Textbook Question
a. Consider the AF3 molecules in Exercise 9.27. Which of these will have a nonzero dipole moment?
Ch.9 - Molecular Geometry and Bonding Theories
Chapter 9, Problem 43b
Dichloroethylene (C2H2Cl2) has three forms (isomers), each of which is a different substance. (b) Which of these isomers has a zero dipole moment?
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Understand that dichloroethylene (C2H2Cl2) has three isomers: 1,1-dichloroethylene, cis-1,2-dichloroethylene, and trans-1,2-dichloroethylene.
Recall that a molecule's dipole moment is determined by the vector sum of the individual bond dipoles. A zero dipole moment means the bond dipoles cancel each other out.
Visualize or draw the structures of the three isomers: 1,1-dichloroethylene has both Cl atoms on the same carbon, cis-1,2-dichloroethylene has Cl atoms on adjacent carbons on the same side, and trans-1,2-dichloroethylene has Cl atoms on adjacent carbons on opposite sides.
Analyze the symmetry of each isomer: trans-1,2-dichloroethylene is symmetrical, which allows the dipoles to cancel out, resulting in a zero dipole moment.
Conclude that trans-1,2-dichloroethylene is the isomer with a zero dipole moment due to its symmetrical structure.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Isomerism
Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but different structural arrangements or spatial orientations. In the case of dichloroethylene (C2H2Cl2), the isomers can differ in the position of the chlorine atoms or the arrangement of the carbon-carbon double bond, leading to distinct chemical and physical properties.
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Dipole Moment
The dipole moment is a measure of the separation of positive and negative charges in a molecule, indicating its polarity. A molecule with a dipole moment has an uneven distribution of electron density, resulting in a positive and a negative end. Molecules with symmetrical structures can have a zero dipole moment, as the individual bond dipoles cancel each other out.
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Molecular Symmetry
Molecular symmetry plays a crucial role in determining the dipole moment of a molecule. Symmetrical molecules, such as the trans isomer of dichloroethylene, can have their dipole moments cancel out due to equal and opposite charge distributions, resulting in a net dipole moment of zero. Understanding the symmetry of a molecule helps predict its physical properties, including polarity.
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Related Practice
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