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Lesson Plan Lesson: Resonance and Polyatomic Ions
Aim: To investigate bond length, bond energy and the Lewis structures of polyatomic ions Learning Outcomes : At the end of the lesson, students will be able to : 1. define bond length and bond energy. 2. compare the bond length and bond energy of covalent bonds (single, double and triple). 3. explain the Lewis structures of ions such as SO 42-, CO32-, NO3- and CN- ions. Assumed prior knowledge : Students should already : 1. be familiar with the formation of single and multiple covalent bonds. 2. know how to draw the Lewis structures structur es of molecules and ions.
Underlying Principles 1. 2.
Making the invisible, visible. Enabling students to know what to look for.
Time taken to complete the activities : 80 minutes
Differentiation Questions in the student notes are designed to enable all students to complete the activity. The pop-up answers are provided for the students to view when they have considered their responses. Worksheet questions include questions that require recall, understanding and application of the new concepts learned .
Steps Set Induction. (Ascertaining prior knowledge and introducing lesson topic for the day).
Strategy •
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Student Activity
Teacher to quiz students on the difference between single, double and covalent bonds (in terms of number of pairs of electrons, sigma and pi bond formation). Teacher to point out to students that there are other differences between the bonds mentioned above. Teacher to point out the presence of resonance structures in some cases. Teacher to go through Activities 1 - 4 with the students.
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Evaluation
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4
Extension activity
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Resources
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Courseware
Activity 1 : Bond length and bond energy Students get to compare the bond length and bond energy of single, double and triple covalent bonds. Activity 2 : Structures of sulphur dioxide and nitrogen dioxide Students get to view the structures of SO2, NO2 and N2O4. Activity 3 : Polyatomic ions Students get to view the different possible structures of CO 32-, NO3- and SO42-. Activity 4 : Writing formulae Students get to revise how to name and write the formulae of ionic and covalent compounds. Students to answer questions in the student worksheet on their own. Students to go through the extension activities on their own.
1.1 Water has a higher bond energy. The H-O bond length in water is shorter than the H -N bond length in ammonia. The bond energy is inversely proportional to the bond length. 1.2 The C -C bond length in ethane (H 3C-CH3) is longer than the C -C bond length in ethene (H 2C - CH2). This is because it is a single bond ( bond) whereas in ethene, the C-C bond is a double bond ( and bonds).
2.
Structure of sulphur dioxide and nitrogen dioxide
2.1 a.
The Lewis structure of nitrogen dioxide molecule is
There is an unpaired electron on N. Thus, two molecules of nitrogen dioxide dimerise by pairing the unpaired electrons.
b.
The Lewis structure of dinitrogen tetraoxide is
c.
The Lewis structure of nitrogen dioxide may be represented as two canonical structures.
Each resonance structure shows N O single bond (bond order = 1) and N O double bond (bond order = 2). The actual structure is a resonance hybrid of the two resonance structures ( N O bond order = 1.5).
The N-O bond length in nitrogen dioxide is expected to be the same as N -O bond length in dinitrogen tetroxide. The Lewis structure of dinitrogen tetroxide is
There is resonance in the molecule. The bond order is 1.5, which is the same as for NO 2.
3.
Polyatomic ions
3.1 The resonance structures of NO 3- ion are
3.2 The three C O bonds in CO32- ion are of equal length and strength because of resonance. There are three resonance structures.
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The three resonance structures contribute equally to the actual Lewis structure of the ion.
