CHEMISTRY NOW: Water Is a Polar Molecule (Grades 9-12) Print

Objective:

Students will understand why water is a polar molecule by examining its molecular composition


Introduction Notes:

CHEMISTRY NOW: Water Is a Polar Molecule

 

Subject Area: Chemistry

Grade Level:  High School Chemistry

Lesson Title: Water is a Polar Molecule

National Science Standards:

Science as Inquiry: 5-8

Science as Inquiry: 9-12

 

Physical Science Standards:

properties and changes of properties: 5-8

structure of atoms: 9-12

structure and properties of matter: 9-12

chemical reactions: 9-12

 

Suggested Prior Knowledge: Lewis Dot Structures, Shapes of Molecules, Valence Electrons

 

Purpose: To give students an understanding of water's polarity by examining its Lewis dot structure, discussing the strength of the covalent bonds, and performing a class experiment.

Key Vocabulary:

 

 polar - a characteristic of a molecule such that it has a greater electron density at one end than the other

 polarity - possessing two opposed poles; a characteristic of molecules which have unequal distributions of charge; water is polar because the oxygen has a partial negative charge and the hydrogen atoms each have a partial positive charge; polar molecules interact with other polar and charged molecules and ions.

 polar molecule - a molecule that has a greater electron density at one end than the other

  covalent bond - An attraction between two atoms in which electrons are shared between them.

valence electrons - The amount of electrons in the outermost shell of an atom. Generally, valence electrons are equal to the elements group number in the periodic table.

Objective:

Students will understand why water is a polar molecule by examining its molecular composition

Materials:

Acrylic rod, wool fabric, faucet or burette on ring stand, safety glasses; included handout, video from NBC Learn.com of water molecules appropriate to this lesson. (Note:  Proper lab safety equipment and protocols should be followed).

Procedure:

Begin with a leading question and follow-up:

 

         What do you think of when you hear the word polar? (North and South Pole of the Earth may be an answer.)

 

     Utilize some of the answers that the students give you and ask them to create a definition for a molecule that is polar. Consider this to be a working definition for a polar molecule that they are free to revise as the lesson progresses.

 

     Have students look at the Lewis dot diagram on their worksheet. This part of the lesson is easier if students have some understanding of valence electrons. Discuss with students how the valence electrons indicate the reactivity of the atom. It is important for students to understand how many electrons are required in both the hydrogen and oxygen atoms to make their valence shells complete. In the diagram below, the valence shell of the hydrogen is stable with the shared electron from oxygen (indicated in red). The following questions will facilitate the discussion:

 

         How many electrons are required for hydrogen to complete its outermost shell? The answer is two since hydrogen only requires one electron to complete its outermost shell.  If students have difficulty, then review group numbers on the periodic table.

 

         If hydrogen has only has 1 electron but needs one more to complete its shell, then where ist the second electron being borrowed or shared from? Answer: Hydrogen shares an electron from oxygen to complete its shell.

 

         Let's look at oxygen: How many electrons are in the valence shell of oxygen? Answer: 6

         How many electrons are required for oxygen to complete the octet rule in its valence shell? Answer: 2 more

 

         How does oxygen complete its shell? Answer: by sharing two electrons, one from each of two hydrogen atoms.

 

     Once students have an understanding that electrons are shared between hydrogen and oxygen, you can identify these as covalent bonds. Have students fill in the missing word in the definition of a covalent bond on the worksheet.

 

     At this point, have students understand that the electron pairs surrounding the oxygen atom are equidistant from each other because of the negative repulsive charges of each electron cloud. Most tetrahedral molecules are 109.5°. However, in a water molecule, the lone unshared electron pairs are held closer to the center of the oxygen atom and therefore push the covalent bonds closer together. The result is a distorted tetrahedral arrangement in which the H—O—H angle is 104.5°.  Use the diagram below as a reference for this explanation:

 

     Explain how water gets its polarity. The oxygen atom in a water molecule has eight electrons in its valence shell. Most of those tend to stay away from the hydrogen atoms, and cause the outside of the oxygen atom to have a negative charge. For the same reason, the outside of the hydrogen atom tends to be positively charged. Draw in the positive and negative ends of the molecule as seen below.

 

 

Conduct the following class demonstration:

 

     This demonstration can be completed as a teacher demonstration or as a class. Explain to your class that they will conduct an experiment that will help them understand polarity in molecules. Take your plastic rod or rigid tube and rub it vigorously in wool cloth. The plastic rod will pick up electrons from the wool and become negatively charged. Turn on the faucet that is in clear view for the class to see. Run the faucet so a thin continuous stream is flowing (If you do not have a faucet, use a burette mounted on a ring stand). Bring the charged rod close to the stream without touching it. You will notice that the stream of water moves towards the rod demonstrating the polarity of water.

 

     Take a  moment to ask for student input and hypotheses with the following leading questions:

 

  • - Why does the water bend towards the acrylic rod? Students may answer that it’s  because water is polar and one end is positive while the other end is more negative.
  • - What end of the water molecule is attracted to the acrylic rod? If the rod has a negative charge it would be attracted to the positive side of a water molecule. The question can be further explained by examining the definition of a polar molecule. As the definition  suggests, the electron density is concentrated to one side of the water molecule, therefore making that end electrically neutral and non- reactive in this situation. The opposite is true for the positively charged side of the water molecule.

 

 

     Now that students have a better understanding of water and what makes it a polar molecule, examine if any of their initial definitions make sense. Discuss the accuracy of their initial answers. See if students now understand why water is polar by calling on some students to explain why water "bends" towards the rod. Their answers should include the key vocabulary indicated at the top of this lesson plan where possible.

 

Finally, have the students complete their definition for a polar molecule and enter an explanation for the phenomenon on their worksheets as either a take-home assignment or lab component.

Additional Resources:

http://www.lpi.usra.edu/education/explore/ice/background/allAboutWater/index.shtml

http://www.chem1.com/acad/sci/aboutwater.html

http://www.middleschoolchemistry.com/multimedia/chapter2/lesson2#models_of_water_molecules

 

 

 

 

 

 

 

 

 

 

Student Worksheet for: Water is a Polar Molecule

 

1. What does the word polar mean to you? Where have you heard the word polar before?

 

__________________________________________________________________

 

2.  Molecules can be polar. What do you think it means to have a polar molecule?

 

__________________________________________________________________

 

3. Based on what you know about polarity create a definition that describes a polar molecule.

 

_________________________________________________________________

 

4. Examine the Lewis dot structure of the water molecule below and describe how oxygen and hydrogen complete their outermost valence shell:

 

__________________________________________________________________

 

 

5, Complete the following definition: Covalent Bond - An attraction between two atoms in which electrons are ______ between them.

 

     Most tetrahedral molecules are 109.5°. However, in a water molecule, the lone pairs are held closer to the center of the oxygen atom and therefore push the covalent bonds closer together. The result is a distorted tetrahedral arrangement in which the H—O—H angle is 104.5°. The oxygen atom in a water molecule has eight electrons in its valence shell. Most of those tend to stay away from the hydrogen atoms, and cause the outside of the oxygen atom to have a negative charge. For the same reason, the outside of the hydrogen atom tends to be positively charged. Draw in the positive and negative ends of the molecule below:

 

 

6. After completing the experiment and examining the composition of a water molecule, has your definition about what makes a molecule polar changed? If so, how has it changed?

 

__________________________________________________________________

 

7. Complete the following definition: A polar molecule is a molecule that has a greater ______ density at one end than the other.

 

8. Using the words polarity, covalent bond, and valence electrons, explain what happened in the bending stream of water experiment:

 

__________________________________________________________________

 

 

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