AP Chemistry

1
AP Chemistry

• Ch 8 Bonding

2
Vocab (Ch 8)

• Isoelectronic series
• Dipole
• Dipole moment
• Formal charge
• Resonance structure
• Bond enthalpy

3
Lewis Dot Structures

• Create structures that satisfy the Octet Rule.
• This shows us how the electrons are distributed
  within a molecule.
• Page 278

4
Formal Charge

• The Formal Charge of an atom is the charge that
  an atom in a molecule would have if all atoms had
  the same electronegativity.
• There may be instances when you can draw 2
  structures for the same molecule. Then you would
  want to use the Formal Charge Method to determine
  which structure is the most feasible.

5
Calculating Formal Charge

• 1. All of the unshared (nonbonding) electrons
  are assigned to the atom on which they are
  found.
• 2. Half of the bonding electrons are assigned to
  each atom in the bond.
• 3. Formal Charge
• of valence e- in the isolated atom
• minus
• of electrons assigned to the atom in the
  Lewis Structure.
• See pages 280-281 for examples.

6
Resonance Structures

• These are individual Lewis dot structures in
  cases where 2 or more Lewis structures are
  equally good descriptions of a single molecule.
• The resonance structures are equivalent except
  for the placement of electrons.

7
Resonance Structures

• Use double-headed arrows to indicate that the
  structures are resonance structures.
• Molecules that have resonance structures can not
  be described accurately using only one Lewis
  structure. The true arrangement of the electrons
  must be considered as a blend of 2 or more
  Lewis structures.
• The molecule does not oscillate between 2 or
  more Lewis structures.
• See page 283

8
Resonance Structures

• commons.wikimedia.org/wiki/FileNitrate_ion_r...

9
Exceptions to the Octet Rule

• There are 3 main types of exceptions
• Molecules with an odd number of electrons.
• Molecules in which an atom has less than an
  octet.
• Molecules in which an atom has more than an
  octet.
• Page 285-287

10
Strengths of Covalent Bonds

• The stability of a molecule is related to the
  strengths of the covalent bonds it contains.
• The strength of a covalent bond between 2 atoms
  is determined by the energy required to break
  that bond.
• We will use bond enthalpy to determine the
  strength of the bonds.

11
Enthalpy and Bond Strength

• The bond enthalpy is always a positive value.
• Energy is always required to break chemical
  bonds.
• Energy is released when a bond forms between 2
  gaseous atoms or molecular fragments.
• The greater the bond enthalpy, the stronger the
  bond.
• A molecule with strong chemical bonds generally
  has less tendency to undergo chemical change than
  a molecule with weak bonds.

12
Bond Enthalpy and Bond Strength

• ?H S(bond enthalpies of bonds broken)
• minus
• S(bond enthalpies of bonds formed)
• One method of doing this is to break all of the
  bonds on the reactant side and add them up. Then
  break all of the bonds formed on the product side
  and add them up. Subtract products from
  reactants.
• Use the chart on page 289 for Average Bond
  Enthalpy values.

13
Exam Topics

• Concept Ionic vs Covalent vs Metallic bonding
• Lattice energy problems
• Isoelectronic series (nuclear charge)
• Electronegativity, bond polarity, dipole, dipole
  moment
• Bond length
• Formal charge
• Resonance structures
• Exceptions to the Octet Rule
• Bond Enthalpy Calculations
• Draw Lewis Dot Structures

14
Problems to Try

• Ch 8
• 9-11, 19, 20, 23, 27, 33, 34, 40, 41, 43, 44,
  46-48, 50, 51, 54, 60, 61, 63, 64, 66-68, 79, 81,
  85, 87
• AP Exam Problems to Try
• 1999 8
• 2000 7 (last section)
• 2002 6
• 2003 8
• 2004 7 8