Experiment #5 & 6 - note: not everything is right but I know I did get higher than 80/100 on all PDF

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note: not everything is right but I know I did get higher than 80/100 on all my labs! My grade ranged from 85-99 for all my labs, keep that in mind when referencing. ...

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Cindy Huynh Matthew Depouli September 26th, 2020

Experiment 5: M  odels to the Rescue ~ Introduction: We always visualize molecules in a two-dimensional aspect during general chemistry but never in a three-dimensional aspect. This prevents us from fully seeing all the properties that the molecule has, as well as understanding those properties. A three-dimensional structure can help us determine chemical and physical properties as well as the bonds they have. In this lab, the molecules we are going to work on include H2O, NH3, CH4, OCl2, CS2, BF3, NF3 and SF6. Using the Avogadro Program, we will be able to verify the molecular geometries predicted from the VSEPR theory, visualize molecular structures in 3D, and obtain the bond distances and bond angles from the calculations.

~ Materials: ● Laptop w/ Avogadro Program: helps us verify the molecular geometries predicted from VSEPR ● Lab manual ● Notebook for drawing Lewis Structures

~ Observations and Experimental: Part 1: Predictions of Molecules using VSEPR Molecules

H2O

NH3

Lewis Structure

Prediction Electron Geometry: Tetrahedral Molecular Geometry: Bent Bond Angles: 109.5°

Electron Geometry: Tetrahedral Molecular Geometry:Trigonal Pyramidal Bond Angles: 109.5°

CH4

Electron Geometry: Tetrahedral Molecular Geometry: Tetrahedral Bond Angles: 109.5°

OCl2

Electron Geometry: Tetrahedral Molecular Geometry: Bent Bond Angles: 109.5°

CS2

BF3

Electron Geometry: Linear Molecular Geometry: Linear Bond Angles: 180°

Electron Geometry: Trigonal Planar Molecular Geometry: Trigonal Planar Bond Angles: 180°

Electron Geometry: Tetrahedral Molecular Geometry: Trigonal Pyramidal Bond Angles: 109.5°

NF3

Electron Geometry: Octahedral Molecular Geometry: Octahedral Bond Angles: 90°

SF6

Part 2: Using “Avogadro” to verify molecular structures. Molecules

H2O

NH3

Structures

CH4

OCl2

CS2

BF3

NF3

SF6

~ Discussion and Conclusion: This lab helped us properly see and understand the components of molecules in a three-dimensional aspect. We used the Avogadro Program to look at the geometric shape of the molecule and concluded that if the amount of lone pairs decreases, the bond angle increases. The VSEPR model allowed us to see what water’s bonds angle as 104.5 degrees with a bent shape. Methane was a tetrahedral resulting in a bond angle of 109.5 degrees. With the ability of the Avogadro Program, these molecules can be seen to show a trend between bond angles.

~ References: ● Smeureanu, G. & Geggier, S. (2019). General Chemistry Laboratory . New York, NY Zumdahl, S. (2014). Chemistry. 9 th ed. Belmont, California: Cengage Learning ● “Avogadro Chemistry.” Avogadro , July 2018, avogadro.cc/. Accessed 3 Oct. 2020. ● “Periodic Table - Ptable.” Ptable.Com , 2020, ptable.com/?lang=en#Properties. Accessed 9 Oct. 2020. ● Creative Chemistry, Tetrahedral molecules. “Tetrahedral Molecules | Creative Chemistry.” Creative Chemistry , 8 June 2017, www.creative-chemistry.org.uk/molecules/tetrahedral#:~:text=The%20H%E2%80%94C %E2%80%94H%20bond,the%20tetrahedral%20angle%2C%20109.5%C2%B0.&text=T he%20bond%20angles%20in%20ammonia,their%20lone%20pairs%20of%20electrons. Accessed 9 Oct. 2020.

~ Focus Questions: 1. What is an element? A pure substance which cannot be broken down by chemical means, consisting of atoms which have identical numbers of protons in their atomic nuclei. 2. What is a molecule? A molecule is a group of atoms bonded together.

3. How can you construct a model of a molecule based on a chemical formula? The chemical formula tells us the type and number of atoms that are in the molecule. We can construct a model by calculating the number of bonds and how they are bonded. When we do this, we have to keep the octet and duet rule in mind as most elements (except Hydrogen) are most stable when they have eight electrons. 4. Do these images look like what you expected? (i.e. compared to images you’ve seen in lectures or in your textbook?). The images came out like I expected because all the shapes that I predicted were shown. The images that I see in textbooks are also the same as what I saw on the program. 5. What does such a picture of an orbital represent? Explain this as clearly and concisely as you can. Each picture of the orbitals represents the orbitals bonding. They show the region between the nucleus where the electrons orbit. The models illustrate the bonding and ant-bonding electrons whether its pi and/or sigma bonds.

~ Post Lab Questions: 1. Fill in the following table using data calculated by Avogadro’s software in the lab: Compounds

Angle VSPER predicted (degrees)

Angle before optimization (degrees)

Angle after optimization (degrees)

Angle Literature value (degrees)

Bond Length before optimization (Å)

Bond Length after optimization (Å)

Water

109.5

109.471

104.5

104.5

0.97

0.99

Ammonia

109

109.47

108.32

107

1.02

1.04

Methane

109.5

109.47

109.47

109.5

1.07

1.11

2. Do you see a trend for water, ammonia, and methane bond angles? Explain your answer in terms of their VSEPR structures. In the data collected, I did not observe any trends. However, a trend can be found using the VSEPR theory. We see that water has 2 bonding groups and 2 lone pairs and can conclude that it has a bond angle of 104.5 degrees. Ammonia has 1 lone pair and 3 bonding groups and has a bond angle of 107.8 degrees. As we observe methane we know that it has 4 bonding groups and no lone pair. This means that we should’ve seen a difference in their angle column, but we did not.

3. What is the main idea behind the VSEPR theory? The main idea behind the VSEPR theory is electron repulsion. Their theory tells us how atoms are bonded to a central atom and how the number of bonding groups affect the shape and bond angle of a molecule. If there are more lone pairs, there would be a stronger electron repulsion causing the bond angles to shrink. 4. Which bond angle was the closest to the literature value (after “clean up” or “ab initio optimization”)? Explain your answer. The bond angle closest to its bond angle after cleanup is methane. Methane has 4 bonding pairs to its central atom resulting in its tetrahedral’s structure. This tetrahedral shape is stable and after we cleaned it up, the bond angle remained the same (109.47). 5. What is the bond order in the nitrogen molecule as predicted by the MO theory? Is this in agreement with the Lewis Structure? Explain and show calculations of bond order. The bond order in a nitrogen molecule is 3. There is a triple bond between the diatomic nitrogen after drawing the lewis structure and following the octet rule. We can see a triple bond between nitrogen. (Bonding Order = # of bonding electrons - # of antibonding electrons) / 2) (Bonding Order = (6-0)/2 = 3)

Experiment 6: H  ow does it look? ~ Introduction: The VSEPR theory is a model used to determine the geometry of individual molecules from the number of electron pairs surrounding their central atoms. The  central atom is the one who is most electronegative relative to the other atoms in the molecule. The theory also assumes that each atom in a molecule will achieve a geometry that minimizes the repulsion between electrons in the valence shell of that atom. In  this experiment, we will use the VSEPR model to determine the shape, bond length, and bond angle specific molecules had. The experiment will also introduce the specific lone pairs and electron density for the specific molecules.

~ Materials: ● Laptop ● Lab Manual ● Notebook for drawing Lewis Structures

~ Observations and Experimental: Part 1: Basic Shapes Molecule

Shape

PF Octahedral

[BrF ]Octahedral

Bond Angles Observed

Predicted Bond Angle (degrees)

89.9 90.1 90 89.4

90

90 (2) 91.1 86.6

90

The bond angles from observed are all close to or exactly 90. We see 2 angles that are exactly 90 degrees.

180

Predicted is the exact angle measurement as the observed molecule.

180

I Linear In(CH ) Trigonal Planar

Observations based on VSEPR

109.4 (2) 109.5

109.5

VSEPR predicts that the bond angle for an octahedral would be 90 degrees and the bond angles observed are all close to 90 and exactly 1 is 90 degrees.

The bond angles observed are all extremely close to the predicted bond angle that a trigonal planar shape would have.

Tetrahedral

108.6 109.7 108.6

109.5

Tetrahedral

114.9 110.3 109.5

109.5

[BeF ]^2-

NH +

[SbF ]Tetrahedral

92.1 88.2 91 88.7

90

Generally we see that the bond angle observed is close to the bond angle predicted from a tetrahedral shape of 109.5. We have one observed angle that is fairly close to the The observed bond angles are fairly close to the expected bond values.

Part 2: Effect of lone pairs on the molecular shape Molecule Name

Shape

XeF -

Pentagonal Planar

H O

Bent

[CIF ]-

Square Planar

SbBr ^2-

Square Pyramidal

How will the presence of lone pairs affect molecular shape? The presence of lone pairs changed the geometry of the molecule. The more lone pairs the molecule has, the more it will repel which causes the change in shape and angles. An example would be water (H2O) because without the 2 lone pairs, the molecule would have been linear but with the 2 lone pairs, the molecular geometry became bent. Part 3: Effect of lone pairs on bond angles Molecule

Predicted Geometry

Actual Geometry

Predicted Bond Angle (degrees)

Actual Bond Angles

Di-bromodimethylselenium

Seesaw

Seesaw

90

90 90.6 91.7

SO

Bent

Bent

120

109.5

NH

Trigonal Pyramidal

Trigonal Pyramidal

109.5

100.4 101.3 102.9

Dichloro-diphenyl-selenium

Trigonal Bipyramidal

Trigonal Bipyramidal

90

90.8 92.4 100.4

Boric Acid

Trigonal Planar

Trigonal Planar

120

119 120.2 120.7

Comment on how closely the observed bond angles agree with the expected idea values? Can you account for any deviations for the idea values? The observed angles were fairly close to the expected idea values (ex: boric acid where it was...