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Week 6 CHEM 1310 - Sections L and M 1
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 Friday
Week 6 CHEM 1310 - Sections L and M 2
Order of Filling Subshells
4s levelfilled before
3d
Figure from Chemistry the Easy Way by Joseph A. Mascetta
What is theorder in whichsubshells are
filled?
2
Week 6 CHEM 1310 - Sections L and M 3
What is Bond Energy?
Bond energy is the energy required to makeor 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 4
Bond Energies, kJ/mol
Single bonds have lower
bondenergy
in general
Higher bondenergy
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Week 6 CHEM 1310 - Sections L and M 5
Bond Lengths & Energies
Multiple bonds are shorter than single bonds.
Week 6 CHEM 1310 - Sections L and M 6
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?
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Week 6 CHEM 1310 - Sections L and M 7
Bond Enthalpy
For a chemical reaction,Δ H = Σ Energybonds broken - Σ Energybonds formed
Bond enthalpy = Δ H in a reaction in which achemical bond is broken in the gas phase.
CCl2F2 (g) + 2 H2(g)
What is the overall bond enthalpy for this reaction?
CH2Cl2 (g) + 2 HF(g)
Week 6 CHEM 1310 - Sections L and M 8
Enthalpy of Reaction
More energy is supplied from bond formation than that released from bond breaking. Thus, overall reaction is endothermic.
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Week 6 CHEM 1310 - Sections L and M 9
1. Count # of valence electrons available - “A”
Write the Lewis structure for POCl3
Element Atomic # Shell Occupancy # Valence e-
P 15 2 + 8 + 5 5 O 8 2 + 6 6 Cl 17 2 + 8 + 7 7
SUM = 5 + 6 + (7x3) = 32 valence electrons
Drawing Lewis Structures
Week 6 CHEM 1310 - Sections L and M 10
2. Count total # of electrons needed - “N”
Element # e-Needed P 8 O 8 3Cl’s 24
SUM = 40 valence electrons
Drawing Lewis Structures
Write the Lewis structure for POCl3
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Week 6 CHEM 1310 - Sections L and M 11
3. Calculate # e-’s shared: S = (e- Needed) - (e- Available)
S = 40 - 32 = 8 valence electrons shared
Write the Lewis structure for POCl3
Drawing Lewis Structures
4. Draw skeleton w/ shared e-
(8 shared electrons) P ClCl
ClO
Week 6 CHEM 1310 - Sections L and M 12
5. Assign remaining shared electrons as double or triplebonds. (NOT REQ’D IN THIS EXAMPLE)
Write the Lewis structure for POCl3
P ClCl
ClO
Drawing Lewis Structures
7
Week 6 CHEM 1310 - Sections L and M 13
6. Assign remaining electrons as lone pairs.
Write the Lewis structure for POCl3
P ClCl
ClO Shared
electrons
Lone-pair electrons
Drawing Lewis Structures
Week 6 CHEM 1310 - Sections L and M 14
7. Calculate the formal charge on each atom
Write the Lewis structure for POCl3
P ClCl
ClO
Phosphorus
5 - 0 - (8) 12
# valenceelectrons
# lonepair
electrons
# bondingelectrons
Formal Charge on Phosphorus: +1
Drawing Lewis Structures
8
Week 6 CHEM 1310 - Sections L and M 15
7. Calculate the formal charge on each atom
Write the Lewis structure for POCl3
P ClCl
ClO
Chlorine
7 - 6 - (2) 12
# valenceelectrons
# lonepair
electrons
# bondingelectrons
Formal Charge on Chlorine: 0
Drawing Lewis Structures
Week 6 CHEM 1310 - Sections L and M 16
7. Calculate the formal charge on each atom
Write the Lewis structure for POCl3
P ClCl
ClO Oxygen
6 - 6 - (2) 12
# valenceelectrons
# lonepair
electrons
# bondingelectrons
Formal Charge on Oxygen: -1
Drawing Lewis Structures
9
Week 6 CHEM 1310 - Sections L and M 17
7. Calculate the formal charge on each atom
Write the Lewis structure for POCl3
P ClCl
ClO Check that formal charges sum to a
net charge of 0 for the molecule
(+1)Phosphorus + (0)Chlorine + (-1)Oxygen
Formal Charge on the molecule: 0
Drawing Lewis Structures
Week 6 CHEM 1310 - Sections L and M 18
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.
PRS Question
What does B represent?
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Week 6 CHEM 1310 - Sections L and M 19
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.
PRS Question
What does B represent?
Week 6 CHEM 1310 - Sections L and M 20
PRS Question
What does B represent?
A = in fig represents interspecies separation that is tooclose together. Energy is relatively high because of repulsive forces.
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Week 6 CHEM 1310 - Sections L and M 21
PRS Question
What does B represent?
C = in fig represents interspecies separation that is toofar apart. Energy is relatively high. Attractive forces cause the species to want to move closer together.
Week 6 CHEM 1310 - Sections L and M 22
PRS Question
What does B represent?
B = in fig represents distance at which a two species are most energetically stable. At that energy, the attractiveforces and the repulsive forces counteract each other.
Correct Answer: 3
12
Week 6 CHEM 1310 - Sections L and M 23
PRS Question
Which of the following bondshas the lowest enthalpy?
1. C−C
2. C=C
3. C=O
4. C=N
Week 6 CHEM 1310 - Sections L and M 24
PRS Question
Which of the following bondshas the lowest enthalpy?
1. C−C 347
2. C=C 614
3. C=O 745
4. C=N 615
Because single bonds generallyhave lower bond energies than
double bonds
kJ/mol
