molecule is linear the
dipole moments are equal and in opposite directions. Therefore CO 2
is non-polar. The dipole moment between H-C points in the direction
of C. The dipole moment points between C-N points in the direction
of the N. Therefore the dipole vectors are additive and HCN is
polar CO 2 SO 2 is a polar molecule because the S-O dipole Moments
dont cancel each other out due to the angle SO 2
Slide 10
10.2 Which of the following molecules are polar (have a dipole
moment)?H 2 O, CO 2, SO 2, and CH 4 O H H dipole moment polar
molecule S O O CO O no dipole moment nonpolar molecule dipole
moment polar molecule C H H HH no dipole moment nonpolar
molecule
Slide 11
CCl 4 is non-polar CHCl 3 is polar Molecules with 4 Atoms
Slide 12
Formation of Intermolecular H- bonds 1.Between non-identical
molecules a. alcohol and water b. chloroform and pyridine
Slide 13
c. acid and water 2. Between identical molecules a. NH 3
molecule
Slide 14
b. hydrocyanic acid c. alcohols
Slide 15
d. acids e. amines
Slide 16
Intramolecular H- bond results to ring formation or chelation.
Ortho substituted are usually involved in chelation, because meta
and para substituents are already distal group as far as effective
H- bonding is concerned.
Slide 17
A. B. C.
Slide 18
STRUCTURE AND PHYSICAL PROPERTIES 1. MELTING POINT 2. BOILING
POINT 3. SOLUBILITY
Slide 19
BOILING POINT One of the most revealing of all physical
properties for a chemical substance is its boiling point. Boiling
point reflects the strength of the intermolecular attractive forces
that hold the molecules of a substance together in a condensed
phase, and as such, it is useful to compare the boiling points for
related compounds to see how structural differences account for the
differences in intermolecular attractions. The trends in boiling
points for various groups of compounds helps in understanding how
size, shape, and functional group polarity affect boiling
point.
Slide 20
The boiling point is the temperature at which the (equilibrium)
vapor pressure of a liquid is equal to the external pressure. The
normal boiling point is the temperature at which a liquid boils
when the external pressure is 1 atm. The boiling point is the
temperature at which the (equilibrium) vapor pressure of a liquid
is equal to the external pressure. 11.8
Slide 21
FACTORS THAT AFFECTS BOILING POINT/ MELTING POINT 1. Molecular
Weight/Size The bigger the molecule, the higher its boiling point.
Thus, for a series of related compounds, the higher the molecular
weight, the higher the boiling point. Note the trend for the first
five straight-chain alkanes:
Slide 22
CompoundFormula (mol. wt.)B.P. methaneCH 4 (16)-164C ethaneC 2
H 6 (30)-88C propaneC 3 H 8 (44)-42C n-butaneC 4 H 10 (58)0C
n-pentaneC 5 H 12 (72)36C Van der Waals attractive forces increases
as the hydrocarbon chain increases so that the boiling point is
high.
Slide 23
2. Branching in the hydrocarbon lowers the boiling point In the
straight chain hydrocarbon molecules can approach each other so
stronger Van der Waals attractive force. In branched chain
hydrocarbon, there will be no close approach of molecules so that
there is weak van der waals attractive force.
