Chem experiment 11 - This is lab work for students taking chem 211 and the lab section. PDF

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This is lab work for students taking chem 211 and the lab section....

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Ruth Leilago G# 01275662 Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020

Experiment 11: VSEPR and Molecular Modeling Reference: Suzanne Slayden, 2018, Chemistry 213, 214, 272 Laboratory Experiments, 7th Edition, pg. 101107 George Mason University, Department of Chemistry and Biochemistry. VSEPR and Molecular Modeling; 2020 VSEPR and Molecular Modeling, Distance Learning Summer 2020, accessed via Blackboard Bergwerf, H. S(Cl)(Cl)(f)(f)(f)f https://molview.org/?cid=154124169 (accessed Nov 16, 2020).

Purpose: The purpose of this experiment is to create an accurate three-dimensional (3D) models of a number of molecules or construct ball-and-stick models using the Valence Shell Electron-Pair Repulsion (VSEPR) theory, that would help to predict the shape of molecule from a the determined chemical formula of the molecule, and which ultimately will be useful to determine if two or more compounds are geometrically isomers (Slayden, 2018). The anticipated outcome of this experiment is to learn how to construct 3D models for the number of molecules and to gain better understanding of the molecular shapes.

Materials: • •

MolView (Website used to construct the geometric shapes) Ball-and-Stick molecular modeling kit

Procedure: Part I: In this part, five basic geometrical shapes will be constructed based on the VSEPR theory. The geometrical shapes are liner, trigonal planar, trigonal bipyramidal, and octahedral. 1. Get a proper ball that contains the proper number and enough placement holes, that would represent each central atom from the molecular model set. 2. To construct the wanted figure, insert the rods into the holes. Make sure to make it tight so that it will not keep failing.

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 3. Draw the molecular models that was constructed. 4. Using a protractor, measure the angles between all adjacent electron pairs in each geometrical model. •

Note: If the model is created online using the website MolView, it would automatically show the angle of the molecular model, so there is no need of measurement.

5. Differentiate between the axial and equatorial positions and angles for the trigonal bipyramidal shape. 6. Differentiate between the two sets of angels for the octahedral shape. Do not forget to record the measurements. Part II: In this part, modes of the molecules that are listed below would be constructed using the VSEPR theory to predict the geometrical shape. BeH2

NH3

XeF4

OH2

PCl3F2

SF4Cl

For all the molecules: 1. Calculate the valence electron numbers in the molecule. 2. Draw a Lewis electron dot structure for the molecule. 3. Determine the values of x and y in the ALxNy notation. Do not include non-bonded pairs on the ligands, since y refers to the number of non-bonded electron pairs on the central atom only. 4. Name or state the geometric arrangement of the electron pairs corresponding to (x + y), where x refers to the number of ligands. 5. Using different colored balls to represent different atom or ligands, build the molecular model. 6. Draw the molecule in its correct geometry. Also, write the symbol for each atom or nonbonding pair of electrons in the drawing.

Part III: A. First, build molecular models of the two possible isomers of PCL4Br to investigate geometrical isomerism. Remember: Even though the isomers contain the same atoms and bond types, the two cannot be superimposed and would be different compounds. Draw these isomers. B. Second, after drawing Lewis electron dot structures, construct models of all possible isomers for each of the molecules or ions that are listed below. TeCl3Br SF4Cl2 XeF2

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 • Use the model that was constructed and the sketches to decide the number of isomers there may be for each case and record it, along with geometry and sketch of each isomer. Note: Even though isomers may be drawn, or model may be built for many molecules, there are chances that all the possible isomers may not exist.

Part IV: After having a clear idea of the VSEPR theory and understand it well, apply its concept and determine how many isomers could be proposed for C3H6O Hint: Ketone (acetone) Ether Aldehyde Alcohol

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 Data: VSEPR Theory Basic Geometric arrangements and Angle Measurements Electron Pairs on Central atom

2

Geometric Arrangement

Lewis Structure

Ball-and-Stick Drawing

Angel Measurement

Linear 180o

3

Trigonal Planar

120o

4

Tetrahedral

109.5o

5

Trig Bipyramidal

90o

6

Octahedral

90o

Table 1:

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 VSEPR Theory Prediction of Geometrical Arrangements for different Molecules Molecule

Valence Electrons

ALxNy Notation

Lewis Structure X

Ball-and-Stick Drawing

Geometric Arrangement

Y

BeH2

4

2

0

Linear

NH3

8

3

1

Tetrahedral

XeF4

36

4

2

Octahedral

OH2

8

2

2

PCl3F2

40

5

0

Triagonal Bipyramidal

SF4Cl2

48

6

0

Octahedral

Tetrahedral

Table 2:

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 Part III

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 Part IV

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 Observations: This experiment was basically a hand on lab because students were able to experience the geometrical shapes of molecules by constructing a model. One thing that could be mentioned here is the difference between in the angle measurement of each geometrical arrangement of molecules. For instance, linear arrangement has 180-degree measurement while octahedral and trigonal bipyramid have 90 without any lone pairs. As the lone pairs increase, the degree measurement of the molecule also decreases.

Calculation: Sample Calculation for Valence electrons (NH3)

Valence electrons of N (Periodic table) = 5 Valence electrons of H (Periodic table) = 1 Valence electron of NH3 = Valence electrons of N + Valence electrons of H Valence electron of NH3 = 5 + 1(3) since there is 3 Hydrogen, it has to be multiplied by 3 Valence electron of NH3 = 8

Result: The VSEPR theory had been applied to identify the geometrical arrangements of different molecules. The valence electrons of the molecule were determined by adding the valence electron of the two or more elements as it is shown in the sample calculation. Table 1 represents the VSEPR theory basic geometric arrangements and shapes of any type of molecules depending on the valence electrons of the molecule. Table 2 represents the six different molecules geometrical arrangement and the number of bonded and non-bonded electrons represented as X and Y in the table.

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 Discussion: One of the major parts of this experiment was drawing a Lewis structure for the different molecules that were given. A Lewis structure is a considerably basic representation of the electrons of the valence shell in a molecule. It was used to explain how the electrons in a molecule are structured around individual atoms. The electrons were shown as a line between the two atoms as dots or bending electrons. However, Lewis structures can only provide 2-diamnetional views of molecules, but since molecules are 3-diamnetional, this view could be misleading. For example, the Lewis structure for the molecule BeH 2 would be represented as H Be H as the two electrons of Be bonded with 2H, even though hydrogen itself has 1 valence electron, the bonding shows that there are a total of 5 electrons. In order to differentiate the bonding and non-bonding pairs of electrons in a molecule, the notation ALxNy was used. L represented the ligands that are attached to the central atom and N represented the non-bonded electron pairs. X and Y represented the numbers of ligands and the numbers of non-bonded electron pairs on the central atom. Learning this has helped me understand how to develop the possible geometrical arrangement for the molecules. For instance, the molecule XeF4 has 4 ligands and 2 non-bonded electron pairs on the central atom when its Lewis structure is drawn or could simply be determined by looking up at the periodic table for the valence electrons. Xe is a noble gas and it’s on the 8th group, so it would have eight valence electron and F is on the 7th group and the 4 fluorine (F) would bond with the four Xe electrons and 2 pairs of electrons will remain unbonded. The angle measurement of the molecules depends on the geometric arrangement. For instance, a molecule with a trigonal planar arrangement and without any lone pair would have 180o. By using VSEPR and MolView, I was able to contrast the ideal angle measurements with the observed angle measurements. When a molecule did not have a lone pair, it was much closer to the ideal bonds than the molecules that did have a lone pair. For example, NH3, which has one lone pair was about 8.2 degrees away from the ideal measurement which was 109.5o. Understanding the molecular geometry is important because once the molecular geometry of a molecule or compound has been figured out, the geometrical isomers of a molecule could also be determined if a molecule has one. These isomers are compounds with the same structural formula but different relative spatial positions of the ligands. For example, the molecule PCl4Br had two isomers. That means that this compound could be found into different structural form because the positions of the ligands vary in each structure. Generally, in this experiment, I have learned the VSEPR theory and making a model for different molecules and the geometrical arrangements of the molecules.

Ruth Leilago Instructor: Ms. Moll CHEM Lab 213-230 11/16/2020 Conclusion: The purpose of this experiment was to experience the geometrical shapes of molecules by constructing their model by applying the VSEPR theory to visualize the 3D relationship of atoms in a molecule or polyatomic ion. The Lewis structures provide 2D views, but molecules since molecules are 3D, the Lewis views are inherently misleading. The Valence Shell Electron Pair Repulsion (VSEPR) theory was applied because it is used to predict the shape of the molecule from knowledge of its chemical formula and its electron distribution as depicted in Lewis structure. Even though the apparatus that was supposed to be used to determine the ball-and-stick structure of the molecules was the kit, the website “MolView” was used instead. The purpose of this experiment was met and the anticipated outcome as well. Since the anticipated outcome was to learn the geometrical arrangements of molecules applying the VSEPR theory. For a given molecule, the Lewis structure could be drawn or structured by simply using the valence electrons of the molecule and creating a proper structure for the molecule. The valence electron of the molecule could be determined by observing the valence electrons of each atom from the periodic table and adding it without leaving out the subscript of every atom. For example, the valence electrons of the molecule XeF4 is 36 (8 electrons of Xe + 7(4) F). Once the valence electron of the molecule is determined, then the geometrical shape or arrangement of the molecule could be determined by using the ALxNy notation which would be useful to differentiate the bonding and non-bonding pairs of electrons in a molecule. By understanding the molecular geometry, the geometrical isomers of a molecule could also be determined if a molecule has one. These isomers are compounds with the same structural formula but different relative spatial positions of the ligands. For example, molecule C3H6O has isomers such as ketone, alcohol, ether, propanal, aldehyde and few other more. These compounds have the same structural formula, but different relative spatial positions of ligands as shown above in part 3. Because online source was used to create the ball-and-stick structure of the molecules, errors may occur and must be taken into consideration. For example, the observed angle measurements were derived by the student dragging the line and clicking the line on the molecule. This could cause error because the student may or may not have measured in the right way or misplaced the line that measures the degree. For the future, using the ball-and-stick model and measuring using protractor might reduce the chance of errors occurring even though it does not guarantee the result would be 100% correct....