Download IntermolecularForcesSE (1)

Name:
Yassin Farih
Date: 11-1-21
Student Exploration: Polarity and Intermolecular Forces
Vocabulary: dipole, dipole-dipole force, dipole-induced dipole force, electronegativity,
intermolecular force, ionic bond, London dispersion force, molecule, nonpolar, nonpolar
covalent bond, partial charges, polar, polar covalent bond, valence electron
Prior Knowledge Questions (Do these BEFORE
using the Gizmo.)
1. A big bully is having a tug-of-war with a small child.
There is a ball attached to the middle of the rope.
Toward whom will the ball move?
bully
Big
2. Two equally strong kids are having a tug-of-war. What do you expect to happen to the ball in
this situation?
The ball will stay in between them,
Gizmo Warm-up
Just like in a tug-of-war, atoms that are bonded to one another pull
on the electrons they share. In the Polarity and Intermolecular
Forces Gizmo, you will explore how these opposing forces relate to
bond types and the forces between molecules.
To begin, drag the Na (sodium) and Cl (chlorine) atoms into the simulation area. Turn on Show
valence electrons. A valence electron is found in the outermost energy level of the atom.
1. Click Play (
). What do you notice?
The Na atom shrink the Cl expands
2. Which atom seems to be pulling more on the sodium’s one valence electron?
How do you know?
The chlorine atom wins the tug of war for the valence
electron
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3. What happens to the colors of the simulated atoms, and what does this indicate?
The
Na atom turns blue indicating a positive charge the Cl atom turns red indicating a negative
charge
Activity A:
Bond polarity
Get the Gizmo ready:
 On the BOND POLARITY tab, click Reset (
 Drag the atoms out of the simulation area.
).
Introduction: A neutral atom has the same number of protons as electrons. Atoms that gain
electrons become negatively charged, while those that lose electrons become positive. A polar
bond forms when shared electrons are pulled closer to one atom than another, causing the
bonded atoms to become partially charged. In a nonpolar bond, electrons are shared equally.
Question: What causes bonds to be polar or nonpolar?
1. Observe: Select the Show polar molecule inset checkbox. The animation shows the
probable location of electrons (orange dots) in a polar molecule.
A. What do you notice about the distribution of the electrons?
electrons on the left side than on the right
There is more
B. How does this electron distribution affect the charges of the bonded atoms? Due to
the fact they are attracted to the atom with the higher electronegativity the atom
holds onto the elctrons more causing it to be negatively charged
2. Observe: Turn on the Show nonpolar molecule inset.
A. How are the electrons in this molecule distributed?
distributed between the two atoms
The electrons are evenly
B. Why do the bonded atoms remain neutral?
Since the electronegativity is the
same , they cancel eachother out, neither is stronger, the result is a neutral charge.
3. Experiment: Turn off Show polar molecule inset and Show nonpolar molecule inset.
Check that Show valence electrons is turned on. Drag the Na and Cl atoms into the
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simulation area. Click Play. Note the colors. Red indicates a negative charge, while blue
indicates a positive charge.
A. Does a polar or nonpolar bond form? ionic bond
B. Try several other metal/nonmetal combinations. What do you notice about the bonds
that form?
the valence electrons are transferred to the non-metal to become
a full stable orbit and it becomes negatively charged
Ionic bonds are polar bonds that form between metal and nonmetal atoms. In this
bond, valence electrons are transferred from a metal to a nonmetal. Drag each of
these metal/nonmetal combinations into the Ionic bond bin on the upper right.
(Activity A continued on next page)
Activity A (continued from previous page)
4. Experiment: Now try forming bonds between different combinations of nonmetals.
A. What do you notice?
There is sharing of electrons, and the number of electron
that one element needs to have a full valence shell is given from the other element
B. Are all of these bonds polar? Explain.
No, some of them are non-polar,
some of the charges cancel out whereas some do not
C. Which nonmetal appears to attract electrons the most? chlorine
5. Experiment: Turn on Show electronegativity. Electronegativity (EN) describes how
strongly an atom attracts a pair of shared electrons. The higher the EN value, the greater
the tendency of an atom to hold onto electrons in a bond and become negatively charged.
Electronegativity difference is found by subtracting the EN value of one atom from another.
Choose two nonmetals with a small (or no) EN difference between them. Click Play.
What happens to the shared electrons in this bond?
The amount of attraction between
each atom is the same causing it to cancel out, this makes it neutral
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When the shared electrons experience the same attraction from each atom, the result is a
nonpolar covalent bond. Drag the bonded atoms to the Nonpolar covalent bond bin.
6. Experiment: Choose two nonmetals with a large electronegativity difference. Click Play.
What happens to the valence electrons in this bond?
pulled closer to the more electronegative atom
In this bond the shared electrons are
A bond in which the electrons are shared unequally is a polar covalent bond. Notice the
small δ+ and δ– symbols, which indicate partial charges. Drag the resulting combination to
the Polar covalent bond bin.
7. Classify: Use the Gizmo to categorize the remaining element combinations as forming either
ionic, polar covalent, or nonpolar covalent bonds. List your results below.
Ionic
(EN difference > 1.67)
Polar covalent
(EN diff. < 1.67 and > 0.40)
Nonpolar covalent
(EN difference < 0.40)
CaO
MgCl2
CO2
NH3N
CH4
NCl3
N2
Activity B:
Molecular polarity
Get the Gizmo ready:
 Select the MOLECULAR POLARITY tab.
Introduction: Ionic bonds, like those found in NaCl, form crystalline solids. Covalent bonds, on
the other hand, usually form discrete molecules. A polar molecule, while neutral overall, has a
slight positive charge on one end and a slight negative charge on the other.
Question: What determines the polarity of molecules?
1. Observe: Notice that the molecules containing polar covalent bonds are grouped together at
lower left, and the molecules containing nonpolar covalent bonds are at lower right. Drag the
H2O molecule into the simulation area.
A. Are the individual bonds in this molecule polar or nonpolar?
B. Click Turn on electric field. What do you notice?
up.
Polar
The H2O molecules are lined
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C. The electric field consists of a positively charged plate on one side and a negatively
charged plate on the opposite side. Which side of the H2O molecule is attracted to
the positive plate, and why do you think this attraction occurs?
The O atom is attracted to the positive plate because the O atoms has a partial
negative charge. And the two H atoms are attracted to the negative plate because
they have a partial positive charge.
D. Click Reverse field. Why does the H2O molecule rotate 180°? Since the molecule is
not symmetrical the oxygen atom on the outside is attracted to the opposite side of
the plate causing the rotation
A polar molecule, such as H2O, changes its orientation when placed in an electric
field. The positive end of the molecule is attracted to the negative plate, while the
negative end is attracted to the positive plate. Drag this molecule into the Polar bin.
2. Observe: Drag the CH4 molecule into the simulation area.
A. Do you think this molecule will rotate in the electric field? Why or why not?
The CH4 molecule contains non-polar bonds so it will not rotate
B. Turn on the electric field. Is this molecule polar or nonpolar?
Nonpolar
Drag the CH4 molecule into the appropriate bin.
(Activity B continued on next page)
Activity B (continued from previous page)
3. Classify: With the Gizmo, test and categorize the remaining molecules. Report your findings.
Polar molecules
Nonpolar molecules
H2O, NCl3, CO, OCl, NH3
CH4, O2, H2, Cl2, N2, CCl4, CO2
Did the polarity of any of these molecules surprise you? Explain. Yes, it is surprising that
NCl3 is polar because it contains nonpolar bonds
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4. Explain: Drag the CCl4 molecule into the simulation area.
A. Does this molecule contain polar or nonpolar bonds?
Polar
B. Turn on the electric field. Why do you think the molecule does not rotate in this field?
The Molecule does not rotate because it is nonpolar. And the bonds are
symmetrically arranged. So the effects of the partialcharges from the polar bonds
cancel out
If a molecule is symmetrical, the effect of the partial charges on either side cancel
out. In this case, even though it contains polar bonds, the molecule as a whole may
be nonpolar.
C. What other nonpolar molecule contains polar bonds?
Carbon Dioxide
5. Challenge: Find an example of a polar molecule that contains nonpolar bonds. Drag this
molecule into the simulation area.
A. Which molecule did you select?
NCl3
B. Turn on Show valence electrons. What do you notice at the very top of the nitrogen
atom, and how does this feature explain why the molecule is polar?
There is a pair of unshared valence electrons at the top of the nitrogen atoms.
These electrons give this part of the nitrogen atom a partial negative charge.
For the NH3 molecule, the lone pair of valence electrons shown at the top spread
out, bending the chlorine atoms downward due to electron repulsion. As a result, the
molecule is slightly polar.
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Activity C:
Intermolecular
forces
Get the Gizmo ready:
 Select the INTERMOLECULAR FORCES tab.
Introduction: The polarity of molecules give rise to the forces that act between them. These
intermolecular forces, or IMFs, affect many physical properties including boiling point,
solubility, viscosity, and surface tension.
Question: How does polarity affect the forces between molecules?
1. Observe: Select the Show polar molecules inset checkbox. What do you notice?
form between the positive end of one molecule and the negative end of the other
A bond
Each polar molecule is known as a dipole. The attraction between the positive end of one
dipole and the negative end of another is called a dipole-dipole force.
2. Observe: Turn on the Show nonpolar molecules inset. What do you notice?
The
nonpolar molecules become momentarilypolar and a bond forms between the temporarily
positive end of one molecule and the temporarily negative end of the other.
Even when the molecules are nonpolar, random variations in the distribution of electrons
can cause parts of these molecules to become slightly charged. This imbalance leads to
very tiny, short-lived attractions between molecules called London dispersion forces.
3. Experiment: Turn off Show polar molecule inset and
Show nonpolar molecule inset. Drag two H2O
molecules into the simulation area, and click Play.
Click Pause ( ) when you see a bond form between
molecules. Sketch the molecules, partial charges, and
the bond between them in the space to the right.
Why is a hydrogen atom in one H2O molecule attracted
to the oxygen atom in an adjacent H2O molecule?
The hydrogen atom has a partial positive
charge, so it is attracted to the partially negatively
charged oxygen atom on the adjacent H2O molecules.
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Drag the H2O-H2O molecule combination into the correct bin on the right. Which type of
intermolecular force causes attraction between H2O molecules?
Hydrogen Bonding
(Activity C continued on next page)
Activity C (continued from previous page)
4. Experiment: Drag two O2 molecules into the simulation area, but do not click Play yet.
A. What force do you expect to see between O2 molecules?
No attractive force
B. Click Play and observe. What do you notice? Even though O2 Molecules are
nonpolar as they interact, they can become temporarily polar and form bonds
C. Which force causes attraction between O2 molecules?
The dispersion
5. Experiment: Drag an O2 molecule and an H2O molecule into the simulation area. Click Play,
and then click Pause when you see a bond form.
A. What happens to the end of the O2 molecule that is closest to the positive end of the
H2O molecule?
This end of the O2 molecules gains a partial negative charge
B. As the bond forms, does the polarity of the O2 molecule change?
yes
C. Classify this combination of molecules. (It might take a few tries to get it right.)
Which type of intermolecular force is acting here?
force.
A dipole-induced dipole
When a polar molecule approaches a nonpolar molecule, the electron cloud of the nonpolar
molecule may become distorted, causing the nonpolar molecule to become temporarily
polar. For example, if the negative end of the polar molecule approaches the nonpolar
molecule, the electron cloud of the nonpolar molecule will be repelled, causing a slight
positive charge at that end of the nonpolar molecule. The resulting attractive force is called a
dipole-induced dipole force.
6. Classify: Drag out different combinations of molecules in the Gizmo and categorize them.
Give at least three examples of molecule combinations for each intermolecular force.
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Dipole-dipole forces
Dipole-induced dipole forces
London dispersion forces
H2O-H2O, CO-NH3
Any combination of polar
molecules
H2O-O2, CO-Cl2
Any combination of polar
and nonpolar molecules
H2-H2, CO2-H2
Any combination of
nonpolar molecules.
7. Summarize: Fill in the blanks to summarize the patterns you see.
Dipole-dipole
forces arise between polar molecules.
Dispersion
forces arise between nonpolar molecules.
Dipole-induced dipole
forces arise between polar and nonpolar
molecules.
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