CH 2: Polar Covalent Bonds; Acids and Bases

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CH 2 Polar Covalent Bonds Acids and Bases

• Renee Y. Becker
• CHM 2210
• Valencia Community College

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Electronegativity and Bond Polarity
Greater ?EN means greater polarity
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Polar Covalent Bonds

• Polar Covalent Bonds
• Form between a non-metal/non-metal of different
  electronegativities

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Polar Molecules

• Polar Molecules
• Just as bonds can be polar, molecules as a whole
  can be polar
• Net sum of individual bond polarities and
  lone-pair contributions

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Dipole Moment

• 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
• ? Q x r
• Expressed in debyes, D, where
• 1 D 3.336 x 10-30 coulomb meters

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Ionic Character

• Which of the following has the highest ionic
  character?
• HI
• HBr
• HCl
• HF
• WHY?

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Example 3

• Tell which of the following compounds are likely
  to have a dipole moment and show the direction of
  each.
• a) SF6 b) CHCl3
• c) CH2Cl2 d) CH2CH2

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Intermolecular Forces

• 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
• Dipole-dipole
• Ion-dipole
• Dispersion forces
• Hydrogen bonding

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Dipole-dipole

• Neutral but polar molecules experience
  dipole-dipole forces as a result of electrical
  interactions among dipoles on neighboring
  molecules.
• 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

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Ion-dipole

• 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

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London Dispersion Forces

• a) Result from the motion of electrons
• b) At any given time more electrons may be in a
  particular area of the molecule
• 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

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Hydrogen Bonding

• 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
• 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.
• 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

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Hydrogen Bonding

• e) Water is able to form a vast 3D network of
  hydrogen bonds because each H2O molecule has two
  hydrogens and two electron pairs

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Intermolecular Forces
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Example 4

• Identify the likely kinds of intermolecular
  forces in the following
• A)   HCl
• B) CH3CH3
• C) CH3NH2
• D) Kr

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Example 5

• Of the substances Ar, Cl2, CCl4 and HNO3 which
  has
• a)  The largest dipole-dipole forces?
• b)   The largest hydrogen-bond forces?

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Resonance

• Only electrons can be moved (usually lone pairs
  or pi electrons).
• Nuclei positions and bond angles remain the same.
• The number of unpaired electrons remains the
  same.
• Resonance causes a delocalization of electrical
  charge.

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Resonance Example

• The real structure is a resonance hybrid.
• All the bond lengths are the same.
• Each oxygen has a -1/3 electrical charge.
  
  

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Major Resonance Form

• has as many octets as possible.
• has as many bonds as possible.
• has the negative charge on the most
  electronegative atom.
• has as little charge separation as possible.
• The more resonance forms the more stable

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Example 6

• Draw the important resonance forms, label major
  and minor
• a) H2CCN-
• b) HCONH2
• c) H2COH
• d) H2CNO2-

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Example 7
Which is the Major Contributor?? Why?
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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 24C
• pH -log H3O
• Strong acids and bases are 100 dissociated.

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BrØnsted-Lowry Acids and Bases

• Acids can donate a proton
• Bases can accept a proton
• Conjugate acid-base pairs
• Label acid, base, conj. acid, conj. base

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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

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Determining Relative Acidity

• Electronegativity (different groups)
• Size (same groups)
• Resonance stabilization of conjugate base
  (oxoacids, HNO3 HNO2)
  
  

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Electronegativity
As the bond to H becomes more polarized, H
becomes more positive and the bond is easier to
break.
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Size

• As size increases, the H is more loosely held and
  the bond is easier to break. (weak bond strong
  acid)
• A larger size also stabilizes the anion.

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Resonance

• 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
  stabilization.

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Example 8
Which of the following is the strongest acid
using pKa values?

• NH3 pKa 33 2. CH3OH pKa 15.5
• 3. HCl pKa -2.2 4. CH3COOH pKa 4.74

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Lewis Acids and Bases

• Acids accept electron pairs electrophile
• Bases donate electron pairs nucleophile

Dative or coordinate covalent bond formed
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• Electrophile
• Electron poor
• Could be neutral or partially positive or
  positive
• Nucleophile
• Electron rich
• Could be neutral or partially negative or
  negative
• Has to have lone pair or pi bond

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Example 9
Which of the following is an electrophile? Why?
1. NH3 2. CH3O- 3. H 4. B-