Master SQL Worksheet

MASTER SQLEXP 10. DETERMINATION OF MOLECULAR GEOMETRIES USING LEWIS DOT
STRUCTURE AND VSEPR MODELS
I. Introduction
1. What are molecules and polyatomic ions, and what are they made of?
2. What are the factors that dictate the physical and chemical properties of molecules and
polyatomic ions?
3. What are valence electrons and molecular shapes, and why are they important in understanding
the relationship between structure and properties of molecules and polyatomic ions.
4. What are Lewis Dot Structures? What are VSEPR models?
5. What were the objectives of this experiment?
6. How did you use the ball-and-stick model to determine the molecular shape and polarity of
molecules?
7. What did you learn from this experiment that reinforces your knowledge about VSEPR theory,
molecular shapes and polarity of molecules?
II. Results Analysis
A. Data and Calculations
Complete all the activity by hand and take photos of the molecular models, if necessary. Submit the
signed datasheet you completed in class.
B. Discussion
Part I Molecular Geometry Determination of Simple Binary Molecular Compounds
1. What is the molecular geometry of H2Se, CCl4?
2. Are they polar or non-polar? Briefly explain.
3. After you examined the ball-and-stick model, what influenced the angle and shape of these
molecules? Briefly explain using the VSEPR theory.
Part II Molecular Geometry Determination of Other Molecular Compounds
1. What is the molecular geometry of H2CO and HCN?
2. Are they polar or non-polar? Briefly explain.
3. After you examined the ball-and-stick model, what influenced the angle and shape of these
molecules? Briefly explain using the VSEPR theory.
Part III Molecular Geometry Determination of Polyatomic Ions
1. What is the molecular geometry of NH4+ and OH-?
2. Are they polar or non-polar? Briefly explain.
3. After you examined the ball-and-stick model, what influenced the angle and shape of these
molecules? Briefly explain using the VSEPR theory.
Part IV Molecular Geometry Determination of Organic Compounds
1. What is the molecular geometry of the individual central atom of CH3COCH3 and CH3COOH?
2. What is the overall polarity of each molecule? Briefly explain.
3. After you examined the ball-and-stick model, what influenced the angle and shape of these
molecules? Briefly explain using the VSEPR theory.
III. Conclusions
1. Why is ball-and-stick model useful in understanding the relationship between Lewis Dot
Structures and molecular shapes of molecules and polyatomic ions?
2. What do electronic geometry represent and how did you use it in conjunction with the VSEPR
theory in determining the angle and shape of the studied molecules and polyatomic ions?
3. How did the ball-and-stick models assist you in predicting the symmetry and polarity of the
molecules?
4. What are the factors that influence the polarity of the molecules studied?
5. Why is the study of molecular shapes important in understanding the properties of matter? Why
is it important to know the polarity of molecules? Give an example of its application.
EXP 10. Determination of Molecular Geometries Using Lewis
Dot Structure and VSEPR Models
OBJECTIVES
•
Familiarize with Lewis Dot Structures, Valence Shell Electron Pair Repulsion (VSEPR) Theory and
the three-dimensional structures of covalent molecules
•
Use Lewis Dot Structures and VSEPR Theory to construct molecular models to predict shapes
and polarity of small molecules and polyatomic ions
•
Able to use the molecular models in predicting and explaining some properties of substances
LECTURE TOPIC REFERENCES
Review the following before performing the experiment:
Tro, N. (2017). Chemistry: A Molecular Approach. 4th Edition. Boston: Pearson. Chapter 9, Sections 9.5,
9.6, 9.7, and Chapter 10, Sections 10.2 and 10.3, 10.4, and 10.5.
CONCEPTS
In previous experiments, although you learned to identify the composition and chemical
formula, and the difference between physical and chemical properties of substances, they are
not sufficient to predict or explain the properties of most molecular compounds. According to
the basic concept of atomic theory, the physical (e.g. solubility) and chemical (e.g. reactivity and
interaction of atoms in molecules) properties of substances are dictated by the distribution of
outer-shell electrons (valence electrons) in its atoms, and the spatial arrangement of these
atoms in its structure – the molecular shape.
Molecular shape is very important in understanding the relationship between structure and
properties of molecules and polyatomic ions. Molecules and polyatomic ions are threedimensional aggregates of atoms. In molecules and polyatomic ions, atoms are bonded by
sharing pairs of valence electrons. You learned from your lecture that electrons repel one
another and they try to stay away from each other. The best arrangement of a given number
of electron pairs is the one that minimizes the repulsion among them. This outer-shell
electron behavior is the basis of Valence shell Electron Pair Repulsion or VSEPR model.
Repulsions occur among the regions of electron density (that is, among pair of bonding
electrons and lone pair electrons), which control the angles between bonds from a central atom
and to its surrounding atoms. The molecular shapes result from minimizing the electron pair
repulsion as predicted by VSEPR model.
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Since atoms are too small to see with the human eye, it is necessary to use large models to
visualize their physical arrangements in molecules and polyatomic ions. These models allow
the study of the shapes and sizes, and spatial relationships of atoms, molecules and ions that
make up a substance. The simple way of predicting the molecular shapes requires the
combination of the Lewis Dot Structure and Ball-and-Stick” molecular models. The Lewis Dot
Structure, consisting of the chemical symbol and electron dots, provides the two-dimensional
representations of chemically active valence electrons, needed in predicting the threedimensional shapes of molecules and polyatomic ions. The ball-and-stick” molecular models
can be physically constructed using a kit as shown in Figure 1.
Figure 1 Molymod Inorganic/Organic Student Set
In this experiment, you will use the Lewis Dot Structure and “ball-and-stick” molecular models
to determine the shapes of molecules and polyatomic ions. You will then evaluate the
molecular shape or geometry to predict the polarity of the molecules. Molecular models will
help you understand the connection between Lewis Dot Structure and VSEPR models in
studying the interactions of atoms in molecules, and predicting the symmetry and polarity of
some molecules using the following steps:
(a) Determine the Lewis Dot Requirement (See Attachment 1 handout)
(b) Draw the Lewis Dot Structure (See Attachment 1 handout)
(c) Draw the Electronic Geometry and give the description (See Attachment 2
handout)
(d) Assemble the Model
(e) Draw the Molecular Geometry (See Attachment 2 handout) in terms of the
angular arrangement of the bonding pairs, which corresponds to the arrangement
of bound atoms. Give the description
(f) Assemble the Model
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(g) Identify the Symmetry (See Attachment 3 handout)
(h) Predict the Polarity (See Attachment 3 handout)
The polarity of molecules influences the physical and chemical properties of molecules. You
will determine the molecular polarity by observing the molecular geometry or shape of the
molecule. If the shape of the molecule is completely symmetrical, a molecule is non-polar, if no
(asymmetrical), then the molecule is polar. The polarity of molecules is influenced by the
presence of lone pairs (non-bonding pairs) of electrons on the central atoms. Electronegative
atoms (atoms that attract electrons more or take a greater share of the electron density)
connected to the central atoms may re-enforce or oppose the effect of the lone pair electrons.
It is therefore imperative that you pay close attention to these relevant information when
predicting the symmetry of molecules.
PROCEDURE
Materials: Molymod Inorganic/Organic Student Kit, Optional Laptop /w internet (http://molview.org)
Part I Molecular Geometry Determination of Simple Binary Molecular Compounds
1. Determine the dot requirement (VE = total valence electrons, BE = bonding electrons or bond
pairs, NE = non-bonding electrons or lone pairs, and draw the Lewis dot structures of the
following compounds: H2Se, CCl4
2. Find out the number of bond pairs and lone pairs to determine their electron geometries.
3. Using the molecular model kit, find out the molecular geometries and angles of the given
compounds.
4. Draw the same compounds using the Molview program to verify your answers (optional).
5. Identify whether it is symmetrical and polar. Complete Part I table in Attachment 4.
Part II Molecular Geometry Determination of Other Molecular Compounds
1. Determine the dot requirement and draw the Lewis dot structures of the following compounds:
H2CO, HCN
2. Repeat steps 2-5 of Part I. Complete Part II table in Attachment 4.
Part III Molecular Geometry Determination of Polyatomic Ions
1. Determine the dot requirement and draw the Lewis dot structures of the following compounds:
NH4+, OH2. Repeat steps 2-4 of Part I. Complete Part III table in Attachment 4.
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Part IV Molecular Geometry Determination of Organic Compounds
1. Determine the dot requirement and draw the Lewis dot structures of the following compounds:
CH3COCH3, CH3COOH
2. Repeat steps 2-5 of Part I. Organic compounds contain multiple central or internal atoms.
Identify the geometries for each of this central atom. Complete Part IV table in Attachment 4.
BIBLIOGRAPHY
Bishop, M. (2013). Introduction to Chemistry. Molecular Polarity. Retrieved November 8, 2018, from
http://preparatorychemistry.com/Bishop_molecular_polarity.htm
New Jersey City University (2013). CHEM 1105: General chemistry I laboratory and recitation: miniscale
experiments. Boston, MA: Pearson Learning Solutions.
New Jersey City University (2002). CHEM 1105: General chemistry I laboratory and recitation. Pacific
Grove, CA: Wadsworth Group.
Tro, N. (2017). Chemistry: A molecular approach, 4th Edition. Boston, MA: Pearson.
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ATTACHMENT 1: STEPS IN WRITING LEWIS DOT STRUCTURES
STEPS
Example 1: CO2
Example 2: NH3
Source: Tro (2017, Page 401)
There are compounds that cannot be represented by these steps for Lewis Dot structures of formulas.
The central atom may have less than 8 electrons (BF3) or more than 8 electrons (PCl5, SF6, XeF4, etc.). For
most of these compounds, the central atom and each outer atom are bonded by single bonds consisting
of one electron from the central atom and one electron from the outer atom. If there are any extra
electrons on the central atom, they are grouped as unshared pairs on the central atom.
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ATTACHMENT 2: MOLECULAR GEOMETRY (Tro, 2017)
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ATTACHMENT 3: DETERMING THE SYMMETRY AND POLARITY OF MOLECULES
Determine the symmetry of the molecule using the following steps.
•
Describe the polar bonds with arrows pointing toward the more electronegative
element. Use the length of the arrow to show the relative polarities of the different
bonds. (A greater difference in electronegativity suggests a more polar bond, which is
described with a longer arrow.)
•
Decide whether the arrangement of arrows is symmetrical or asymmetrical
•
If the arrangement is symmetrical and the arrows are of equal length, the molecule is
nonpolar.
•
If the arrows are of different lengths, and if they do not balance each other, the
molecule is polar.
•
If the arrangement is asymmetrical, the molecule is polar.
Source: Bishop (2013)
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ATTACHMENT 4: DATA SHEET
Part I: Molecular Geometry Determination of Simple Binary Molecular Compounds
Note: Electron Type and Number: VE= Total Valence Electrons, BE= Bonding Electrons, NE = Non-bonding Electrons/Lone Pair
Formula
Electron
Type &
Number
Electronic Geometry
(Sketch & Description )
Molecular Geometry
(Sketch)
Angle & Shape
Circle One
Symmetrical?
Yes
No
Sample:
NF3
H2Se
Lewis Dot Structure
Polar?
Yes
No
Symmetrical?
Yes
No
VE =
BE =
Polar?
Yes
No
NE =
CCl4
VE =
BE =
Symmetrical?
Yes
No
Polar?
Yes
No
NE =
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Part II: Molecular Geometry Determination of Other Molecular Compounds
Formula
Electron
Type &
Number
Lewis Dot Structure
Electronic Geometry
(Sketch & Description )
Molecular Geometry
(Sketch)
Angle & Shape
Circle One
Symmetrical?
Yes
No
Sample:
CHBr3
Polar?
Yes
No
Symmetrical?
Yes
No
H2CO
VE =
BE =
Polar?
Yes
No
NE =
Symmetrical?
Yes
No
HCN
VE =
BE =
Polar?
Yes
No
NE =
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Part III: Molecular Geometry Determination of Polyatomic Ions
Formula
Electron
Type &
Number
+ 1 e- from
Sample: charge
BrO2-
NH4+
Lewis Dot Structure
Electronic Geometry
(Sketch & Description )
Molecular Geometry
(Sketch)
Angle & Shape
Circle One
Symmetrical?
Yes
No
Polar?
Yes
No
Symmetrical?
Yes
No
VE =
BE =
Polar?
Yes
No
NE =
Symmetrical?
Yes
No
OHVE =
BE =
Polar?
Yes
No
NE =
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Part IV: Molecular Geometry Determination of Organic Compounds
Formula
Electron
Type &
Number
Electronic Geometry
(Sketch & Description )
Molecular Geometry
(Sketch)
Angle & Shape
Circle One
Symmetrical?
Yes
No
Sample:
C2H2
CH3COCH3
Lewis Dot Structure
Polar?
Yes
No
Symmetrical?
Yes
No
VE =
BE =
Polar?
Yes
No
NE =
Symmetrical?
Yes
No
CH3COOH
VE =
Polar?
Yes
No
BE =
NE =
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