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3.3 Polar Bonds and Polar Molecules
In Chapter 2 we focused on the chemical bonds (forces of attraction) between ions or
atoms in chemical compounds ( Table 1 ). In this section, we will review polar bonds
and explore how they influence the properties of a substance.
Bond polarity
In Section 2.3 you learned about bond polarity: whether a covalent bond is polar or
non-polar. Atoms with significantly different electronegativities form polar covalent
bonds. In this case, there is unequal sharing of the bonding electron pair. Electrons
are in continual motion. In a polar covalent bond the shared electrons tend to be clus-
tered closer to the strongly electronegative atom than to the other atom. The “electron
density” is greater around the more electronegative atom. This end of the molecule
therefore has a negatively charged end or “pole.” The other end of the molecule, where
the electron density is less, is the positive pole ( Figure 1 ).
Table 1 Summary of Chemical Bonds
Ionic bonds Covalent bonds form between ions
form between atoms are created by transfer of valence electrons between atoms to form ions
are created by sharing of pairs of valence electrons between atoms result in large crystal lattices
result in individual molecules
Reviewing Previous Material When you see a reference to earlier material, it is a good idea to go back and reread it. In particular, make sure that you understand important definitions. Relevant vocabulary here includes polar covalent bond; non-polar covalent bond; and electronegativity difference, DEN.
learning Tip
Look again at Figure 5 in Section 2.3 to see the spectrum of bonds, from non-polar
covalent through ionic. You can see that the bond in a molecule of hydrogen chloride,
HCl (DEN 5 1.0), is classified as polar covalent.
The extent to which a bond is polar depends directly on the electronegativity dif-
ference. For example, consider the covalent bonds that form between the following:
hydrogen and hydrogen, nitrogen and hydrogen, and oxygen and hydrogen ( Figure 2 ).
See how these bonds would fit on the bonding continuum in Section 2.3.
δ^ δ
Figure 1 When two atoms are joined by a polar covalent bond, the negative pole, where the electron density is greater, is indicated by “d^2 .” The positive pole, where the electron density is less, is indicated by “d^1 .” The grey shading represents the electron density.
A phenomenon of polar Liquids
Scientists have discovered that different liquids behave in rather interesting ways when
poured, in a thin stream, near a charged object. The following Mini Investigation lets
you observe this phenomenon.
δ�
δ�
�EN � 0.
(a) H–H, a non-polar covalent bond
�EN � 0.
(b) N–H, a polar covalent bond
�EN � 1.
(c) O–H, a very polar covalent bond
δ�
��EN 1.
(c) O–H, a very polar
EN � 1.41.
δ�
δ�
Figure 2 A covalent bond’s polar nature depends on the electronegativity difference between the bonding atoms.
102102 Chapter 3 • Molecular Compounds and Intermolecular Forces (^) NEL
HANDBOOKSKILLS A6.
Evidence for polar Molecules
Mini investigation
Skills: Performing, Observing, Analyzing, Communicating (^) HANDBOOKSKILLS A1.2, A2.2, A
In this investigation you will observe the effects of charged objects on thin streams of various liquids (Figure 3). You will look for differences in behaviour between polar and non-polar liquids.
liquid
burette
charged strip
retort stand
beaker
tray
Figure 3 Testing a liquid with a charged strip
Equipment and Materials: chemical safety goggles; lab apron; nitrile gloves; tray; 50 mL glass burette clamped to a stand; small funnel; 400 mL beaker; acetate strip; vinyl strip or balloon; small covered bottle containing water, ethanol, or propanone (acetone); paper towel Some of the liquids used in this investigation are flammable. They should be used only in a well-ventilated area. There should be no open flames or sparks in the laboratory.
- Copy Table 2 into your notebook, adding one more column for your observations.
- Put on your chemical safety goggles, lab apron, and gloves.
- Your group will be assigned one of the liquids. Using a funnel, fill the burette with your assigned liquid and set it up securely on the tray.
- Rub the acetate strip back and forth several times in a piece of paper towel. Paper attracts electrons away from the acetate strip. The acetate will therefore acquire a positive charge.
- Allow a stream of the liquid to pour into the beaker below.
- As the liquid is running, hold the charged acetate strip close to the stream of water. Observe the stream closely. Record your observations. 7. Repeat Steps 3 to 6 with a charged vinyl strip or inflated balloon. Vinyl has a greater tendency to gain electrons when rubbed. It therefore acquires a negative charge. 8. Observe the other groups testing the other liquids and examine Figure 4.
tetrachloroethene charged strip
beaker
tray
Figure 4 Tetrachloroethene flowing past an acetate strip
- Empty your burette into a beaker and, using the funnel, return the liquid to its small bottle. Hand the bottle to your teacher.
Table 2 The Effect of Electric Charge on Molecular Liquids
Liquid Chemical formula Polar or non-polar
water H 2 O(l) polar ethanol C 2 H 5 OH(l) polar
propanone (acetone) CH 3 COCH 3 (l) polar
tetrachloroethene C 2 Cl 4 (l) non-polar
A. Is there any connection between the behaviour of the liquids and whether they are polar or non-polar? If so, describe the relationship. T/i B. What difference occurred when you used the acetate strip compared to the vinyl strip? Did you expect this result? T/i C. Try to propose an explanation for your observations. T/i D. Predict what you would observe if you held a vinyl strip beside a stream of hexane (a non-polar liquid). T/i
NEL 3.3 Polar Bonds and Polar Molecules3.3 Polar Bonds and Polar Molecules^103103
δ�^ δ
�
H Cl
O
H H
δ�^ δ�
δ�
Figure 6 The Lewis structures and molecular shapes of (a) hydrogen chloride, HCl, and (b) water, H 2 O. Both are polar molecules.
(b)
(a)
has a positively charged end and a negatively charged end. The whole molecule is
therefore polar ( Figure 6(a) ).
What about water, H 2 O? Are water molecules polar? Remember that a water molecule
is made up of an oxygen atom and 2 hydrogen atoms. There are two O]H bonds. The dif-
ference between the electronegativities of oxygen and hydrogen atoms is 1.4, so each bond
is a polar covalent bond. The oxygen atom attracts the bonding electrons most strongly.
There is therefore a negative charge around the oxygen atom and a positive charge around
each hydrogen atom ( Figure 6(b) ). Since the water molecule is lopsided, or bent, we can
consider that it has two ends: a negatively charged oxygen end and a positively charged
hydrogen end. Thus, the water molecule is polar.
non-polar Molecules
Oxygen, O 2 , and carbon dioxide, CO 2 , are two examples of non-polar molecules
( Figure 7 ). The oxygen molecule, O 2 , is diatomic: it contains only 2 atoms. Since both
of the covalent bonds in an oxygen molecule are non-polar, the whole molecule is
non-polar. To be polar, a molecule must have at least one polar covalent bond. Diatomic
molecules made up of identical atoms are always non-polar.
The carbon dioxide molecule, however, is polyatomic: it contains more than two
atoms. To be specific, a molecule of carbon dioxide is composed of 3 atoms joined by
four covalent bonds: two double bonds, each between an oxygen atom and a carbon
atom. The electronegativity difference between oxygen and carbon is 1.0, with oxygen
having the higher electronegativity. Consequently, each C 5 O bond is polar. But look
closely at the shape of the CO 2 molecule in Figure 7(b). Notice that the two polar
covalent C 5 O bonds are arranged symmetrically about the central carbon atom. In
a symmetrical, linear molecule like carbon dioxide, the positive and negative charges
cancel each other out. The molecule is non-polar overall. Note that a molecule can
include polar covalent bonds but still be non-polar overall if it is symmetrical.
O O
(a)
O C O
δ�^ δ�^ δ�
Figure 7 The Lewis structure and molecular shape of (a) oxygen, O 2 , and (b) carbon dioxide, CO 2. Both are non- polar molecules.
(b)
You have just painted your room with a fresh coat of oil-based paint. You carefully wash the paintbrushes in turpentine. Now what are you going to do with the leftover paint and used turpentine? Should you pour them down the sink? Seal them into a container and put them in the garbage? Take them to a hazardous-waste centre? We are often faced with tough waste-disposal issues. As consumers, how are we to know what to do with hazardous household waste? How can we maintain safe and clean environmental conditions? Are there any guidelines or regulations regarding hazardous-waste disposal? Who develops and enforces them?
- Choose a consumer product, such as paint, turpentine, motor oil, or batteries, that should not be placed in the regular garbage. 2. Research the correct way to dispose of this product, and the dangers to the environment if it is improperly disposed of. 3. Research any guidelines or regulations regarding disposal of this product in your municipality. 4. Research to find out whether most people comply with the guidelines or regulations. A. Summarize your research. Do most people follow the rules for proper disposal? Why or why not, in your opinion? T/i^ A B. What else could be done to protect the environment from hazardous household waste? Justify your position. A^ C
Waste Wisdom
Research This
Skills: Researching, Analyzing, Communicating, Defining the Issue, Defending a Decision
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HANDBOOK^ SKILLS A5.
How to Determine if a Molecule is polar or non-polar
It is very useful to know whether a molecule is polar or non-polar. This knowledge
helps us to predict a compound’s physical properties, such as its solubility in other
substances. We can predict whether a molecule is polar by considering whether it
contains polar covalent bonds and the orientation of these bonds around the central
atom in the molecule.
NEL^ 3.3 Polar Bonds and Polar Molecules^105
Tutorial 1 Determining the Polarity of Molecules There is a series of steps you can follow to determine whether a molecule is polar or non-polar. The flow chart in Figure 8 presents the steps visually.
- Determine how many atoms of which elements make up a molecule.
- Draw the Lewis structure for the molecule.
- Determine how many covalent bonds there are in the molecule.
- Determine the electronegativity difference for each covalent bond in the molecule. Indicate whether each bond is polar (0.0. ∆EN. 1.7) or non-polar (∆EN 5 0.0).
- If there are polar covalent bonds in the molecule, indicate the partial charges. Write “d^1 ” by the atom with the lower electronegativity and “d^2 ” by the atom with the higher electronegativity.
- Interpret your diagram. If the molecule only has one polar covalent bond, the molecule is polar. If the molecule has more than one polar covalent bond, the molecule may or may not be polar. Examine the shape of the molecule. Is it symmetrical or asymmetrical? Table 3 shows which types of compounds tend to be polar and non-polar molecules. Note that there are some exceptions to these general rules.
Does the molecule have one bond or more than one?
All covalent molecules
Diatomic molecule
This molecule is polar. Examples: HCI, HI
This molecule is polar. Examples: NH 3 , H 2 O
This molecule is non-polar. Examples: CO 2 , SiCl 4
This molecule is non-polar. Examples: Cl 2 , S 8
One bond More than one bond
Is the shape of the molecule symmetrical? Yes, symmetrical
No, asymmetrical
Polyatomic molecule
Molecule has polar covalent bonds.
What is the ∆EN value for each of the bonds within a molecule?
Molecule has only non-polar covalent bonds.
Figure 8 This flow chart can help you to determine whether a molecule is polar or non-polar. Table 3 Some Rules for Determining Polarity of Polyatomic Molecules
General chemical formula Polarity Examples
diatomic; 2 different atoms polar HCl, CO nitrogen and 3 other atoms of the same element polar NH 3 , NF 3 oxygen and 2 other atoms of the same element polar H 2 O, OCl 2 carbon and other atoms of two or more elements polar CHCl 3 , C 2 H 5 OH diatomic; 2 identical atoms non-polar N 2 , O carbon and 2 or more atoms of the same element non-polar CH 4 , CO 2
There are many powerful computer programs that help chemists draw structures for molecular compounds. To explore some of these programs,
web Link
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3.3 Summary
- Whether a bond is polar covalent or non-polar covalent depends on the
electronegativity difference between the two bonded atoms.
- Molecular polarity depends on the polarity of the bonds within the molecule
and on the shape of the molecule.
- A diatomic molecule will be polar if the covalent bond is polar and non-polar
if the bond is non-polar.
- A polyatomic molecule with only non-polar covalent bonds will be non-polar.
- A polyatomic molecule with several polar covalent bonds will be polar if the
molecule is asymmetrical and non-polar if the molecule is symmetrical.
- Molecular diagrams and models help us to visualize whether a molecule is
symmetrical or asymmetrical.
3.3 Questions
- Arrange the following sets of bonds in order of decreasing polarity (from most polar to least polar). Refer to the electronegativity values in the periodic table at the back of this textbook. k/U (a) K–Br, H–Br, C–H, O–F (b) C–O, C–F, O–H, H–H
- For the most polar bonds in Question 1, draw the bond and indicate which atom would have the partial positive charge, d^1 , and which atom would have the partial negative charge, d^2. k/U^ C
- Draw a diagram showing the orientation of water molecules when a thin stream of water is deflected by a positively charged acetate rod. T/i^ C
- Determine whether each of the following molecules is symmetrical or asymmetrical. Explain your reasoning. k/U^ T/i (a) O (^2) (b) H 2 O (c) CO (^2) (d) CH 4 (e) NH 3
- Determine whether each of the following molecules is polar or non-polar: k/U^ T/i (a) CCl 4 (b) H 2 O (c) HF (d) CF (^4) (e) CH 3 Cl
- Table 4 shows the results of an investigation similar to the Mini Investigation: Evidence for Polar Molecules. Predict whether liquids (a) to (d) are polar or non-polar. Give your reasons for each prediction. T/i Table 4 Observations of the Effect of Electric Charge
Liquid
Chemical formula
With acetate strip
With vinyl strip
(a) carbon tetrachloride
CCl 4 (l) unaffected unaffected
(b) methanol CH 3 OH(l) attracted attracted (c) hexane C 6 H 14 (l) unaffected unaffected (d) nitrogen trichloride
NCl 3 (l) attracted attracted
- Research companies that are using carbon dioxide for dry cleaning. Summarize the pros and cons of replacing the conventional cleaning fluid with carbon dioxide. T/i^ C
- Two types of computer-generated images (CGI) are commonly used to represent the three-dimensional shapes of molecules: the ball and stick model and the space-filling model. Research these two models, find examples, and list the pros and cons of each model. T/i^ C
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