Molecular Structure 2 (Worksheet)

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3159
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Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.

Q1

For each of the following, (i) draw a Lewis diagram, (ii) count the number of electron groups around the central atom and the number of bonded electron groups, (iii) draw a three-dimensional representation of the molecule, (iv) give the values of the ideal bond angles, and (v) give the name of the electron-pair and molecular geometries.

Substance i ii iii iv v
CH4
NO3
PCl5
SO42–

Modeling can be done with physical models such as balls and sticks or electronically with computers. Molecular models are very useful for visualizing molecular shapes and conceptualizing how molecular shapes relate to physical and chemical properties such as solubility and chemical reactivity. Physical models and computer generated graphics are equally valuable visualization tools. Computer graphics are particularly good for large, complex molecules, such as proteins, where building a physical model is difficult.

2.    Refer to the references cited in: http://ep.llnl.gov/msds/Chem120/vsepr.html

Complete the following table for the molecular images found on the Web page to include:

1. Lewis Structures,
2. An accurate description of the shape of the molecule,
3. Identifying the molecule as being polar or non-polar. For example, see 4A in the table.

A B C
1
2
3
4

Bent, Polar

a) Draw all possible resonance structures for 1B and 4A. The bond lengths of the sulfur to oxygen bonds are equal (1.57 Angstroms) for every sulfur to oxygen bond in both moleclues.  Briefly explain this fact despite an S-O single bond is >1.57 Angstroms and an S=O double bond is < 1.57 Angstroms.

b) Calculate the formal charge for each oxygen atom in the following molecules and clearly indicate their respective charge.

c) Answer the tutorial's practice problems: 3, 9 and 11 from the Purdue Web-site: http://www.chem.purdue.edu/gchelp/vsepr/

Refer to: http://ep.llnl.gov/msds/Chem120/hv-globalCO2.html

3.    a) Prepare a table below with a list of the respective frequencies (wavenumber, cm-1) of the infrared (IR) absorbances and their corresponding percent transmission in the spectrum for your assigned greenhouse gas.

b) Using IR Tutor (downloadable from: http://chemistry.beloit.edu/Warming/.../infrared.html), view the IR spectrum of your assigned greenhouse gas. For each of the absorbances (peaks) that you listed in question #1, add a column or row to the table with the type of molecular vibration that corresponds to each of the absorbances in the spectrum. Eg. anti-symmetrical stretching vibration. Use a separate page if necessary.

Table:

A molecular model kit consists of colored centers that correspond to various common geometries and connecting joints to link the centers. The connecting joints come in longer and shorter lengths to represent longer and shorter bonds. Use the shorter lengths to indicate bonds to hydrogen and the longer lengths to indicate bonds between any other two atoms.

The colored centers usually follow a standard correlation with atoms:

 Black C Red O White H Blue N Purple P Yellow S Silver metals Green halogens

4.    Complete the following table by:

1. finding an example of a molecule or ion with the given structure
2. predicting the molecular geometry
3. building a model
4. estimating the bond angles.

A represents a central atom, B represents a terminal atom, and E represents an unshared electron pair on the central atom. The first line is given as an example.

Structure Example Molecular Geometry Bond Angles
AB2 CO2 linear 180°
AB2E
AB3
AB4
AB3E
AB2E2
AB5
AB6

5.    Build models of each of the following: H2O, H2S, CO2.

a) Consider the electronegativities of the elements and the three-dimensional molecular geometry of each of the three molecules, and match each of the following dipole moments to its corresponding molecule: 0 D, 0.95 D, 1.85 D. Indicate the electronegative and electropositive regions of each polar molecule.

6.    Build a model of acetic acid, CH3COOH.

a) Draw a Lewis diagram of the molecule.

b) Identify the electron-pair and molecular geometry around each atom in the acetic acid molecule, i.e. consider each atom separately as the central atom.

d) Does the molecule have only one unique shape? What factors contribute to its shape or shapes?

The Dutch chemist Jacobus Van’t Hoff (1852–1911) and the French chemist Joseph Le Bel (1847–1930) reasoned that carbon with four covalent bonds must have a tetrahedral shape. This was a tremendous step forward in conceptualizing the chemistry of carbon compounds. Information and experimental methods available to them at the time were limited and sparse compared to today's scientific resources.   A molecular model kit will help you to appreciate Van't Hoff's insight into carbon bonds and three dimensional  shapes which are universally accepted and used today.

Some molecules have the same formula, but can be separated into compounds that exhibit different physical and chemical properties. These compounds are called isomers. Since isomers have different properties, they cannot have exactly the same molecular structure.

An instrument that was used to examine compounds in the late nineteenth century was a device known as a polarimeter. It worked by passing a beam of polarized light through a solution to an analyzing filter. Certain types of compounds had isomers with three dimensional shapes that would interact with the light. These compounds are referred to as being optically active. As an example, if a pair of isomers are mirror images of one another, the light of a polarimeter will rotate the beam in one direction for one of the isomers and rotate the beam by the same amount in the opposite direction for the other isomer.

Van’t Hoff examined a number of carbon compounds with a polarimeter. Compounds without an isomer did not rotate light, while others with did. Consider the following data table.

Compound Number of Isomers
CH4 1
CH3Cl 1
CH2Cl2 1
CH2ClBr 1
CHClBrF 2

The compound CHClBrF was shown to have two isomers that were mirror images of each other. Build a molecular model of each of them.

The two structures are stereoisomers. They have the same connectivity, or order of attachment among the atoms, but a different orientation of their atoms in space. Enantiomers are stereoisomers whose molecules are mirror images of each other. One enantiomer will rotate polarized light in one direction whereas the other enantiomer rotates polarized light by that same amount but in the opposite direction. What do you think will happen to a beam of polarized light if you have a 50:50 mixture of both enantiomers, which is called a racemic mixture?

Consider your hands. Are they mirror images of each other? You may want to use a mirror to check.

7.    Sketch three-dimensional representations of the respective CHClBrF enantiomers to show that they are mirror images. Solid lines are in the plane of the paper, dotted lines are behind the plane, and solid wedges come out from the plane.