Bonding: General Concepts
Ionic Bonds
Sections 13.2 - 13.6
Covalent Bonds
Section 13.7
Covalent Bond Energy & Chemical Reactions
– Section 13.8 - 13.9
Lewis Structures
Sections 13.10 - 13.12
VSEPR Theory
Section 13.13
Week 6
Friday
CHEM 1310 - Sections L and M
1
Order of Filling Subshells
What is the
order in which
subshells are
filled?
4s level
filled before
3d
Figure from
Chemistry the Easy Way
by Joseph A. Mascetta
Week 6
CHEM 1310 - Sections L and M
2
1
What is Bond Energy?
 Bond energy is the energy required to make
or break 1 mol of a chemical bond.
– Energy is released when bonds are broken:
Exothermic
– Energy is supplied when bonds are formed:
Endothermic
Week 6
CHEM 1310 - Sections L and M
3
Bond Energies, kJ/mol
Higher bond
energy
Single bonds
have lower
bond
energy
in general
Week 6
CHEM 1310 - Sections L and M
4
2
Bond Lengths & Energies
Multiple bonds are shorter than single bonds.
Week 6
CHEM 1310 - Sections L and M
5
Enthalpy
What is enthalpy (H)?
– A state function: pathway does not matter
– H = E + PV, where
• E = internal energy of a system
• P = pressure of the system
• V = volume of the system
– Δ H = qp the energy flow as heat at constant pressure
How can enthalpy help us understand chemical bonding?
Week 6
CHEM 1310 - Sections L and M
6
3
Bond Enthalpy
For a chemical reaction,
Δ H = Σ Energybonds broken - Σ Energybonds formed
Bond enthalpy = Δ H in a reaction in which a
chemical bond is broken in the gas phase.
CCl2F2 (g) + 2 H2(g)
CH2Cl2 (g) + 2 HF(g)
What is the overall bond enthalpy for this reaction?
Week 6
CHEM 1310 - Sections L and M
7
Enthalpy of Reaction
More energy is supplied from bond formation than that released from
bond breaking. Thus, overall reaction is endothermic.
Week 6
CHEM 1310 - Sections L and M
8
4
Drawing Lewis Structures
Write the Lewis structure for POCl3
1. Count # of valence electrons available - “A”
Element
P
O
Cl
Atomic #
15
8
17
Shell Occupancy
2+8+5
2+6
2+8+7
# Valence e5
6
7
SUM = 5 + 6 + (7x3) = 32 valence electrons
Week 6
CHEM 1310 - Sections L and M
9
Drawing Lewis Structures
Write the Lewis structure for POCl3
2. Count total # of electrons needed - “N”
Element
P
O
3Cl’s
# e-Needed
8
8
24
SUM = 40 valence electrons
Week 6
CHEM 1310 - Sections L and M
10
5
Drawing Lewis Structures
Write the Lewis structure for POCl3
3. Calculate # e-’s shared: S = (e- Needed) - (e- Available)
S = 40 - 32 = 8 valence electrons shared
4. Draw skeleton w/ shared e(8 shared electrons)
Week 6
O
Cl P Cl
Cl
CHEM 1310 - Sections L and M
11
Drawing Lewis Structures
Write the Lewis structure for POCl3
5. Assign remaining shared electrons as double or triple
bonds. (NOT REQ’D IN THIS EXAMPLE)
O
Cl P Cl
Cl
Week 6
CHEM 1310 - Sections L and M
12
6
Drawing Lewis Structures
Write the Lewis structure for POCl3
6. Assign remaining electrons as lone pairs.
O
Cl P Cl
Cl
Week 6
Shared
electrons
Lone-pair
electrons
CHEM 1310 - Sections L and M
13
Drawing Lewis Structures
Write the Lewis structure for POCl3
7. Calculate the formal charge on each atom
O
Cl P Cl
Cl
Week 6
Phosphorus
5
-
# valence
electrons
0
# lone
pair
electrons
- 1 (8)
2
# bonding
electrons
Formal Charge on Phosphorus: +1
CHEM 1310 - Sections L and M
14
7
Drawing Lewis Structures
Write the Lewis structure for POCl3
7. Calculate the formal charge on each atom
O
Cl P Cl
Cl
Chlorine
7
-
# valence
electrons
6
# lone
pair
electrons
- 1 (2)
2
# bonding
electrons
Formal Charge on Chlorine: 0
Week 6
CHEM 1310 - Sections L and M
15
Drawing Lewis Structures
Write the Lewis structure for POCl3
7. Calculate the formal charge on each atom
O
Cl P Cl
Cl
Oxygen
6
-
# valence
electrons
6
# lone
pair
electrons
- 1 (2)
2
# bonding
electrons
Formal Charge on Oxygen: -1
Week 6
CHEM 1310 - Sections L and M
16
8
Drawing Lewis Structures
Write the Lewis structure for POCl3
7. Calculate the formal charge on each atom
Check that formal charges sum to a
O
net charge of 0 for the molecule
Cl P Cl
(+1)
+ (0)
+ (-1)
Cl Formal Charge on the molecule: 0
Phosphorus
Week 6
Chlorine
CHEM 1310 - Sections L and M
Oxygen
17
PRS Question
What does B represent?
1. Attractive forces dominate over repulsive forces.
2. Repulsive forces dominate over attractive forces.
3. Attractive and repulsive forces roughly balance.
4. There are no repulsive or attractive forces.
Week 6
CHEM 1310 - Sections L and M
18
9
PRS Question
What does B represent?
1. Attractive forces dominate over repulsive forces.
2. Repulsive forces dominate over attractive forces.
3. Attractive and repulsive forces roughly balance.
4. There are no repulsive or attractive forces.
Week 6
CHEM 1310 - Sections L and M
19
PRS Question
What does B represent?
A = in fig represents interspecies separation that is too
close together. Energy is relatively high because of
repulsive forces.
Week 6
CHEM 1310 - Sections L and M
20
10
PRS Question
What does B represent?
C = in fig represents interspecies separation that is too
far apart. Energy is relatively high. Attractive forces
cause the species to want to move closer together.
Week 6
CHEM 1310 - Sections L and M
21
PRS Question
What does B represent?
B = in fig represents distance at which a two species are
most energetically stable. At that energy, the attractive
forces and the repulsive forces counteract each other.
Correct Answer: 3
Week 6
CHEM 1310 - Sections L and M
22
11
PRS Question
Which of the following bonds
has the lowest enthalpy?
1. C−C
2. C=C
3. C=O
4. C=N
Week 6
CHEM 1310 - Sections L and M
23
PRS Question
Which of the following bonds
has the lowest enthalpy?
kJ/mol
1. C−C
347
2. C=C
614
3. C=O
745
4. C=N
615
Week 6
Because single bonds generally
have lower bond energies than
double bonds
CHEM 1310 - Sections L and M
24
12