Lesson Plan Molecular Shape

Posted by: kazzuya on: 14 November 2010

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LESSON PLAN

I. Identity

School	:	SMA
Subject	:	Chemistry
Year/semester	:	XI IA/1
Time Allotment	:	3 hours (3 x 45minutes)

II. Standard Competence

Understanding atomic structure to predict periodicity of elements, molecular structure, and properties of compounds.

III. Basic Competence

1.            Explain theory of electronic pair around atomic nucleus and theory of hybridization to predict molecular shapes

IV. Indicators

1.2.1        Predict the shapes of molecules according to pair electron theory

1.2.2        Predict the shapes of molecules according to hybridization theory

V. Learning Outcomes

1.      Student be able to predict molecular shape by using VSEPR theory

2.      Student be able to predict molecular shape by using hybridization theory

VI. Teaching and Learning Topics

1. Domain Electron Theory and VSEPR Theory
2. Molecular shapes
3. Hibridization Theory

Contents

The shape of a molecule deal with the position of atoms in the molecule. In this case shapes of molecules describe the position of atoms in the three dimensional space and the magnitude of the bond angle which occurs at the covalent bond in a molecule.

1. Electron Domain Theory

Electron domain theory is one away on predict the molecular shape according to repulsion of electron in out-shell centre atom. Electron domain is electron position or occupied electron area. The number electron domain is determined such as:

Each bonded atom is counted as one region of high electron density, whether the bonding is single, double, or triple.

Each unshared pair of valence electrons on the central atom is counted as one region of high electron density.

2. VSEPR (Valence Shell Electron Pair Repulsion) Theory

VSEPR (Valence Shell Electron Pair Repulsion) Theory is a concept used to predict geometric shape formed by atoms in molecules which are covalently bonded. This theory is based on the idea that all electron pairs which are directly attached to an atom, those are bonded electron pairs and non-bonded electron pairs (free electron pairs) at around the central atom, will arrange their position to be as far away from each other as possible. Bonded electron pairs are valence electron share by atoms in a molecule, while non-bonded electron pairs or free electron pairs are valence electrons in which a molecule at locations where those electrons are not involved in the bonding process.

According to VSEPR theory, non-bonded electron pairs or free electron pairs have a greater repulsion than bonded electron pairs, so these non-bonded electron pairs occupy the larger space than bonded electron pairs. They will adopt a shape so repulsions between those electron pair will be evenly distributed and at a minimum. As a result of these repulsions, atoms will form the certain geometries; those are linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral shapes.

The basic ideas of the valence shell electron pair repulsion (VSEPR) theory are:

Each set of valence shell electrons on a central atom is significant. The sets of valence shell electrons on the central atom repel one another. They are arranged about the central atom so that repulsions among them are as small as possible.

3. Predicting Shape of Molecule Using VSEPR Theory

The five basic molecular shapes.

a. Linier. A linier arrangement of atoms occurs when they are all in a straight line. The angel formed between two bonds that go to the same central atom, which we call the bond angle is 180^o.

b. Planar triangular. A planar triangular arrangement of four atoms has them all in the same plane. The central atom is surrounded by three others located at the corners of a triangle. The bond angels are all 120^o.

c. Tetrahedral. A tetrahedron is a four-sided pyramid having equilateral triangles as faces. In a tetrahedral molecule, the central atom is located in the center of this tetrahedron and four other atoms are located at the corners. The bond angels are all equal and have values of 109.5^o.

d. Trigonal bipyramid. A trigonal bipyramid consist of two triangular pyramids (similar to tetrahedrons) that share a common face. In a trigonal bipyramid molecule, a central atom is surrounded by five other. The central atom is located at the five corners. In this kind of molecule, the bond angels are not the same. Between any two bonds that lie in the central triangular plane, the bond angel is 120^o. The angle is only 90^o between a bond in the central triangular plane and a bond that points to the top or bottom of the trigonal bipyramid.

e. Octahedral. An octahedron is a geometrical figure that has eight faces. We can think of it as two square pyramids that share a common square base. Notice that octahedron has only six corners even thought it has eight faces. In an octahedral molecule, the central atom is surrounded by six others. The central atom is located in the center of the square plane that passes through the middle of the octahedron. The six atom bonded to it are at the six corners of the octahedron. The angel between any pair of adjacent bonds is the same, and has a value of 90^o.

Molecular shape will as same as whit the electron geometric in central atom if there were not non-bonding electron. The steps to predict molecular shape such as:

1. Write down the electron configuration each atom in molecule
2. Determine the valence electron all atoms in molecule
3. Draw electron-dot structure (Lewis structure)
4. Determine bonded electron pairs and lone pairs electron of central atom
5. Determine molecular shape

The formula of electron pairs in a molecule is symbolized as follows.

AX_nE_m

Information           A = central atom

X = bonded electron pairs

E = lone pairs electron

n = the number of  bonded electron pairs

m = the number of  lone pairs electron

Example : H₂O molecule

Electron configuration of  ₈O = 1s² 2s² 2p⁴ (valence electron 6)

Electron configuration of  ₁H = 1s¹ (valence electron 1)

One atom O are bonded with 2 atoms H  with lewis structure such as =

Bonded electron pairs: 2

Lone pairs electron: 2

Formula of electron pairs: AX₂E₂

Molecular shapes : bent (V)

1. Hybridization of Atomic Orbital Theory

According to Pauling orbital of valence electrons can form a new set orbital called hybrid atomic orbital or hybrid orbital. The process of hybrid orbital formation done by an atom is called hybridization. These hybrid orbital influences the shape of a molecule formed when the atom combines with other atoms.

a.       The sp Hybrid Orbital

The sp orbital are possible states of electrons in atom when the atom is bonded to others and these electrons states have half 2s orbital and half 2p orbital characters. Mathematically, there are two ways to combine the 2s and 2p atomic orbital.

b.      The sp² Hybrid Orbital

Energy states of valence electrons in atoms of the second period are in the 2s and 2p orbital. If we combine two of the 2p orbital with a 2s orbital, we will obtain three sp² hybridized orbital.

c.       The sp³ Hybrid Orbital

If we combine three of the 2p orbital with a 2s orbital, we will obtain four sp³ hybridized orbital. For example, CH₄ molecule which is tetrahedral in shape can be explained by using the sp³ hybrid orbital concept.

d.      The d²sp³ Hybrid Orbital

If two 3d orbital; one 3s orbital; and three 3p orbital are combined, six d²sp³ hybrid orbital are resulted. For example, SF₆ is a molecule which is d²sp³ hybridized. If an atom makes use the d²sp³ hybrid orbital to bond six other atoms, geometry of the molecule is octahedral.

e.       Hybridization with Free Electron Pairs

Among of sp²; sp³; dsp³; and d²sp³ hybridized orbital, one of the orbital may be occupied by pair or a single electron. It can influence the shape of the molecule. For example, the C, N, and o atoms in CH₄, NH₃, and H₂O molecules use the sp³ hybrid orbital. CH₄ molecule has no free electron pair, so it is tetrahedral in shape. Meanwhile, NH₃ has a free electron pair and H₂O has two free electron pairs, so NH₃ is trigonal  pyramidal in shape and H₂O is bent.

VII. Teaching-Learning Method

1.      Class discussion (use visualization)

2.      Ask and Question

VIII. Teaching and Learning Process

1.      Preparation (± 10 minute)

a)      Greetings and Introductions

b)      Checking students attendance

c)      Presenting the learning outcomes and the assessment procedure

d)     Apperception and motivation (focusing) about lewis structure

2.      Mains Activities (± 120 minutes)

Teacher Activities	Student Activity	Time Allocation	Location
Exploration			Class room
Motivate the student with some questions related to the topics that will learn.	Students answer the teacher question.	15 minutes
Elaboration
–    Presenting the topics, collecting information and problem that have students  –    Guiding the students to investigate understanding the shape of molecule according to VSEPR theory and hybridization theory –    Observing the student’s activity	Paying a good attention to the teacher’s explanation, Give information answer the teacher’s question	60 minutes
Confirmation
–    Giving some exercises  –    Giving a comment about the asks and giving a explanation if there is misconception –    Post test	–      Doing exercises given by teacher  –      Correcting the ask according to the teacher’s comment	45 minutes

3.      End Activities (± 5 minutes)

a.       Making the conclusion about the topic together (the students and the teacher)

b.      Giving homework

c.       Giving information of the next topic

d.      Greetings

IX. TEACHING AND LEARNING RESOURCES

1.      Chemistry book :

a.       Brady, James E and Gerard E Humiston. 1982.General Chemistry Principles and Structure. New York: John Wiley and Sons.

b.      Chang, Raymond. 2005. Kimia Dasar Konsep Konsep Inti Jilid II. Jakarta: Erlangga.

c.       Parning, Horale, Tiopan. 2008. Kimia SMA XI Semester Pertama. Jakarta: Yudistira.

d.      Sunardi. 2008. Kimia Bilingual Kelas XI SMA. Bandung: Yrama Widya.

2.      Student worksheet (Exercise)

3.      Power Point

X. MATERIAL

a. Student Worksheet

b.Laptop

c. LCD

XI. ASSESSMENT

1. Evaluation Method:    Test, individual assignment, small test.

2. Type of evaluation: Essay and multiple choice test

3. Instrument: cognitive and affective assessment rubric

By

Putu Eka Surya Putra

Jurusan Pendidikan Kimia

Universitas Pendidikan Ganesha