14modeling-v2 - Laboratory notes and report. Helpful notes and annotations PDF

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Experiment-14: The VSEPR Theory and Three-Dimensional Modeling of Molecular Compounds  Predict molecular geometry and polarity of a molecule using Lewis structures and the VSEPR theory  Gain experience in building 3D models of covalent molecules Molecules, Molecular Geometry and Covalent Bonds A group of atoms held together by covalent bonds is called a molecule. Although we represent molecules on paper as being two-dimensional for convenience, they are actually threedimensional. The three-dimensional arrangement of atoms in a molecule is known as molecular geometry or molecular shape, which determines most of the properties of a molecule, including its role in nature. Molecular geometry contributes toward determining a compound’s boiling point, freezing point, viscosity, solubility, types of reactions it can participate in, and a host of other physical and chemical properties. The covalent bonds holding the atoms of a molecule together can be classified as three different types. Covalent bonds are classified by comparing the differences in electronegativities of the two bonded atoms. Electronegativity is a scale used to determine an atom’s attraction for an electron in the bonding process. Bond Polarity Differences in electronegativities are used to predict whether the bond is nonpolar covalent, polar covalent, or ionic. The range of electronegativity differences and bond classification typically is: 0.0-0.4 (non-polar covalent); 0.4-2.1 (polar covalent); 2.1-4.0 (ionic). In a polar covalent bond, the electrons are more attracted to the atom with the greater electronegativity. This results in a partial negative charge on that atom. The atom with the smaller electronegativity value acquires a partial positive charge.

Molecular Polarity Molecules composed of covalently bonded atoms may also exhibit polar or nonpolar properties. For the molecule to be polar, it must, of course have polar bonds. But the key factor in determining the polarity of a molecule is its shape, or geometry. If the polar bonds are arranged symmetrically around the central atom they will offset each other and the resulting molecule is nonpolar. However, if the arrangement of the polar bonds is not symmetrical around the central atom, the electrons will be pulled to one end of the molecule and the resulting molecule is polar. Predicting Molecular Geometry The model used to determine the molecular shape is referred to as the Valence Shell Electron pair Repulsion (VSEPR) model. The model is based on an arrangement that minimizes the repulsion of bonding and lone pairs of electrons around the central atom. By examining the central atom and identifying the number of atoms bonded to it and the number of lone electron pairs surrounding it, one can determine the geometry, shape, of a molecule.

Models To study covalent molecules, chemists find the use of models helpful. In this experiment, you will construct three-dimensional models to help you visualizing molecular geometry. You will use ball-and-stick type models in which colored balls represent atoms, and connectors represent bonds. Single bonds are represented by wood connectors, while double and triple bonds will be represented by metallic springs. The balls have holes molded into them to accept the connectors. The holes are bored at angles that approximate the accepted bond angles, and the number of holes in the ball represents the maximum number of bonds that a given atom can have. The balls are color-coded so that atoms of different elements can be distinguished.

EQUIPMENT Molecular model building set. COLOR KEY (amount) Black (10) Yellow (28) Red (6) Blue (2) Green (4) Orange (2) Purple (2)

NUMBER OF HOLES 4 1 2 3 1 1 1

ELEMENT Carbon Hydrogen Oxygen Nitrogen Chlorine Bromine Iodine

SAFETY PRECAUTION Do not place model pieces in your mouth – they present a choking hazard.

Suggested Videos https://www.youtube.com/watch?v=DBrq31w8vC4 https://www.youtube.com/watch?v=keHS-CASZfc https://www.youtube.com/watch?v=xNYiB_2u8J4 CHEM1110-L Pre-Lab #14 Last Name: Akash__ First: Sm Arifuzzaman ___

1. Explain what molecular geometry is. Answer: First of all, when we say molecule, we are referring to a group of atoms that are held together with covalent bond and has certain chemical, physical and structural properties. Usually, when we write a molecule, we write them as being two dimensional for our convenient while atoms of the molecules are arranged three dimensionally. Now, this three-dimensional arrangement of atoms in a molecule is known as molecular geometry or molecular shape which is very significant for the molecule.

2. Why is molecular geometry important? Answer: Molecular geometry is very important since this three-dimensional arrangement of atoms determines most of the properties of a molecule and also determine the molecule’s role in nature. Additionally, molecular geometry contributes toward determining a compound’s boiling point, freezing point, solubility, viscosity, types of reactions it can participate in, and a host of other physical and chemical properties.

3. How do we predict molecular geometry from the Lewis structure? Answer: We predict the molecular geometry from the Lewis structure by using the Valence Shell Electron Pair Repulsion or the VSEPR model. This model is mainly based on an arrangement that minimizes the repulsion of bonding and lone pairs of electrons around the central atom of the molecule. When we examine the central atom, identify the number of atoms bonded to it and the number of lone electron pair surrounding it, we can easily figure out the molecular geometry or the molecular shape of the molecule.

4. What are the two factors determining the polarity of a molecule? Answer: As I mentioned earlier, a molecule is an arrangement of atoms that are held by the covalent bond. When we classify further the covalent bond, we put the molecule in either polar or nonpolar covalent bond category. In a polar covalent bond, the electrons are more attracted to the atom with the greater electronegativity while the non-polar covalent bond is the contrast. Now, if we want to determine the polarity of a molecule we must look for polar covalent bonds in the molecule, but the key factor in determining the polarity of a molecule is its shape, or geometry because if the polar bonds we found are arranged symmetrically around the central atom, they will offset each other and the resulting molecule will be nonpolar. However, if the arrangement of the polar bonds is not found to be symmetrical around the central atom, the electrons will be pulled to one end of the molecule and the resulting molecule must be polar. Therefore, the two polarity determining factors will be the polar bonds and the molecular geometry.

PROCEDURE 1. Complete the table below by a. Drawing the Lewis structure for each molecule. b.

Determining the number of bonding pairs and lone pairs around each central atom.

c. Drawing a 3-D sketch of the molecule.

d.

Determining the geometry of the molecule at each central atom using the VSEPR model.

e.

Determining whether the bonds between the central atom and the other atoms are nonpolar, polar, or ionic.

f. Determining whether the molecule is polar or nonpolar.

2. Obtain a molecular model and build those molecules with tetrahedral, trigonal pyramidal, linear or bent geometry.

Lab Report#14: The VSEPR Theory and Three-Dimensional Modeling of Molecular Compounds Last Name____________________, First___________________________, Date____________

Formula

Lewis Structure

Formula

Lewis Structure

OF2 Cl2

H 3O + HBr

H 2O NH3

CO2 SO42-

CH2Cl2

# of Bonding # of Pairs Bonding Pairs

# of Lone # of Pairs Lone Pairs

3D Drawing 3D Drawing

Molecular Geometry Molecular Geometry

Molecule Polar or Molecule Non-Polar Polar or Non-Polar

Formula

Lewis Structure

# of Bonding Pairs

# of Lone Pairs

3D Drawing

N2

SbI5

CCl4

SO3

XeF4

Questions: 1. What molecules from the list have polar bonds but are non-polar because of the geometry?

2. Which molecules from the list have all non-polar bonds?

Molecula Geometr

3. Which molecular geometries from the list appear to produce polar molecules?...