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Molecular Geometry and Polarity Flipbook PDF
55 Laboratory 1 Molecular Geometry and Polarity 1–6 Figure 3 shows different molecular geometries with observed bond ang
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Molecular Geometry and Polarity Objectives Station 1. Lewis Structures and Molecular Models • Correlate Lewis structures with electron domain geometries. Station 2. VSEPR and Balloon Models: effect of lone pairs on molecular shape • View the effect of lone pairs of electrons on molecular shape. Station 3. Polarity of Solvents: mixing of solvents of different polarities • Study the effect of polarity on the miscibility of liquids. Station 4. “Like Dissolves Like”: determination of polarity of an unknown solid • Determine the type of solvent that generally dissolves ionic compounds. • Determine the type of solvent that generally dissolves polar covalent compounds. • Determine the type of solvent that generally dissolves nonpolar covalent compounds. • Investigate the effect of adding a polar liquid solute to a nonpolar liquid solvent.
Materials Needed Station 1. Lewis Structures and Molecular Models • 5 molecular models representing the 5 basic electron-domain geometries (prepared by instructor) Station 2. VSEPR and Balloon Models: effect of lone pairs on molecular shape • 13 balloon models representing the 13 different molecular geometries (prepared by instructor) Station 3. Polarity of Solvents: mixing of solvents of different polarities Equipment
test-tube rack test tubes stoppers to fit test tubes pipettes waste container
Chemicals
water ethanol cyclohexane
Station 4. “Like Dissolves Like”: determination of polarity of an unknown solid Equipment
test-tube rack test tubes stoppers to fit test tubes pipettes unknown white solid
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Chemicals
water ethanol cyclohexane sodium chloride (table salt)
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waste container naphthalene (moth balls) unknown white solid
sucrose (table sugar)
Safety Precautions Standard safety precautions (goggles, gloves) are satisfactory for this lab. Background Function depends on structure. The three-dimensional (3D) structure of a molecule (e.g., how its atoms are connected and arranged in space) is critical in determining its chemical and physical properties. As an example, a molecule’s geometry determines whether or not it will be polar; polarity has a large impact on boiling and freezing points as well as chemical reactivity. The senses of smell and taste are largely governed by
molecular structure. How drugs work and their specificity or toxicity are also controlled by geometry. It is therefore important to be able to predict the structure of molecules so that we can understand chemical and biological phenomena. To determine the correct 3D structure for a molecule, the following strategy is used:
Step 1 – Draw the molecule’s Lewis structure. Step 2 – Determine the electron-domain geometry around the central atom. Step 3 – Based on the number of lone pairs on the central atom, determine the molecular geometry. Each of these steps is outlined next. STEP 1. Draw the Lewis structure. A review of drawing Lewis structures is located at end of this lab’s workshop.[[Author: Question 1 or is something missingGregg and John, please answer so?]] STEP 2. Determine electron-domain geometry around the central atom. Chemists utilize the valence shell electron-pair repulsion (VSEPR) theory to predict the 3D structure of molecules. VSEPR is a straightforward method that relates a molecule’s Lewis structure (2D) to its molecular geometry, which can then be used to predict molecular properties such as polarity. VSEPR is based on the simple idea that electron (e–) pairs (both bonding and nonbonding, or lone pairs) on an atom repel each other and try to stay as far apart from each other in space as possible. To minimize
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repulsion, e– pairs will distribute evenly around an atom, generating specific geometric arrangements, depending on the number of pairs. We use the term electron-domain geometry to describe this geometric arrangement around an atom. An electron domain is composed of a single lone pair of electrons or a chemical bond; a single, double, or triple bond each counts as a single electron domain. As just described, using e– pairs as an example, electron domains on a single atom will repel each other and therefore arrange to minimize electrostatic repulsions. There are five basic electron-domain geometries possible (Fig. 1): 1. Linear: In this arrangement, there are only two electron domains on the middle atom. The two domains will orient themselves on opposite sides of the middle atom, generating a linear geometry with a bond angle of exactly 180°. Note that all two-atom molecules (A2 or AB) will be linear molecules—there is no need to consider electron domains. 2. Trigonal Planar (or planar triangular): In this arrangement, all electron domains of the central atom are located in the same plane as the central atom, at the corners of a triangle (the central atom is located in the middle of the triangle). This, in turn, places all atoms in the molecule in the same plane, with bond angles of 120°. The three electron-domain positions are equivalent. 3. Tetrahedral: The four electron domains arrange at the corners of a pyramid, which contains four trigonal faces. The central atom is located in the center of the pyramid, with bond angles of 109.5°. As we saw for the trigonal planar electron-domain geometry, all four of the domain positions are equivalent. 4. Trigonal Bipyramidal: For this arrangement, there are five electron domains. As the name implies, this geometry can be thought of as containing two pyramids that share one trigonal face; the trigonal bipyramid has six trigonal faces, with one domain at each corner; and the central atom is located in the center of the equatorial triangle. This geometry contains two different types of positions: three equatorial positions (corners of shared trigonal face) and two axial positions (one above and one below the shared trigonal face). There are also two different bond angles: 120° between the equatorial positions and 90° between an axial position and each of the equatorial positions. 5. Octahedral: Six electron domains arrange themselves at the corners of an octahedron, with the central atom in the center equidistant from the six corners. An octahedron can be thought of as two square pyramids that share their square bases (eight sides total). This creates 90° bond angles between all corners (all domain positions are equivalent). STEP 3. Determine the molecular geometry. The electron-domain geometry describes where the electron domains are on the middle atom; the molecular geometry (or shape) of the molecule describes where the atoms (or nuclei) are in the molecule. The distinction comes from the fact that an electron domain may be involved in a bond between the middle atom and an outer atom, or else may simply be a lone pair on the middle atom and therefore will occupy space but will not be involved in a bond. A single electron-domain geometry can be the basis for multiple molecular geometries, depending on the number of lone pairs on the middle atom (Fig. 1 and Table 1): 1. Trigonal planar electron-domain geometry: • Trigonal planar molecular geometry (no lone pairs on middle atom) • Bent molecular geometry (one lone pair on middle atom) 2. Tetrahedral electron-domain geometry: • Tetrahedral molecular geometry (no lone pairs on middle atom) • Trigonal prismatic molecular geometry (one lone pair) • Bent molecular geometry (two lone pairs) 3. Trigonal bipyramidal electron-domain geometry: • Trigonal bipyramidal molecular geometry (no lone pairs on middle atom)
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• Seesaw or distorted tetrahedral molecular geometry (one lone pair): the five positions are not
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equivalent; the lone pair goes into an equatorial position • T-shape molecular geometry (two lone pairs): the two lone pairs go into equatorial positions • Linear molecular geometry (three lone pairs): all three lone pairs go into equatorial positions 4. Octahedral electron-domain geometry: • Octahedral molecular geometry (no lone pairs on middle atom) • Square pyramidal molecular geometry (one lone pair): all positions are identical, so it does not matter where the single lone pair goes • Square planar molecular geometry (two lone pairs): the two lone pairs will locate opposite each other
Fig. 1 Electron-domain geometries and molecular geometries[[Comp: If possible, please fix art label see-saw to be seesaw]]
Table 1 The VSEPR structures for molecules and ions. Valence electron pairs
Bonding electron pairs
Nonbonding electron pairs
Approx. bond angles
Electron- domain geometry (geometry)
Molecular geometry (shape)
2
2
0
180°
Linear
Linear
3
3
0
120°
Trigonal planar
Trigonal planar
2
1
< 120°
Trigonal planar
Bent
4
4
0
109.5°
Tetrahedral
3
1
< 109.5°
Tetrahedral
Trigonal pyramidal
2
2
< 109.5°
Tetrahedral
Bent
5
5
0
90° and 120°
Trigonal bipyramidal
4
1
> 90° and < 120°
Trigonal bipyramidal
Seesaw (irregular tetrahedral)
3
2
< 90°
Trigonal bipyramidal
T-shaped
2
3
180°
Trigonal bipyramidal
Linear
6
6
0
90°
Octahedral
5
1
< 90°
Octahedral
Square pyramidal
4
2
90°
Octahedral
Square planar
Tetrahedral
Trigonal bipyramidal
Octahedral
Deviation from Ideal Bond Angles Lone pairs of electrons occupy more space (i.e., repel better) than bonding pairs of electrons (the bonded pairs are attracted to two nuclei, while a lone pair is only attracted to a single nucleus and thus spreads out). This leads to a distortion of the ideal bond angles in molecular geometries that contain one or more lone pairs on the middle atom. Similar distortions are generated when a double or triple bond is located at one of the electron domain positions; the generally accepted trend in terms of degree of distortion is shown in Fig. 2. Fig. 2 Distortions caused by different electron domains
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Figure 3 shows different molecular geometries with observed bond angles; the lone pair on the N in NH3 pushes the three H atoms away, decreasing the H–N–H angles from 109.5° to 107°; the C=O double bond repels the H atoms, decreasing the H–C–H angle from 120° to 116°; similar distortions are generated by a single lone pair on S in SF4 and on Br in BrF5. Fig. 3 Deviations from ideal bond angles in molecular geometries
Molecular Geometry and Polarity Polarity results from the molecular geometry (shape) of a molecule. To determine whether a molecule is polar, one must first consider the polarity of each individual bond in the structure. If the molecule/ ion is diatomic (like HCl), then the polarity of the molecule will be equivalent to the polarity of the single bond. With more than two atoms, we must look at the arrangement of the polar bonds to see if there is a net dipole for the structure. Two specific examples follow. The first is carbon dioxide (CO2). This molecule contains two polar C=O bonds; the two dipoles point exactly opposite of each other, however, so the dipoles counter each other. The net result is no net molecular dipole—that is, CO2 is not polar. The second example is sulfur dichloride (SCl2). This molecule again contains two polar S– Cl bonds; due to lone pairs of electrons on the S atom, the molecule adopts a bent shape, so the S–Cl bonds are not pointed directly opposite, so they do not cancel each other completely. The net result is an overall dipole; thus, SCl2 is a polar molecule. In general, a molecule will be nonpolar if (1) there are no lone pairs of electrons on the middle atom, and (2) all of the bonds have similar dipole moments and so cancel out. A molecule has a good chance of being polar if the middle atom contains one or more lone pairs, and there is an asymmetric arrangement of peripheral atoms. The polarity of the molecule ultimately depends, however, on the arrangement of the polar bonds around the middle atom (i.e., molecular geometry). Polarity affects the boiling point, melting point, and many other properties of a molecule. Polarity also plays an important role in determining if two substances can combine to form a homogenous mixture (called a solution). Homogeneous means uniform throughout. When a solution forms, the attractive interactions between the particles (ions, molecules, atoms) of each substance must be disrupted and replaced with interactions with the other substance in the mixture. Those attractive interactions are based largely on the polarity of each substance. A general rule is that “like dissolves like,” indicating that substances with similar polarities will mix favorably and form a solution, while two substances with very different polarities will usually not mix effectively. People who work in the drycleaning industry, for example, utilize solvents to remove stains from clothing. In many cases, the nature of the stain will determine the polarity of the dry-cleaning solvent that is used. Procedure/Data Collection
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Stations: Groups of students will rotate between four stations. Groups will spend 20 to 25 minutes at each station. Within a group, students are encouraged to work together, but each student must fill in (and turn in) a separate worksheet for this lab. Worksheets are turned in at the end of the lab today. _________________________________________________________________________________ Station 1. Lewis Structures and Molecular Models In your workshop, you were assigned one of the sets of eight substances (shown in Table 2) for which you drew Lewis structures. Table 2 Sets of substances. Set
Substances
A
CF3Cl
ClO2–
AsF6–
SeBr3+
AlF63–
H3O+
B
SnCl4
BrO2–
C
ClO4–
OI2
D
ClF2O2+
E
CFCl3
SCl2 ClF2+
SbF6–
PF3
BrF6+ SeF6
I3– XeF2 Br3–
SeI3+ SBr3+
O3
ICl4+
AsO2– NO2–
SF4 TeCl4
ICl2–
SO2
KrF2
SeO2
XeBr4 ICl4– KrF4
SeBr4 ClF4+
ClF4– XeI4
For each of the eight Lewis structures from your workshop (Table 2), you are to match the structure to one of the five molecular models on the lab bench, each of which represents one of the possible electron-domain geometries. In Table 3, for each, • Identify the correct model number. • Name the electron-domain geometry (geometry). • Draw the 3D structure for your Lewis structure (do not indicate potential distortions to the ideal geometry)—make it clear!! • Label all ideal bond angles in your structure. • Table 3 Electron-domain geometries. Formula
Model
Electron-domain geometry name
Drawing with angles labeled
_________________________________________________________________________________ Station 2. VSEPR and Balloon Models: effect of lone pairs on molecular shape For each of the eight Lewis structures from your workshop (Table 2), you are to match the structure to one of the balloon models that represent various VSEPR structures. The balloons represent the space occupied by bonding pairs or lone pairs. The central atom resides at the central knot where the balloons are joined. In Table 4, for each of your structures from your workshop, • Identify the correct VSEPR model number. • Name the molecular geometry (shape). • Draw the 3D structure for your Lewis structure, clearly indicating expected distortions. • Label all bond angles in your structure, indicating distortions using “” signs. • Table 4 Molecular geometriesComp: fix table b reak so table is on one page. so. Formula
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Model
Molecular geometry name
Drawing with angles labeled
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_________________________________________________________________________________ Station 3. Polarity of Solvents: mixing of solvents of different polarities 1. 2. 3. 4. 5. 6.
Add one full pipet of water to test tubes 1 and 2. Add one fill pipet of ethanol to test tubes 1 and 3. Add one full pipet of cyclohexane to test tubes 2 and 3. Stopper the open end of each test tube and agitate the liquids to thoroughly mix. Examine what happens to the liquids after agitation and record your observations in Table 5. Dispose of liquids properly and clean your glassware.
Table 5 Mixing liquids. Solvent mixture tested
Results observed
Water/ethanol Water/cyclohexane Ethanol/cyclohexane
_________________________________________________________________________________ Station 4. “Like Dissolves Like”: determination of polarity of unknown solid 1. Add one full pipet of water into test tubes 1 through 4. 2. Add one full pipet of ethanol into test tubes 5 through 8. 3. Add one full pipet of cyclohexane into test tubes 9 through 12; you should now have a 3 x 4 grid of solvent-filled tubes. 4. In tubes 1, 5, and 9 add enough NaCl to cover the bottom of the tube. Stopper those tubes and invert to agitate the mixture. Repeat several times until no further changes take place. Carefully examine the contents of each tube and record your observations in Table 6. 5. Repeat step 4 with sucrose (tubes 2, 6, and 10). 6. Repeat step 4 with naphthalene (tubes 3, 7, and 11). 7. Repeat step 4 with the unknown solid (tubes 4, 8, and 12). 8. Dispose of tube contents properly and clean your glassware. Table 6 Mixing solids and liquids. Solutes
Solvents
Water
Ethanol
Cyclohexane
NaCl Sucrose Naphthalene Unknown
__________________________________________________________________________
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Questions 1. For each solvent, draw its Lewis structure (the skeletons for ethanol and cyclohexane are given).
2. Based on the Lewis structures in question 1, determine the type for each (polar or nonpolar).Water ________________________Ethanol ________________________Cyclohexane ________________________ 3. Based on the Lewis structures you drew in your workshop (Table 2), predict whether each compound is polar or nonpolar. [Note: A polyatomic ion can also be considered polar or nonpolar, depending on the arrangement of polar bonds in the structure. An ion is charged, meaning it has a different number of electrons versus protons; something is polar, on the other hand, if its electron density is unevenly distributed over its structure. Soluble ionic substances will dissolve in polar solvents and not in nonpolar solvents (whether or not the individual ions are polar) because the individual ions are charged.] Formula
Polar or nonpolar?
4. What general trend appears in Table 6 with regard to which type of solute dissolves in which type of solvent? 5. Classify the known solids in Table 6 as ionic, polar covalent, or nonpolar covalent. 6. Attempt to classify the unknown solid as ionic, polar covalent, or nonpolar covalent. 7. Explain which solvent from this experiment you would use to remove road salt stains from a pair of jeans. 8. What general rule can be followed when choosing a type of solvent to dissolve a particular solid? Connection
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Based on your experience in this lab, draw a connection to something in your everyday life or the world around you (something not mentioned in the background section).
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