CH 2: Polar Covalent Bonds;
Acids and Bases
Renee Y. Becker
Valencia Community College
Electronegativity and Bond Polarity
Greater EN means greater polarity
Polar Covalent Bonds
• Polar Covalent Bonds
– Form between a non-metal/non-metal of
• Polar Molecules
– Just as bonds can be polar, molecules as a
whole can be polar
– Net sum of individual bond polarities and lone-
• Dipole moment, , (ionic and covalent)
– Measure of net molecular polarity
– The magnitude of the charge Q at either end of
the molecular dipole times the distance, r,
between the charges
– Expressed in debyes, D, where
1 D = 3.336 x 10-30 coulomb meters
Which of the following has the highest %
Tell which of the following compounds are likely to have
a dipole moment and show the direction of each.
a) SF6 b) CHCl3
F S F Cl Cl
c) CH2Cl2 d) CH2CH2
Cl H H H
• Van der Waals forces – intermolecular forces as a
whole, all are electrical in origin and result from
the mutual attraction of unlike charge or mutual
repulsion of like charges.
4 main types
• Dispersion forces
• Hydrogen bonding
a) Neutral but polar molecules experience dipole-dipole forces as
a result of electrical interactions among dipoles on neighboring
b) Forces can be attractive or repulsive, depending on the
orientation of the molecules.
c) These forces are weak 3-4 kJ/mol and only significant if
molecules are close
HH H H H Ion-dipole
O Na O Cl
H O H H H
H H O
Result of electrical interactions between an ion and the partial
charges on a polar molecule
b) Particularly important in aqueous solutions of ionic substances
such as NaCl, in which polar water molecules surround the ions
London Dispersion Forces
a) Result from the motion of electrons
b) At any given time more electrons may be in a particular area of
c) This gives the molecule an instantaneous dipole
d) This short lived dipole can affect the electron distribution in
neighboring molecules and induce temporary dipoles in them
e) More electrons a molecule has the stronger the dispersion forces
a) Attractive interaction between a hydrogen atom bonded to a very
electronegative atom (O, N, F) and an unshared electron pair on
another electronegative atom
b) Hydrogen bonds arise because O-H, N-H, and F-H bonds are
highly polar with partial positive charge on the hydrogen and
partial negative on the electronegative atom.
c) Hydrogen has no core electrons to shield its nucleus and it is
small so it can be approached closely by other molecules
d) The dipole-dipole attraction between the hydrogen and an
unshared electron pair on a nearby atom is usually strong 12
e) Water is able to form a vast 3D network of hydrogen bonds
because each H2O molecule has two hydrogens and two
Identify the likely kinds of intermolecular forces in the
Of the substances Ar, Cl2, CCl4 and HNO3 which
a) The largest dipole-dipole forces?
b) The largest hydrogen-bond forces?
• Only electrons can be moved (usually lone pairs or
• Nuclei positions and bond angles remain the same.
• The number of unpaired electrons remains the
• Resonance causes a delocalization of electrical
_ _ _
O O O
N N N
O O O O O O
• The real structure is a resonance hybrid.
• All the bond lengths are the same.
• Each oxygen has a -1/3 electrical charge.
Major Resonance Form
• has as many octets as possible*.
• has as many bonds as possible*.
• has the negative charge on the most
• has as little charge separation as possible.
The more resonance forms the more stable 19
Draw the important resonance forms, label
major and minor
Which is the Major Contributor?? Why?
H + H H + H
C N C N
H H H H
Arrhenius Acids and Bases
• Acids dissociate in water to give H3O+ ions.
• Bases dissociate in water to give OH- ions.
• Kw = [H3O+ ][OH- ] = 1.0 x 10-14 at 24°C
• pH = -log [H3O+ ]
• Strong acids and bases are 100% dissociated.
BrØnsted-Lowry Acids and Bases
• Acids can donate a proton
• Bases can accept a proton
• Conjugate acid-base pairs
CH3 C OH + CH3 NH2 CH3 C O + CH3 NH3
Label acid, base, conj. acid, conj. base
Acid and Base Strength
• Acid dissociation constant, Ka
• Base dissociation constant, Kb
• For conjugate pairs, (Ka)(Kb) = Kw
• Spontaneous acid-base reactions proceed from
stronger to weaker
CH3 C OH + CH3 NH2 CH3 C O + CH3 NH3
Determining Relative Acidity
• Electronegativity (different groups)
• Size (same groups)
• Resonance stabilization of conjugate base
(oxoacids, HNO3 HNO2)
As the bond to H becomes more polarized, H
becomes more positive and the bond is easier to
• As size increases, the H is more loosely held and
the bond is easier to break. (weak bond strong
• A larger size also stabilizes the anion.
• Delocalization of the negative charge on the
conjugate base will stabilize the anion, so the
substance is a stronger acid.
• More resonance structures usually mean greater
CH3CH2OH < CH3C OH < CH3 S OH
Which of the following is the strongest acid using
1. NH3 pKa = 33 2. CH3OH pKa = 15.5
3. HCl pKa = -2.2 4. CH3COOH pKa= 4.74
Lewis Acids and Bases
• Acids accept electron pairs = electrophile
• Bases donate electron pairs = nucleophile
Dative or coordinate covalent bond formed
– Electron poor
– Could be neutral or partially positive or positive
– Electron rich
– Could be neutral or partially negative or negative
• Has to have lone pair or pi bond
Which of the following is an electrophile? Why?
1. NH3 2. CH3O- 3. H+ 4. B-