CHM 2045C

Module Four Part II: Chemical Bonding & Molecular Structure (Chapters 9

L.  ____  (05) Bond Angles/Bond Lengths-Section 9.1 Answers

N. ____  (05) Geometry of Molecules-Section 9.2  Answers

O. ____  (05) Polarity of Molecules-Section 9.3  Answers

P._____ (05) Hybrid Orbitals Sections 9.4, 9.5, 9.6  Answers

______(20) Total Chapter 7

 

M-4ii Required Homework:

      ______(20) M-4ii Pretest Hardcopy Homework Packet

      ______(10/20) M-4ii Multiple Choice (MC) Homework/Exam (Blackboard Online)

 

Reference: VSEPR Videos:

  http://www.lsua.info/chem1001/VSEPR/VSEPRtheory.wmv

  http://www.fscj.me/VSEPR/VSEPRTheoryAndShapesOfMolecules.mp4

 

Reference: Polarity Videos:

  http://www.lsua.info/Polarity/01Polarity.rm

  http://www.fscj.me/Polarity/PolarityOfMolecules.mp4  

  http://www.fscj.me/Polarity/InfluenceShapeOnPolarity.mp4

 

Reference: Hybrid Orbital Videos:

  http://www.fscj.me/HybridOrbitals/Hybrid_Orbitals.mp4

  http://www.fscj.me/HybridOrbitals/Sigma_Bonds.mp4

  http://www.fscj.me/HybridOrbitals/PiBondsIEthylene.mp4

What is VSEPR?

 

VSEPR stands for Valence Shell Electron Pair Repulsion.  It's a complicated acronym, but it means something that's not difficult to understand.  Basically, the idea is that covalent bonds and lone pair electrons like to stay as far apart from each other as possible under all conditions.  This is because covalent bonds consist of electrons, and electrons don't like to hang around next to each other much because they have the same charge.

 

This VSEPR thing explains why molecules have their shapes.  If carbon has four atoms stuck to it (as in methane), these four atoms want to get as far away from each other as they can.  This isn't because the atoms necessarily hate each other, it's because the electrons in the bonds hate (repel)  each other.  That's the idea behind VSEPR.

 

What is a Bond Angle?

 

 Let’s look at the water molecule:

http://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/Water_molecule_dimensions.svg/200px-Water_molecule_dimensions.svg.png

 

VSEPR table

The bond angles in the table below are ideal angles from the simple VSEPR theory, followed by the actual angle for the example given in the following column where this differs. For many cases, such as trigonal pyramidal and bent, the actual angle for the example differs from the ideal angle, but all examples differ by different amounts. For example, the angle in H2S (92°) differs from the tetrahedral angle by much more than the angle for H2O (104.5°) does.

Bonding electron pairs

Lone pairs

Electron domains (Steric #)

Shape

Ideal bond angle (example's bond angle)

Example

Image

2

0

2

linear

180°

CO2

Linear-3D-balls.png

3

0

3

trigonal planar

120°

BF3

Trigonal-3D-balls.png

2

1

3

bent

120° (119°)

SO2

AX2E1-3D-balls.png

4

0

4

tetrahedral

109.5°

CH4

AX4E0-3D-balls.png

3

1

4

trigonal pyramidal

107°

NH3

AX3E1-3D-balls.png

2

2

4

bent

109.5° (104.5°)

H2O

AX2E2-3D-balls.png

5

0

5

trigonal bipyramidal

90°, 120°, 180°

PCl5

Trigonal-bipyramidal-3D-balls.png

4

1

5

seesaw

180°, 120°, 90° (173.1°, 101.6°)

SF4

AX4E1-3D-balls.png

3

2

5

T-shaped

90°, 180° (87.5°, < 180°)

ClF3

AX3E2-3D-balls.png

2

3

5

linear

180°

XeF2

AX1E3-3D-balls.png

6

0

6

octahedral

90°, 180°

SF6

AX6E0-3D-balls.png

5

1

6

square pyramidal

90° (84.8°), 180°

BrF5

AX5E1-3D-balls.png

4

2

6

square planar

90°, 180°

XeF4

Square-planar-3D-balls.png

7

0

7

pentagonal bipyramidal

90°, 72°, 180°

IF7

Pentagonal-bipyramidal-3D-balls.png

6

1

7

pentagonal pyramidal

72°, 90°, 144°

XeOF5

Pentagonal-pyramidal-3D-balls.png

5

2

7

planar pentagonal

72°, 144°

XeF5

Pentagonal-planar-3D-balls.png

8

0

8

square antiprismatic

XeF82−

Square-antiprismatic-3D-balls.png

9

0

9

tricapped trigonal prismatic

ReH92−

AX9E0-3D-balls.png

 

Example

Predict all bond angles in the following molecules.

a. CH3Cl   b. CH3CNl     c. CH3COOH

Solution

a. The Lewis structure of methyl chloride is:

http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx73/MeCl.gif

In the Lewis structure of CH3Cl carbon is surrounded by four regions of high electron density, each of which forms a single bond. Based on the VSEPR model, we predict a tetrahedral distribution of electron clouds around carbon, H - C - H and H - C - Cl bond angles of 109.5°, and a tetrahedral shape for the molecule. Note the use of doted lines to represent a bond projecting behind the plane of the paper and a solid wedge to represent a bond projecting forward from the plane of the paper.

http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx73/MeCla.gif

b. The Lewis structure of acetonitrile, CH3CN is:

http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx73/MeCN.gif

The methyl group, CH3-, is tetrahedral. The carbon of the -CN group is in the middle of a straight line stretching from the carbon of the methyl group through the nitrogen.

http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx73/MeCNa.gif

c. The Lewis structure of acetic acid is:

http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx73/AcOH.gif

Both the carbon bonded to three hydrogens and the oxygen bonded to carbon and hydrogen are centers of tetrahedral structures. The central carbon will have 120 7deg bond angles.

http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx73/AcOHa.gif

The geometry around the first carbon is tetrahedral, around the second carbon atom is trigonal planar, and around the oxygen is bent.

 

Reference: VSEPR Videos:

  http://www.lsua.info/chem1001/VSEPR/VSEPRtheory.wmv

  http://www.fscj.me/VSEPR/VSEPRTheoryAndShapesOfMolecules.mp4

 

 

 

 

 

Module Four: Part L Bond Angles           05 points

What is the bond Angle in the following structures:

 

___1. ethanoicAcid  ___4.acetone

___2.                                             ___5.

___3.                                             ___6.

 

___7. CarbonDioxide     ___8. water____9.Ammonia

 

 

___10. ethylene      ____11. acetylene

 

___12. methanal ___13. DimethylEther ___16.ethane

                              ___14.

                              ___15.

 

___17.CarbonicAcid    ___20. HydrogenCyanide

 

Bonus:

 

___21. Cyclopropane  ___23. Cyclobutane

___22.                                   ___24.

Steric Numbers do not predict bond angles within rings of carbons

 

 

 

Types of molecular structure

Main article: VSEPR theory #AXE method

Some common shapes of simple molecules include:

Common shapes you should know

There are a whole bunch of common shapes you need to know to accurately think of covalent molecules.  Here they are:

 

 

Module Four- Part N: Geometry of Molecules      5 points

Use the dot/stick structures on the Part L page to state the geometry of the molecules:

         Steric #2, Steric #3, and Steric #4 Via Octet Rule:

 Bent      Linear      Trigonal Planer  Planer    Trigonal Pyramidal      Tetrahedral

         Steric #5 & Steric #6 Shapes (Rule of 10 and Rule of 12):

Trigonal-bipyramidal      Square Planer     Seesaw      T-shaped    Octahedral

 

_____________1.  H2O

 

 

_____________2.  CO2

 

 

_____________3.  C2H4

 

 

_____________4.  SO2

 

 

_____________5.  SO3

 

 

_____________6.  HCN

 

 

_____________7.  CH4

 

 

_____________8.  NH3

 

 

_____________9.  CH2O

 

 

_____________10. C2H2

 

 _____________Bonus. PF5

 

  _____________Bonus  SF6

 

waterzcarbonDioxidez

ethenez1SulfurDioxidez

sulfurTrioxidez hydrogenCyanidez.

metanez ammoniaz

methanal ethynez

 

 

 

 

Polar Covalent Bonds

•         Covalent bonds result from the sharing of valence electrons.

•         Often, the two atoms do not share the electrons equally. That is, one of the atoms holds onto the electrons more tightly than the other.

•         When one of the atoms holds the shared electrons more tightly, the bond is polarized.

A polar covalent bond is one in which the electrons are not shared equally

 

Electronegativity

 

•         Each element has an innate ability to attract valence electrons.

•         Electronegativity is the ability of an atom to attract electrons in a chemical bond.

•         Linus Pauling devised a method for measuring the electronegativity of each of the elements.

•         Fluorine is the most electronegative element.

•         Electronegativity increases as you go left to right across a period.

Electronegativity increases as you go from bottom to top in a family.   

 

Electronegativity: The ability of an atom in a molecule to attract the shared electrons in a covalent bond.

 

 

 

07_03_Figure

 

 

 

Electronegativity Differences

07_04_Figure

•         The electronegativity of H is 2.1; Cl is 3.0.

•         Since there is a difference in electronegativity between the two elements (3.0 – 2.1 = 0.9), the bond in H – Cl is polar.

•         Since Cl is more electronegative, the bonding electrons are attracted toward the Cl atom and away from the H atom. This will give the Cl atom a slightly negative charge and the H atom a slightly positive charge.

 

Nonpolar Covalent Bonds

 

•         What if the two atoms in a covalent bond have the same or similar electronegativities?

•         The bond is not polarized and it is a nonpolar covalent bond. If the electronegativity difference is less than 0.5, it is usually considered a nonpolar bond.

•         The diatomic halogen molecules have nonpolar covalent bonds.

 

Reference: Polarity Videos:

  http://www.lsua.info/Polarity/01Polarity.rm

  http://www.fscj.me/Polarity/PolarityOfMolecules.mp4 

  http://www.fscj.me/Polarity/InfluenceShapeOnPolarity.mp4

 

 

 

 

 

Module Four II- Part O: Polarity of Molecules      5 points

Use the dot/stick structures and sketch the molecule in three dimensions. Then draw the dipoles for each bond to state if the molecule is polar or nonpolar:

 

Electronegativities: F=4.0; O=3.5;  N=3.0; Cl=3.0; Br=2.7; C=2.5; S=2.5; P=2.1; H=2.1

 

_____________1.  H2O

 

 

_____________2.  CO2

 

 

_____________3.  C2H4

 

 

_____________4. C2H2

 

 

_____________5. SO2

 

 

_____________6. SO3

 

 

_____________7.  CH4

 

 

_____________8.  NH3

 

 

_____________9.  BH3

 

 

_____________10. HCN

 

 

           

 _____________Bonus. PCl5                      

 

 _____________Bonus  SCl6

waterz   carbonDioxidez

ethenez1  ethynez

 

SulfurDioxidez     sulfurTrioxidez

 

     metanez      ammoniaz

 

BoronTrihydride  hydrogenCyanidez

 

 

 

 

 

 

Hybrid Orbitals

http://misterguch.brinkster.net/vsepr.gif

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bonding7Hybrid.jpg

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Module 4II- Part P: Hybrid Orbitals of Molecules      5 points

Use the dot/stick structures in the table to predict the hybrid orbitals that overlap to form the covalent bond:

 

_____________1.  H2O   the sigma bond

                              between either  H-O

 

_____________2.  CO2  the sigma bond

                              between either C=O

_____________2a.  CO2      the pi bond

                               between either C=O

 

_____________3.    C2H4   the sigma bond

                               between the two carbons

_____________3a.  C2H4   the pi bond

                               between the two carbons

 

_____________4.    SO2  the sigma bond

                               between S=O

_____________4a.  SO2  the pi bond

                               between S=O

____________4b.  SO2  the sigma bond

                               between S-O

 

____________ 5.    SO3   either sigma bond

                               between S-O

____________ 5a.  SO3   the sigma bond

                              between S=O

____________ 5b.  SO3   the pi bond

                              between S=O

 

 

____________6.    HCN  the single (sigma)

                              bond between H-C

____________6a.  HCN  the sigma bond

                              between C=N

____________6b.  HCN  either pi bond

                              between C=N

 

____________7.  CH4  the sigma bond

                              between any of the H-C

 

____________8.  NH3  the sigma bond

                              between any of the H-N

 

___________  9.   CH2O  either sigma

                              bond between the H-C

____________9a.  CH2O  the sigma  bond

                              between the C=O

____________9b.  CH2O  the pi  bond

                              between the C=O

 

___________ __10.   C2H2   the sigma bond

                              between the C=C

_________ ____10a. C2H2   either pi bond

                            between the C=C

_____________ 10b. C2H2  the sigma

                   bond between any of the H-C

waterzcarbonDioxidez

ethenez1SulfurDioxidez

 

sulfurTrioxidez hydrogenCyanidez

 

.

metanez ammoniaz

 

 

 ethynez