CHEM 106 Lab 6

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Chemistry

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Apr 3, 2024

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3/29/22 Experiment 6: VSEPR and Molecular Shape “How Does It Look” Observations and Experimental Data for Part 1 Molecule Name Shape Bond Angle Predicted Bond Angle PF6- Octahedral 89.8 90 [BrF6]- Octahedral 91.8 90 I3- Linear 180 180 ln(CH3)3 Trigonal Planar 116.2 120 [BeF4]2- Tetrahedral 109.7 109.5 NH4+ Tetrahedral 110.3 109.5 [SbF6]- Octahedral 89.9 90 Data for Part 2 Molecule Name Shape XeF5- Square Planar H2O Bent [ClF4]- Square Planar SbBr52- Square Pyramidal Data for Part 3 Molecules Predicted Geometry Actual Crystal Structure Predicted Angles Actual Angles Di-bromodimethylsel enium Seesaw Seesaw 180, 90 177.7, 98º SO2 Bent Bent 120 113.0 NH3 Trigonal pyramidal Trigonal pyramidal 109.5 102.9 Dichloro-diphenyl-se Tetrahedral Seesaw 180, 90 175, 100

lenium Boric acid Trigonal planar Trigonal planar 120 120.7 Focus Question 1. What is VSEPR theory, and how can it be used to predict molecular shapes? The VSEPR theory is used to predict the 3D structure of a molecule by analyzing the number of electron groups around the central atom. There are two components of this theory: electron geometry and molecular geometry. Electron geometry only considers the electron groups around the center, including lone pairs as a group, and gives an overall shape for the molecule. Molecular geometry also looks at the electron group around the center but it considers the impact lone pairs can have on a molecular structure. For structures with a lone pair, they have different names from their electron geometry and a lower expected bond angle. 2. a) Can the structure of simple molecular substances be illustrated by drawing or building models? Why are why not? This is possible because when given a simple molecular substance and its molecular formula, its Lewis structure can be drawn, which can help us to determine the number of electron groups around the central atom. This information can then be used to find the molecular geometry, which can be easily drawn. These structures can also be built as shown in this experiment. b) What about more complex molecules? Explain. Complex molecules are harder to draw since their molecular formula tends to have more atoms to each element. For instance, in the experiment, di-bromodimethylselenium and dichloro-diphenyl-selenium were given. From the compound name, it can be hard to know the chemical formula. In drawing the Lewis structure, it gets more complicated to draw an accurate structure. However, these molecules can be built on a program performed in this experiment. Hence, it isn’t impossible to draw and predict the molecular structure, but it can be difficult without the help of a program. 3. How are models and theories useful in helping explain the structure and behavior of matter? Models and theories like the VSEPR theory can help predict the 3D structure of a molecule – its shape, bond angle, and polarity. A molecule with a greater amount of lone pairs will decrease in bond angle and length, impacting the way the molecule looks. It can also show how atoms are positioned, helping us see the molecule’s shape and polarity – whether there is symmetry or if the dipole moments cancel out. Post-Lab Questions 1. What are molecular geometries and how do they differ from electron domain geometry Molecular geometry predicts a molecule’s structure by looking at the number of electron groups around the central atom and the impact lone pairs from the center can have on the overall structure. This is seen by the many names and shapes of molecules with different amounts of lone pairs. For instance, a molecule with 5 electron groups and 1 lone pair is a seesaw structure

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