Understanding Molecular Geometry through VSEPR Theory Lab
School
Joshua High School, Texas**We aren't endorsed by this school
Course
SCI 100
Subject
Chemistry
Date
Dec 10, 2024
Pages
6
Uploaded by ChancellorRain15891
Name__________________________Date_______________Per_________Molecular Geometries as Predicted by VSEPR LabIntroduction:Chemists spend a lot of time trying to understand the structure of molecules. This understanding allows one to explain observations such as bond lengths, dipole moments, and solubilities. It alsoallows one to better control molecules in our attempts to synthesize various products. Like all models that survive the test of time and experimentation, the VSEPR (Valence Shell Electron Pair Repulsion) theory is often, but not entirely, successful. It does a good job predicting the shapes of small, simple molecules, but is not entirely adequate for some more complex organic structures. Nonetheless, it presents a very good foundation upon which to build your 3D understanding of molecular geometry. As you have learned, it is usually essential to start by drawing acceptable Lewis structures, which help orient valence electrons and distinguish between bonded and lone pair electrons. The only problem here is that this is still a two-dimensional description of a three-dimensional problem. With VSEPR, there are two distinct skills to develop:1) After drawing the Lewis structure, one immediately knows the number of electron pairs, both bonded and lone, that surround the central atom. This allows a prediction of the geometrical distribution of electron pairs. The molecular geometry will then be a subcategory under one of these 5 basic electron geometries – linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. 2) Once the basic geometry is known, the specific molecular shape can be predicted on the basis of the number of lone pairs versus bonded pairs. There are 11 basic molecular shapes – linear, trigonal planar, tetrahedral, trigonal pyramidal, bent (V-shaped), trigonal bipyramidal, see-saw, T-shaped, octahedral, square pyramidal, and square planar. The ones in bold are the ones you are responsible for. Three notes worth your attention:1) Lone pairs cause more repulsion than bonded pairs, thereby often compressing ideal bond angles.2) Beryllium and boron can be electron deficient, requiring less than 8 valence electrons.3) Group 16 and Group 17 often use expanded octets for elements in Period 3 and higher. Most common elements to do this are sulfur, phosphorus, arsenic, selenium, and xenon. In this lab, you will ultimately determine the bond angles and molecular geometries of a set of compounds. You are expected to know the 5 basic electron geometries and corresponding molecular geometries.
Pre-Lab (Day 1):Go to the following website: https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule-shapes_en.htmlExplore the Model screenof the simulation. As you explore, answer the following questions. If you click the Molecule Geometry and Electron Geometry checkboxes, the simulation will tell you the names of these geometries. 1) How does adding an atom affect the position of existing atoms or lone pairs?2) How does adding a lone pair affect the position of existing atoms or lone pairs?3) Is the effect of adding bonded atoms and lone pairs to the central atom similar? Explain why this could be the case.We can think of a bond or a lone pair as a “domain” of electrons. Single bonds, double bonds, and triple bonds each count as one domain. 4) How do the electrons in bonds (bonding domains) differ from lone pairs (non-bonding domains)?5) What happens to the bond angle when you add or remove an electron domain?6) Can you force the atoms into new configurations by pushing atoms around? What does this suggest about the configuration of atoms in real molecules?7) What is the difference between Electron Geometryand Molecule Geometry?8) In one or two sentences, write a definition for the term Molecular Geometry.
9) Using the Model screen, add bonding domains () to the central atom (). Sketch each molecule’s shape.
10) In the following table, draw the molecule geometry using the simulation. Label with the Molecular Geometry name.
Name__________________________Date_______________Per_________Molecular Geometries as Predicted by VSEPR LabProcedure (Day 2):Part I:1.For each molecule/ion listed in the accompanying data chart, draw the best Lewis structure. 2.Using the model kit and the data chart, construct a model of each molecule. You will need to consider the number of bonds that each atom can make when you choose pieces for your model. Look to your Lewis diagrams for guidance. ColorMaximum number of bondsWhite1Black4Light Blue3Red2Green1Yellow2Dark Blue5Dark Purple5Light Purple5Silver63.Determine the number of electron domains (bonding and lone pairs of electrons). Describe each structure’s electron geometryas linear, trigonal planar, tetrahedral, trigonalbipyramidal, or octahedral. Record in the data chart. 4.Estimate the bond angle for each molecule. 5.Determine the molecular geometryof each molecule using the chart from Question 10 in the prelab. 6.After you are finished with the models, deconstruct your models and put them back into the kit. Please make sure you don’t lose pieces. Conclusion:1. In your own words, describe VSEPR theory.
2. Open the PHET Simulation you used to complete the prelab. Open the Real Molecules Simulationand choose water. Make sure “Show Bond Angles” and “Show Lone Pairs” are both turned on in the options. What is the bond angle for the real molecule? What is the bond angle for the model?Based on your observations of bond angles in the “Real” vs “Model” simulation, what can you conclude about the difference between lone pairs and bonding pairs in terms of how they affect shape? 3. How do you determine a molecule’s electron geometry? Be detailed. You are writing a summary that you can use to study for your test. 4. How do you determine a molecule’s molecular geometry? Be detailed. You are writing a summary that you can use to study for your test. 5. Now that you’ve finished the lab, would you alter your answer for Number 7 of the Prelab? Why or why not?What is the difference between Electron Geometryand Molecule Geometry?