5 Text Reference Section 16.3 MOLECULAR MODELS EXPERIMENT PURPOSE To investigate the three-dimensional shapes of molecules by building molecular models. Time Required 30 minutes Objectives Construct threedimensional models of molecules. Convert three-dimensional molecular models into two-dimensional molecular drawings. Examine the concept of structural isomerism. BACKGROUND You can represent a molecule on paper with either a molecular formula or a structural formula. owever, molecular formulas, such as N 3, provide no information concerning the actual arrangement of atoms in the molecule. Structural formulas, such as the following, give some information about the arrangement of atoms in the molecule. O N C owever, these structural formulas provide only limited information because they are two-dimensional. Actual molecular shapes are threedimensional. A molecular model is far superior to a structural formula when it comes to visualizing atomic arrangement. Compared to molecular formulas and structural formulas, molecular models provide much more information about the true shapes of the molecules. In this experiment, you will use ball-and-stick models to help you visualize the shapes of molecules. The balls are color-coded and sized to represent different atoms. The balls are also drilled with holes to accept sticks and springs; the number of holes in a ball reflects the maximum number of bonds a given atom can form. Single bonds are represented by short wooden sticks; double and triple bonds are represented by springs. Prentice all, Inc. All rights reserved. Styrofoam balls or balls of clay can be used to represent atoms. Toothpicks or pipe cleaners can be used to represent the bonds that hold the atoms together. Safety goggles need not be required if this exercise is done in a classroom setting. If it is done in the laboratory, though, safety goggles should be required because of the hazards posed to the eye by equipment and materials normally present in the laboratory. MATERIALS (PER 6 STUDENT GROUP) safety goggles 1 ball-and-stick model set SAFETY FIRST! In this lab, observe all precautions. Caution: Wear your safety goggles. (All steps.) Molecular Models 177
PROCEDURE Data Table 1 shows color codes for balls representing different atoms. As you build the models, draw the structural formulas of the molecules in Data Table. DATA TABLE 1 Atom Symbol Color of Ball Number of oles Maximum Number of Bonds hydrogen yellow 1 1 carbon C black 4 4 oxygen O red nitrogen N blue 3 or 5* 3 chlorine Cl green 1 1 bromine Br orange 1 1 iodine I purple 1 1 * If the nitrogen has five holes, connect two adjacent holes with a spring bond. Step 3. N (N N); O (O O); CO (O C O) This is a good time to review the properties of compounds and mixtures. The components of a mixture can be physically separated. 1. Using the ball-and-stick model set, construct models of water, O; ammonia, N 3 ; and methane, C 4. Draw a sketch of each molecule in Data Table. The shape shown by the water molecule is described as bent, the shape of the ammonia molecule is called trigonal pyramidal, and the shape of the methane molecule is termed tetrahedral. Write these names below the matching structures you have drawn in the table.. Construct models of hydrogen sulfide, S; carbon tetrachloride, CCl 4 ; dichlorodifluoromethane, CCl F ; and ethane, C 6. Give the molecular formula for each of these compounds, and draw a sketch of each molecule in Data Table. Name the shape of each molecule. 3. The air above a burning candle contains nitrogen gas, carbon dioxide gas, and oxygen gas. Construct models of these substances. In Data Table, draw a sketch of each molecule. 4. The compound urea has the molecular formula CO(N ). The structural formula of urea is: NCN O 3 Prentice all, Inc. All rights reserved. Construct a model of urea and sketch its shape in Data Table. 178 Experiment 5
Step 5. Inform students that, although they may rotate the molecule about the C C single bonds, the identity of the molecule will be unchanged. To change the model of one molecule into the model of another molecule requires bond breaking and bond making, not bond rotation. Step 6. ave students try to superimpose the two models they have made in Step 6. If the models are superimposable (match up on an atom-to-atom basis), they represent the same molecules. True stereoisomers cannot be superimposed. 5. Construct a model of butane, C 4 10. (int: The carbons are bonded to one another in a continuous, unbranched chain.) Draw a sketch of this molecule in Data Table. Can you construct a model of a different molecule that has the same molecular formula as butane? Make a model of such a molecule and sketch its structure in the data table. The two different compounds having the molecular formula C 4 10 are called structural isomers. They have identical molecular formulas but different structural formulas. They also have different physical and chemical properties. Structural isomers play a very important role in organic chemistry. 6. Construct a model of bromochlorofluoromethane, CBrClF. Sketch the compound in the table. Can you construct an isomer of this compound? (int: Is your left hand identical to your right?) Draw the new compound if you can. The compound and the isomer have the same molecular formula, CBrClF, but they are different from each other in the way that a left hand is different from a right hand. The compounds are mirror images of each other and are called stereoisomers. The phenomenon of handedness exhibited by pairs of stereoisomers is very important in organic chemistry and biochemistry. Prentice all, Inc. All rights reserved. Molecular Models 179
Name Class Date OBSERVATIONS DATA TABLE : SKETCES OF MOLECULES O N 3 C 4 bent triatomic trigonal pyramidal tetrahedral S CCl 4 CCl F C 6 bent triatomic tetrahedral tetrahedral N CO O linear triatomic CO(N ) CBrClF, stereoisomers Prentice all, Inc. All rights reserved. C 4 10, structural isomers Molecular Models 181
Name Class Date GOING FURTER Develop a ypothesis Although the VSEPR model allows one to predict the geometry of a molecule, the prediction must still be verified through experiment. One experimental method involves measuring the dipole moment of a molecule, a quantity that reflects the degree of charge separation in a molecule the polarity. Based on your models of CCl 4 and N 3, propose a hypothesis about the polarities of these molecules. The polar C-Cl bonds are symmetrically distributed around the central carbon atom in CCl 4, the dipoles therefore cancel out and CCl 4 is not expected to be polar overall. The trigonal pyramidal shape of N 3 is expected to produce a net dipole moment. Design an Experiment To test your hypothesis, look up dipole moments of each molecule in a chemistry handbook. The dipole moment of CCl 4 is 0 debye. The dipole moment of N 3 is 1.47 debyes. Prentice all, Inc. All rights reserved. 18 Experiment 5