Lab 2 Bond Polarity & Hydroca
ons
Purpose
· In this exercise we will investigate bond polarity and the structure of various types of hydroca
ons. You will investigate how to draw them, how to interpret them and how different structural representations of chemicals look different or similar.
· We will continue to explore the concept of isomers, adding additional types of isomers to our vocabulary including cis and trans isomers that only exist in alkenes due to the double bond structure.
Learning objectives are:
· Convert between Lewis Structures, Skeletal Structures and Structural Formulas
· Distinguish between Skeletal Structures, Structural Formulas and Lewis Structures.
· Predict what constitutional isomers are possible for a compound with a given chemical formula.
· Differentiate between a 2D representation of a molecule and the actually 3D a
angement of atoms in the molecule.
· Use electronegativity differences between atoms in a bond to determine bond polarity
· Use bond polarity and VSEPR (Valence Shell Electron Pair Repulsion) shape to determine molecular polarity.
Knowledge
This assignment will help you review & strengthen knowledge on the following content:
· Electronegativity and Polarity
· Valence electrons and VSEPR (Valence Shell Electron Pair Repulsion) theory
· Isomers
Skills
After this assignment you should be able to:
a. Differentiate between Lewis Structures, Structural Formulas, Condensed Structural Formulas & Skeletal Formulas
. Make predictions about what isomers of a chemical formula will form.
c. Identify electronegativity, bond polarity and molecular polarity.
d. Differentiate between isomers and the same molecule viewed from a different perspective or using a different structural representation.
Criteria for Passing Grade
In order to achieve a passing grade student must do the following as a minimum:
· Make detailed observations during the completion of the procedure above.
· Answer all of the questions using detailed, complete answers with clear reasoning and evidence used as necessary to support your answer.
· Complete all answers in their own words.
· It is recommended to compare your lab results with other lab teams. Just be sure all your answers are your own original thoughts and in your own words.
Activity 1 Introduction
In this first activity we will investigate bond polarity, note that this may be different from the molecular polarity. We will explore why in a later exercise.
How to Complete Activity 1 Table 1
In order to determine bond polarity you will need to:
· look up the electronegativity of each atom in the bond
· subtract the smaller electronegativity from the larger one (if they are not equal) to determine the electronegativity difference
· use the electronegativity difference to determine the type of bond and its polarity:
· <0.5 electronegativity difference is a nonpolar covalent bond(approximately equal sharing of electrons)
· > XXXXXXXXXXis a polar covalent bond (unequal sharing of electrons)
· >1.9 is an ionic bond (the more electronegative atom takes the valence electrons from the less electronegative atom resulting in two charged ions attracted to one another by their opposing charge)
· Fill in the table below with the data you collect
·
Activity 1
Table 1 - Fill in the blanks
Bonding Pai
Electronegativity Difference
Type of Bond
Bond Polarity
H and H
ex XXXXXXXXXX = 0
ex. covalent
ex. non-polar bond
B and F
C and H
N and H
H and O
O and O
C and O
Fe and O
Na and Cl
How to Complete Activity 1 Table 2
1. Step One: Identify the VSEPR (Valence Shell Electron Pair Repulsion) Shape of the Molecule. This will be important, because the shape can impact the overall polarity of the molecule causing the molecular polarity to sometimes differ from the individual bond polarities.
a. Build the molecular model using your physical model kit or an online modeling software like https:
molview.org or https:
chemagic.org/molecules
. Watch this video to review from Chem 121 how to determine VSEPR shapes to determine the shape of your molecule
c. Enter your answers in the table below
2. Step Two: Determine the Electronegativity of Each Bond
a. Repeat the process you used to determine electronegativity difference in Activity 1 Table 1.
. Enter your answer in the table below
3. Step Three: Determine the Polarity of Each Bond (note for duplicate bonds just list this one time, ie in ca
on dioxide there are two identical C-O bonds so you only need to list this once)
a. Repeat the process you used to determine bond polarity in Activity 1 Table 1.
4. Step Four: Molecular Polarity - this is where stuff gets interesting!!
a. Check out this video for a super simple way to tell if a molecule is polar.
. Check out this video if you’d like to understand more about WHY a molecule is polar or nonpolar.
c. Take what you learned to determine the molecular polarity of each molecule below and enter your answer in the table below.
Table 2 - fill in the blanks
Molecule
VSEPR Shape
Electronegativity of Each Bond
Bond Polarity
Molecular Polarity
CH4
ex. tetrahedral
ex. C-H bond
2.5-2.1= 0.4
ex. Non-pola
ex. Non-pola
NH3
H2O
BeF2
Note this is an Octet Rule Exception
BCl3
Note this is an Octet Rule Exception
CO2
H2CO
Observation Questions
1. Are the molecular and bond polarities always the same for a given molecule? Include an explanation of why or why not in your answer for it to be considered complete. A simple yes or no isn’t sufficient to answer this question.
2. What did you notice about the relationship between lone pairs on the central atom and molecular polarity?
3. What did you notice about the uniformity of the types of atoms around the central atom and its relationship to molecular polarity?
Activity 2
How to Complete Activity 2 Table 1
1. Step One: Identify the VSEPR (Valence Shell Electron Pair Repulsion) Shape around the central atoms in each molecule using the same method you used in Activity 1. Be sure to build the model to help with the later parts of this activity.
2. Identify if the C-C bonds can rotate in each molecule (without
eaking bonds) or not, use the models you built to help you with this if you’re not sure.
3. Rank the relative total strength of each of the following from least to greatest: single C-C bond, double C-C bond and triple C-C bonds. Hint: think about the total strength of each bond type.
4. Rank the relative chemical reactivity of each of the following from least to greatest: single C-C bond, double C-C bond and triple C-C bonds. Hint: think about what happens when more electrons come into close proximity to one another.
5. Record your answers in the table below.
Activity 2
Table 1 - fill in the blanks
Example
Structural Formula
VSEPR Shape
Do Bonds Rotate?
Relative Bond Strength
Relative Bond Reactivity
Ethane
C2H6
Ethene
C2H4
Ethyne
C2H2
Observation Questions
4. What are the similarities and differences between alkanes, alkenes and alkynes?
5. How do the differences between alkanes, alkenes and alkynes impact the VSEPR shapes present in each molecule type?
6. How do the differences between alkanes, alkenes and alkynes impact the way they react or behave chemically?
Activity 2 - Isomers
Table 2 - Fill in the blanks
Alkenes: Isomers of C4H8
Molecule 1
Molecule 2
Molecule 3
Structural Formula
Condensed Structural Formula
ex. CH2=CHCH2CH3
Skeletal Model
ex.
Observation Questions:
7. Can Molecule 1 be made to look exactly like Molecule 2 without
eaking any bonds? Why or why not?
8. Describe the differences in structures between the three alkenes.
9. Are Molecules 1, 2 and 3 chemically identical?
Activity 2 - Isomers
Table 3 Alkynes
Draw all the isomers of the following that contain a C-C triple bond: C4H6
Aromatic Compounds
Benzene
10. Are these benzene molecules isomers, or are they the same molecule? Explain.
11. In image 2 above, what is symbolized by the circle in the center? Why does this representation of benzene most accurately represent the actual structure of benzene? Explain your answer.
Skeletal Models of Di
omobenzene
12. Are these di
omobenzene molecules isomers, or are they the same molecule? Explain.