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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?

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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

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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

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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

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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

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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