Lab 9 Data - Lab assignment for CHEM105a PDF

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Lab assignment for CHEM105a...

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Chemical Bonding: Molecular Shape and Polarity Observations 9

 

You may complete the observations as a group All molecule drawings must be completed individually. Open the Drawings File…

Part 1A: Electron Domains 1. Explore the Model screen of the simulation. As you explore, answer the following questions. a. How does adding an atom affect the position of existing atoms or lone pairs? Adding an atom pushes the other atoms away, and decreases bond angles

b. How does adding a lone pair affect the position of existing atoms and lone pairs? Changes the geometry of the existing atoms, it rearranges them closer together. It’s the same as adding an atom. c. If a single bond is replaced with a double bond or triple bond, does this affect the position of existing atoms and lone pairs? It does not affect the position of the existing atoms or lone pairs 2. Is the effect of adding bonded atoms and lone pairs to the central atom similar? Explain why this could be the case.

Yes, they are similar because the both push the other atoms away to create more space for themselves. We can think of a bond or a lone pair of electrons as a “domain” of electrons. Single bonds, double bonds, and triple bonds each count as one 3. How do the electrons in bonds (bonding domains) differ from lone pairs (non-bonding domains)?

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They differ because electrons in bonds are shared between two atoms and electrons in lone pairs only belong to one atom 4.

What happens to the bond angle when you add or remove an electron domain?

Adding = decreases

Removing = increases

5. Can you force the atoms into new configurations by pushing atoms around? What does this suggest about the configuration of atoms in real molecules? You are able to force atoms into new configurations. This would suggest that although they are technically flexible, their predicted geometry will most likely be the same. 6.

What is the difference between Electron Geometry and Molecule Geometry?

The difference between Electron Geometry and Molecular Geometry is that electron geometry takes into account everything in terms of domains while molecular geometry only takes into account the bonding. 7. In one or two grammatically correct sentences, write a definition for the term Molecule Geometry. Molecular geometry is a 3-D representation of the arrangement of atoms that constitute a particular molecule. These 3-D diagrams include information regarding bond angles, bond lengths, and other information that determines the molecules overall arrangement. Part 1B: Drawing Molecules to Show 3-Dimensionality: Line, Wedge and Dash Drawings Line: In the plane of the paper:

_____

Wedge: Coming forward, in front of the plane of the paper: Dash: Going backward, behind the plane of the paper: 8. Where is each of the 5 atoms in the molecule CHFClBr? In the plane of the paper __C__ __H__ __Br__ In front of the plane of the paper __F__ Behind the plane of the paper __Cl__

Bonding Domains Around Central Atom

Drawing of Shape

Electron Geometry

2

Bond Angles

2

●─○─●

Linear

180º

3

○

Trigonal Planar

120

Tetrahedral

109.5

Trigonal Bipyramidal

90,120,180

Octahedral

90,180

4

○

5

○

6

○

10. In the Model screen, build a molecule with 5 atoms attached to the central atom. Look at the molecule geometry and electron geometry. Predict what will happen to the molecule geometry as you replace atoms with lone pairs. Your Prediction: When you add lone pairs as a replacement to atoms, then the molecular geometry will change

11. In the following table draw the molecule geometry. As a group, make a prediction for each first, and then compare your answers with the simulation. Predict First, Then Compare with the Simulation Total Number of Domains Around Central Atom

1 Lone Pair

2 Lone Pairs

3

3 Lone Pairs

4 Lone Pairs

3

4

5

6

4

Part 1C: Comparing Model vs. Real Molecules 12. Explore the Real Molecules screen. a. List the molecules that show a difference in bond angle between “Real” and “Model”. Note: differences in bond angle may be small. Molecule

Electron Geometry

# of Lone Pair Domains

Real Bond Angle(s)

Model Bond Angle(s)

H2O

Tetrahedral

2

104.5

109.5

SO2

Trigonal Planar

1

119.0

120.0

ClF3

Trigonal Bipyramidal

2

87.5

90.0

NH3

Tetrahedral

1

107.8

109.5

SF4

Trigonal Bipyramidal

1

87.8

90.0

BrF5

Octahedral

1

84.8

90.0

b. What do all of the molecules in the table have in common? All the molecules have at least one lone pair c. What trend do you observe that distinguishes lone pairs from bonding domains? Lone pairs are closer to the central atom than bonding domains are. Lone pairs are also farther apart from bonding domains than bonding domains are from each other.

d. For a given electronic geometry, is there a relationship between the number of lone pairs and the bond angles? Yes, the more lone-pairs there are the smaller the bond angles and vice versa. This is because the lone pairs have a strong repulsion against each other.

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9. Use the simulation to build a system with 5 domains. This is called a trigonal bipyramidal structure. The two different sites in a trigonal bipyramid are labeled as A and B in the drawing to the right. a. Each A atom is adjacent to 3 B atoms. What is the A-C-B bond angle? 90 degrees b. Each B atom is adjacent to 2 A atoms and 2 B atoms. What is the B-C-B bond angle. 120 degrees

c. In a system with 4 atoms and 1 lone pair, predict whether the lone pair will be in a B site or an A site? Explain. The lone pair would be in the B site because there is more space there from other domains and it minimizes repulsion between lone pairs and bonding groups.

d. Examine the molecule SF4 in the Real Molecules screen to check your prediction from question c. Which interactions (bond-bond repulsion, lone-pair bond repulsion, lone pairlone pair repulsion) are more important in determining where the lone pair will go? Our prediction for Part C was correct. The interactions that are more important in determining where the lone pair will go would be the axial-equatorial and equatorialequatorial interactions because both offer 90 degree angle, but axial also has a 180 degree angle and equatorial also has about 2 180 degree angles. In this instance, equatorial interactions are more favored because they have fewer 90 degree angles which allows lone pairs to maximize their distance between domains.

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Part 2A: Diatomic molecules – ‘Two Atoms’ Screen 1. Explain all the ways you can change the polarity of the two-atom molecule.   

Electronegativity differences between atoms Uneven distribution of electron charge Electron withdrawing/donating groups attached to an atom

10. Record your ideas in the table below. How does each change as Representation electronegativity changes?

Dipole moment is a measure of a molecules polarity and therefore, the way electronegativity affects bond dipoles is directly correlated to the polarity of a molecule

Higher electronegativity difference→ greater dipole moment Bond Dipole

Partial Charges

Electrostatic Potential

Electron Density

How does each help you understand the polarity of molecules?

The more electronegative atom will have a more partial negative charge (more electronegativity difference the more significant the partial charges are)

The greater the electronegativity difference → more polarized the electron distribution

Electrostatic potential is the potential energy of a proton at a specific point→ changes in electronegativity therefore effect EP directly

Same reasoning as for electronegativity changes→ for example when a proton interacts with a positive region of an atom/molecule this causes repulsion interactions

The more electronegative atom→ greater electron density (and vice versa)

More electron density on a side= more negative charge= more negative polarity in that region

PART 2B: Triatomic molecules – ‘Three Atoms’ Screen 11. Explain any new ways to change the molecule polarity of the three-atom molecule.

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One way to change the polarity of the three-atom molecule is if we increase the electronegativity of atom A only while keeping the others the same. We can also increase the electronegativity of the A and C atoms to be greater than the B atom.

12. How does the ABC-bond angle effect molecule polarity? Tip: Try changing the bond angle in the simulation.

The more linear we make the bond angles, the smaller the molecule polarity gets. The molecule polarity increases as the bond angle between molecules A and C gets closer to zero.

13. Explain the relationship between the bond dipoles and the molecule dipole.

A bond dipole is a dipole created by a single bond whereas a molecule dipole accounts for all the dipoles created by all the bonds in the molecule. A molecule dipole is the sum of all bond dipoles.

14.

Can a non-polar molecule contain polar bonds? Use an example to explain your answer.

Yes, a non-polar molecule can contain polar bonds because the polarity of a molecule depends on the net dipole movement of the molecule. An example would be BF3. The bonds between B and F are polar but since the net dipole movement cancels out, then the molecule is overall non-polar.

7. Close the simulation. Predict the polarity of the following molecules. Explain your reasoning. Element Electrone gativity

H

B

C

N

O

F

2 . 1

2 . 0

2 . 5

3 . 0

3 . 5

4 . 0

YOUR PREDICTION Chemical Formula

Draw the Molecular Shape: Include Bond Dipoles & Molecule Dipole

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Explain Your Reasoning:

C l 3 . 0

F2

CO2

O3

HCN

CH2O

CH2F2

PART 2C: Real molecules – Predictions 8. Discuss with your group the method(s) that you used to determine the bond dipoles and the molecule dipole. Write your method(s) in complete sentences below. The methods we used to determine the bond dipoles and molecule dipoles are to just change the electronegativity of the atoms and see how that changes the direction and magnitude of the bond dipole arrows as well as the main molecule dipole arrow.

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9. Predict which of the following molecules will have a stronger molecular dipole. Explain your reasoning.

YOUR PREDICTION Use the space provided for any work necessary: NH3

Which has a stronger dipole? Explain Your Reasoning:

BH3 NH3 since it is a more polar molecule

O3

H 2O H2O because the water molecule has a bigger difference in electronegativity

CHCl3

CHF3 CHF3 because it is more electronegative

CHF3

CH3F Identical due to the similar structures

PART 2D: Real molecules – Polarity, Electrostatic Potential and Electron Density 10. Open the Molecular Polarity Simulation and go to the ‘Real Molecules’ Screen. Look at a few different molecules. 10

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15. 2C:

With the Molecular Dipole Box selected, check your predictions that you made in part

Chemical Formula

F2

CO2

Was your prediction correct? Explain any differences. There is no molecular dipole because both atoms have the same electronegativity so our predictions were correct.

There is no molecular dipole because carbon dioxide is non-polar so our predictions are correct.

O3

Even though ozone is made up of a single element it does have a dipole moment because it has one double bond contrary to what we predicted.

HCN

Hydrogen cyanide has a molecular dipole because nitrogen is more electronegative than hydrogen

CH2O

Our prediction was correct and CH2O has a molecular dipole because it is polar and the bonds are not symmetrical.

CH2F2

CH2F2 does have a molecular dipole because it is polar so our prediction was correct.

Comparison

Was your prediction correct? Explain any differences.

NH3 vs BH3

Our prediction was correct, NH3 has a stronger dipole because although BH3 has polar bonds, the symmetrical shape of the molecule cancel them out making BH3 non-polar.

O3 vs H2O

Our prediction was correct, H2O has a stronger dipole than O3 because it is more electronegative.

CHCl3 vs CHF3

Our prediction was correct, CHF3 has a stronger dipole moment because fluorine has a higher electronegativity than chlorine.

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Yes, our prediction was correct and they both have the same dipole moment because of their structures.

CHF3 vs CH3F

16. Compare the following molecules within the simulation. Observe the molecules with both the Electrostatic Potential and Electron Density Surfaces. CH4

CH3F

CH2F2

CHF3

CF4

a. Place these molecules in order of increasing polarity.

CH =CF < CHF < CH F < CH F 4

4

3

2

2

3

b. Describe how the electron density surface changes as the number of fluorine atoms in the molecule changes. Electron density increases with fluorines. Fluorine is very electronegative and has most of the electron density around it.

c. Compare the electron density surface to the electrostatic potential surface. What is the relationship between these models?

Electron potential is inversely proportional to electron density.

See below for images of drawing of molecules

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