GRADE 11/12 Physical Science: Polarity of Molecules PDF

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Unit 5Polarity of MoleculesTable of ContentsTable of Contents 1 Introduction 3 Essential Questions 3 Review 4Lesson 5: Electronegativity 5 Objective 5 Warm-Up 5 Learn about It 5 Web Links 6 Check Your Understanding 6 Challenge Yourself 6 Lesson 5: Determining Polarity of Molecules 7 Objective 7 Warm...

Description

Unit 5

Polarity of Molecules Table of Contents  Table of Contents

1

Introduction

3

Essential Questions

3

Review

4

Lesson 5.1: Electronegativity Objective

5 5

Warm-Up Learn about It

5 5

Web Links Check Your Understanding

6 6

Challenge Yourself

6

Lesson 5.2: Determining Polarity of Molecules

7

Objective Warm-Up

7 7

Learn about It Worked Examples

7 7

Key Points Web Links

7 7

Check Your Understanding Challenge Yourself

8 8

Lesson 5.3: Properties of Molecules Based on Polarity

9

Objective Warm-Up

9 9

Learn about It Worked Examples

9 9

Key Points Web Links

10 10

Check Your Understanding Challenge Yourself

10 10

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  Lesson 5.4: Practice Examples Objective

11 11

Warm-Up Learn about It

11 11

Key Points Web Links

11 11

Check Your Understanding Challenge Yourself

12 12

Laboratory Activity

13

Performance Task

14

Self Check

16

Key Words

16

Wrap Up

17

Photo Credits

18

References

18

Answer Key

20





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  G  RADE 11/12 | PHYSICAL SCIENCE

Unit 5



Polarity of Molecules 

  There are millions of different molecules and there are many ways to sort them. As you have learned from the previous unit, the properties of molecules highly depend on their structure and the arrangement of their atoms in space. Because of this, their classification also varies due to their structural and geometrical differences.  One way to classify them is through their polarity, where molecules can be polar or nonpolar. Polar molecules are also called dipoles, or literally translating to molecules having two distinct poles which are positively and negatively charged. This characteristic in terms of polarity greatly influences the properties of molecules, which includes their boiling point, melting point, and solubility.  One example of polar molecules is water. You might have heard before that water is the universal solvent. It is very much capable of solubilizing a wide variety of substances. Do you know what makes water a very good solvent? 

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 



Essential Questions

 At the end of this unit, you should be able to answer the following questions.  ● What is electronegativity? ● What makes a molecule polar or nonpolar? ● What properties are affected by the polarity of the molecule? ● How do these properties vary based on polarity? ● What makes water a very good solvent? 

 Review  Review the concepts of ionization energy, electron affinity, and covalent bond. ● Ionization energy is the amount of energy needed to remove an electron from an atom in a gaseous ground state. ● Electron affinity is the change in energy level caused by adding an electron to an atom or ion. A greater decrease in energy level makes the atom or ion more stable. ● Covalent compounds are compounds formed from atoms sharing one or more pairs of electrons. The bond that holds atoms in a covalent compound is called a covalent bond. ● Covalent molecules are represented by Lewis structures. These are derived from the constituting elements’ Lewis dot symbols and show whether bonds are transferred or shared between these elements. ● The three-dimensional shape of a molecule can be predicted using its Lewis structure together with the valence-shell electron pair repulsion (VSEPR) or electron domain (ED) model. ● The ED model assumes that the shape of a molecule can be predicted by arranging the electrons in a geometry that keeps them separated as far as possible. ● Vectors are quantities with magnitude and direction. These characteristics of vectors are considered when adding them.  

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 

Lesson 5.1: Electronegativity 

Objectives  In this lesson, you should be able to: ● define electronegativity; ● familiarize the electronegativity trend; and ● compare the electronegativity of different elements.  Not all elements are created equal. They differ in many aspects causing variations among groups. When atoms form covalent bonds, they share electrons. However, sharing may or may not be equal among these atoms. What causes the electrons to be more attracted to certain atoms? 



Warm-Up

 Electronegativity Tug of War The differences in the quantity and arrangement of subatomic particles in elements affect many of their properties, including electronegativity. In this activity, you will observe how bonding electrons are attracted towards a certain atom in a bond.  Materials: ● masking tape ● rope  Procedure: 1. Group yourselves according to the following table. Note that your group must consist of members having the same strength as much as possible.  Group Name

No. of students

Group Name

No. of students

Na

1

N

3

B

2

F

4

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  2. After groups are assigned, the matches of tug of war will be done.  Guide Questions: 1. Which groups won against each which? Why do you think this is so? 2. What do the number of students represent? 

Learn about It   Electronegativity Electronegativity, represented by the lowercase Greek letter chi ( ), is   the measure of the ability of an atom to attract bonding pairs of electrons. The concept of electronegativity originated in the fact that several elements have different abilities to attract electrons in a bond. This means that some elements are better electron attractors than other elements. This concept was first introduced by Linus Pauling in the early 19th century.  Linus Pauling was an American chemist who developed a numerical scale of electronegativity of selected representative elements in the periodic table. The figure below shows a part of the periodic table where the general trend of electronegativity values increases across the periods (i.e. from left to right) and decreases within the groups (i.e. from top to bottom).  Even though electronegativity is associated with the ability to attract electrons, this property is different from electron affinity, or the energy released when an electron is incorporated on an atom. The latter measures the tendency of an individual atom to gain (or attract) electrons while the earlier measures the same tendency of an atom in a covalent bond. 

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 

 Fig. 1. Pauling’s electronegativity values based on thermochemical data.  Noble gases do not usually have electronegativity values. Since their shells are already full, they do not participate in bond formation. However, heavier noble gases such as Kr, Xe and Rn have been discovered to form covalent compounds at low temperatures and high pressures. Neil Bartlett, a British chemist, was able to observe the formation of the very first noble gas compound, xenon tetrafluoride (XeF4).  The higher the electronegativity value of a specific element, the greater is the attraction of electrons to the the atoms of that element. For example, in the Fig. 1, fluorine has an electronegativity value of 4.0, which signifies that it is the most electronegative among the representative elements. Lithium, calcium, and strontium, on the other hand, have electronegativity values of 1.0 which denote that they are the least electronegative among the representative elements.  Metals are considered to be poor electron attractors, which means that their electronegativity values are low. This explains why they lose their electrons most of the time. On the other hand, nonmetals are good electron attractors, which means that they have high electronegativity values. This explains why they gain electrons from external sources most of the time.  Electronegativity values are determined from thermochemical data, and the values vary from one element to another depending on the element’s atomic size, number 

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  of inner shell electrons, and nuclear charge. These factors determine the influence of the nucleus on the electron of interest. The capacity of an atom or an ion to attract electrons is also influenced by its electron affinity as addition of an electron can significantly affect its stability.  It should be noted that the pattern is just a general trend and not absolute, especially for transition metals. For example, the electronegativity of copper (  = 1.9) is higher than the element on its right, zinc (  = 1.6). A possible explanation for this particular example is that copper will be more stable when it gains an electron because it fills up all of its 3  d orbitals while zinc will be more unstable upon gaining an electron because it will have an unpaired electron in its newly occupied 4p orbital.  Determining the Electronegativity Difference The electronegativity difference, , is a very important mathematical quantity in chemistry. It describes the p  olarity of a covalent bond. Since a covalent bond is formed by sharing electrons between two atoms, the electronegativity of each atom significantly affect how electrons will position themselves in the space between the nuclei of the atoms. How electrons arrange themselves in a bond describes the polarity of a molecule, which will be discussed more in depth in the next lesson.  Polarity is directly described by the electronegativity difference, , between the constituting atoms. The following steps show how to calculate the electronegativity difference. Let us take a look at the bond C-H as an example.  Step 1 Identify the electronegativity values of each constituting atoms. You can use Fig. 1 as a reference, but modern periodic tables have these values, too. The bond C-H is consist of C and H atoms. Based on Fig. 1, C has electronegativity value of 2.55 while H has an electronegativity value of 2.20.  Step 2 Write the working equation. The electronegativity difference can be determined using the equation   where

 is the electronegativity difference between two atoms, and

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  and  Step 3

are the electronegativity values.

Evaluate the working equation and determine the difference. Take note that equation is applied inside an absolute value symbol, which means you are taking only the positive difference.

   Hence, the electronegativity difference between C and H is 0.35. 



 Example 1 Calculate the electronegativity difference in the H-H bond.  Solution: Step 1 Identify the electronegativity values of each constituting atoms. The  bond  H-H  is  consist of two H atoms. Based on Fig. 1, H has electronegativity value 2.20.  Step 2 Write the working equation.   Step 3

Evaluate the working equation and determine the difference. 



 Hence, the electronegativity difference on the H-H bond is 0.00. 



L  et us Practice Calculate the electronegativity difference in the F-F bond.  It is intuitive that the electronegativity difference in a bond that is composed of two similar atoms is zero. In this type of bonds, the attraction felt by electrons between the two nuclei is equivalent. This has a significant effect on the property of the 

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  bond.  Example 2 Calculate the electronegativity difference in the N-O bond.  Solution: Step 1 Identify the electronegativity values of each constituting atoms. The  bond  N-O  is  consist of N and O atoms. Based on Fig. 1, N has electronegativity value of 3.00 while O has an electronegativity value of 2.5.  Step 2 Write the working equation.   Step 3

Evaluate the working equation and determine the difference. 



 Hence, the electronegativity difference between N and O is 0.50. 



L  et us Practice Calculate the electronegativity difference in the B-F bond.  There are times that even though the bond is composed of two different atoms, their electronegativity values are quite close to one another. This results to a relatively small electronegativity difference. The electrons somehow feel a relatively equal attraction to both nuclei.  Example 3 Calculate the electronegativity difference in the H-Cl bond.  Solution: Step 1 Identify the electronegativity values of each constituting atoms. The  bond  H-Cl  is  consist of H and Cl atoms. Based on Fig. 1, H has electronegativity value of 2.20 while Cl has an electronegativity value of 3.16.  Step 2 Write the working equation. 

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    Step 3

Evaluate the working equation and determine the difference. 



 Hence, the electronegativity difference between H and Cl is 0.96.

 Most of the time, bonds formed between two different atoms have significant electronegativity difference. In these cases, the electrons are attracted more towards one of the nuclei. This results in a drastic change in the polarity of a bond.  Let us Practice Calculate the electronegativity difference in the H-F bond. 



Key Points

● Electronegativity (χ) is the measure of the ability of an atom to attract bonding pairs of electrons. ● Linus Pauling was an American chemist who developed a numerical scale of electronegativity of selected representative elements in the periodic table. ● The higher the electronegativityvalue of a specific element, the greater is the attraction of the atoms of that element to the bonding pair of electrons. ● The electronegativity difference (Δχ) between two atoms describes the polarity of the bond they can form. 



Web Links



For further information, you can check the following web links: ● Read about the life of Linus Pauling, the developer of the electronegativity scale. 

The Nobel Foundation. 1962. ‘Linus Pauling - Biographical’ https://www.nobelprize.org/nobel_prizes/peace/laureates/1962/pauling-bio.html

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  ● Watch this video about the most electronegative element. 

Periodic Videos. 2010. ‘Fluorine - Periodic Table of Videos’ https://www.youtube.com/watch?v=vtWp45Eewtw

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Check Your Understanding

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A. Calculate the electronegativity difference for the following bonds. 1. H-Cl 2. C-O 3. P-Cl 4. Sb-O 5. Br-F 6. As-Cl 7. B-F 8. C-H 9. Be-Cl 10.N-H  B. Arrange the following in increasing electronegativity. 1. Cl, P, S 2. N, O, Na 3. Ga, Ba, P 4. Al, Cl, C 5. O, Te, Se 

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Challenge Yourself

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Answer the following questions briefly and clearly. 1. How do you rationalize the contrast between the electronegativity of metals and nonmetals? 2. Why do halogens have the highest electronegativity values? 3. What factors affect the electronegativity of an atom? 4. Why do copper and zinc deviate from the trend? 5. How is electronegativity related to electron affinity? 

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Lesson 5.2: Determining Polarity of Molecules 

Objective  In this lesson, you should be able to: ● determine if a molecule is polar or nonpolar given its structure.  Polarity depends on the structure of the molecule. Molecules with uneven electronegativity have partial positive and negative charges which makes them polar. Otherwise, they are nonpolar. There are ways to determine the polarity of a molecule which requires your knowledge on electronegativity and VSEPR theory. What exactly makes a compound polar or nonpolar? 

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Warm-Up

 Charged Attraction Polarity somehow describes how the electrons in a bond orient themselves. Depending on this, bonds in molecules can acquire partial charges. This differentiates polar and nonpolar compounds. In this activity, you will observe how polar and nonpolar compounds behave in the presence of static electricity.  Materials: ● 2 cups ● water ● wool ● vegetable oil ● 2 balloons  Procedure: 1. Put water in a cup. 2. Charge a balloon by rubbing it against a piece of wool. 3. Pour the water to make a steady stream and place the balloon near the stream. Observe what happens to the stream. 4. Repeat steps 1-4 using vegetable oil instead of water.  

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  Guide Questions: 1. Why does the balloon get charged upon rubbing it against the piece of wool? 2. What happened to the stream of water? Explain why. 3. What happened to the stream of oil? Explain why. 

Learn about It  Polarity of Bonds Polarity is the measure of the degree of inequality in the attraction of electrons between atoms in a molecule. Polarity means having dipoles, a positive and a negative end. These partial charges are created based on the orientation of the electrons in a bond, which highly depends on the electronegativities of each constituting atoms.  Based on polarity, bonds can either be polar or nonpolar. In determining the polarity of a bond, it is important to know the electronegati...