Experiment 13- Molecular Geometry PDF

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Download Experiment 13- Molecular Geometry PDF

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Experiment 13: Molecular Geometry Required reading: Ebbing, 11th Edition, Chapter 10. - VSEPR Model -Valence Bond theory Learning Goals: • To predict and study the geometry of a molecule using simulations • To observe how increase in electrons in the valence shell affects the geometry of a molecule. • To study the different geometries a molecule can exhibit. Background information and theory: Atoms bond to satisfy their need for more electrons. Atoms of nonmetals have high electronegativities, when forming a bond they will share electrons to satisfy the Octet Rule – every atom wants 8 electrons to fill the s and p orbitals in the outer energy level. But, as you will see, if the electronegativities are high enough and both atoms unwilling to give up electrons, sometimes atoms can deviate from and not follow) the Octet Rule. Because electrons have a negative charge and atoms occupy space, bonds and electrons will spread out as much as possible. Since we write in a two dimensional plane on paper, it is difficult to visualize the true geometry of these molecules. This activity and the program you are about to use allows us to visualize on a more 3-dimensional scale. To understand more about molecular shapes watch this video: https://www.youtube.com/watch?v=nxebQZUVvTg&t=52s To learn about how to draw 3D structures watch this video: https://lonestar.techsmithrelay.com/ayqJ Materials: Playdoh (at least 3 colors) Toothpicks Procedure: Experiment courtesy of Michael Kwasny via PhET simulations/teaching materials (Original Title: PhET Shapes Inquiry Lab).

Part 1 and 2: Simulation 1) Go to the following website: https://phet.colorado.edu/en/simulation/molecule-shapes 2) Click on the simulation 3) Select Model Spend a few minutes playing with the simulation, add bonds, remove bonds, add lone pair of electrons, etc. 4) Click on reset or end the simulation and start it again to work on the activities for this experiment. 5) Complete the data tables and answer the questions in the next pages.

Part 3: AT HOME 1) Using playdoh and toothpicks build 3D models for the following molecules according to the VSEPR theory: CO2, H2O, HCN, CH4, NH3 , SO3. Before making the model, predict the shape the molecule will exhibit. Use one toothpick to represent one pair of bonding electrons. Use different colors of playdoh to represent the different elements. Don’t forget to consider lone pairs of electrons, when working on the 3D arrangement, but because you are representing molecular geometries (not electron arrangements) you don’t need to show these non-bonding electrons. 2) Print the table in the data table section and place each molecule in the corresponding box. 3) Take a picture of the paper with the 3D models you made and add it to your report.

Name: ___Judy Pham_____ Molecular Geometry Simulation Data Part 1 – Generic Molecules Fill in the chart below by creating the generic molecules below. On your screen in the lower left corner, click on “molecule geometry.” Add atoms and electron pairs as needed to produce the generic formula. Once the molecule is assembled, click and drag the screen to spin the atom around. Click on the “ Show bond angles.” Use the following key: • A – central purple atom – cannot be removed • B – single bonded white atom • C – double bonded white atom • D – triple bonded white atom • E – Electron pairs not bonded In each box: 1. Draw the molecule you create to the best of your ability 2. Write the Molecule Geometry (MG)name in the box 3. Label the bond angle 4. Look at the central atom, is its octet satisfied? AC2

ABE3

MG linear Octet? Yes MG __linear_____________________ Octet? __no____ AB3E

MG __trigonal pyramidal Octet? __no____

AB

MG _linear______________________ Octet? __no____

ACE2

MG _____linear_______________ Octet? __yes____ AB2C

MG __trigonal planar_______________ Octet? _yes_____ AB4

MG _tetrahedral________________ Octet? __yes____

AB2E2

MG ____bent__________________ Octet? ____yes__ AB3

MG trigonal planar ________ Octet? __no____ ADE

MG __linear_____________________ Octet? __no____

Part 2 – Real Molecules Click on the “Real Molecules” tab at the top of the page. Using the pull-down menu, select the molecules below and fill in the chart. Match the molecule to the generic structure above. Fill in the generic bond angles.

Molecule

Generic Formula

H 2O

AB2E2

Generic bond angles (from True Bond Angles part 1) 109.5 104.5

CO2

AC2

180

180

CH4

AB4

109.5

109.5

NH3

AB3E

109.5

107.8

BF3

AB3

120

120

Part 3 – Octet Rule Breakers While still in the “Real Molecules” tab, select the following molecules. Draw each molecule, name the geometry and write how many electrons are on the central atom. Molecule Molecule XeF2 SO2

MG _bent_______________________ Electrons?__6__ ClF3

MG linear _____________________ Electrons?_8___ SF4

MG ____T-shaped_____________ Electrons?_7__

MG _seasaw___________________ Electrons?_6___

BrF5

XeF4

MG _square planar_________________ Electrons?__8__ PCl5

MG _square pyramidal____________ Electrons?__7__ SF6

MG _trigonal bypyramidal____ Electrons?__5__

MG octahedral_________ Electrons?_6___

Analyze the following: 1. What angle is needed to spread 4 bonds as far apart as possible? Hint: look at a molecule with four separate bonds. • 109.5 2. Find the two generic molecules from Part 1 that are made of 3 atoms. a. Compare and contrast these two molecules by listing two similarities and two differences. b. Give a real-life example of each. Molecule AB3 ADE Similarities

Made of 3 atoms

Do not satisfy octet rule

Differences

Different molecular geometry

Different bond angles

Real-life Examples NH4Cl

Nitrogen

3. Who are the 2 most electronegative elements on the periodic table? • Oxygen and Flourine

PART 3: At home: Print this table and place the 3D models on each box. Focus on molecular geometry, not the electron geometry (do not show lone pairs but consider them to define the shape of the molecule) Add a picture to your report. CO2 H 2O

HCN

CH4

NH3

SO3

Name: ___Judy Pham__________ Molecular Geometry Postlab questions: Your instructor may ask you to answer these in your lab notebook, or to answer directly on this page and turn it in, or to include these answers in a formal lab report. Follow your instructor’s directions. 1.

What does VSEPR stand for? Explain its meaning in your own words. •

2.

VSEPR stands for the valence shell electron pair repulsion theory. It serves as a guide or model that demonstrates the shapes/structures of the pairs in an atom. What molecules in Part 1 consisted of only two atoms?

•

AB

3. The program did not give a bond angle to a molecule consisting of only two atoms. Why? Think geometry class. •

Bond angles for two atoms would be invalid because it is not enough information. The two atoms could be put anywhere which would change the angle every time. It would not be reliable.

4. Looking at the table in Part 2, some of the angles stayed consistent while others did not. Compare and contrast the two groups of molecules (those with matching angle measurements to those with different measurements). What is causing the angles to skew? Explain why this might be. •

There could be numerous factors that are causing the angles to skew as demonstrated by part 2 such as the amount of lone pairs and the total amount of electrons. For example, not counting the lone pair in NH3, both it and H2O had a total of 3 electrons and were the only two out of the five that had inconsistent angle readings from the table in part 2....