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Ch.11 - Liquids and Intermolecular Forces
All textbooksBrown 14th EditionCh.11 - Liquids and Intermolecular ForcesProblem 76a
Chapter 11, Problem 76a

Two isomers of the planar compound 1,2-dichloroethylene are shown here.
(a) Which of the two isomers will have the stronger dipole– dipole forces?

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1
Identify the structures of the two isomers of 1,2-dichloroethylene, which are typically cis-1,2-dichloroethylene and trans-1,2-dichloroethylene.
Understand the concept of dipole moments: A dipole moment arises from the difference in electronegativity between two chemically bonded atoms. The greater the difference in electronegativity, the stronger the dipole moment.
Analyze the molecular geometry of each isomer. In the cis isomer, the chlorine atoms are on the same side of the double bond, which results in a net dipole moment pointing from the center of the molecule towards the chlorine atoms. In the trans isomer, the chlorine atoms are on opposite sides, which results in the cancellation of their dipole moments.
Compare the net dipole moments: Since the dipole moments in the trans isomer cancel each other out, it has a zero net dipole moment. The cis isomer, however, has a non-zero net dipole moment because the dipoles do not cancel out.
Conclude which isomer has stronger dipole-dipole forces: The cis-1,2-dichloroethylene, with a non-zero dipole moment, will exhibit stronger dipole-dipole interactions compared to the trans isomer.

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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 1,2-dichloroethylene, the two isomers can be classified as cis and trans, which differ in the positioning of the chlorine atoms around the double bond. This difference in structure can significantly affect the physical and chemical properties of the compounds, including their polarity.
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Dipole-Dipole Interactions

Dipole-dipole interactions are a type of intermolecular force that occurs between polar molecules. These forces arise due to the attraction between the positive end of one polar molecule and the negative end of another. The strength of dipole-dipole interactions is influenced by the polarity of the molecules; more polar molecules exhibit stronger dipole-dipole forces, which can affect boiling points and solubility.
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Polarity

Polarity in chemistry refers to the distribution of electrical charge over the atoms in a molecule. A molecule is considered polar if it has a significant difference in electronegativity between its atoms, leading to an uneven distribution of charge. In the context of 1,2-dichloroethylene, the cis isomer is generally more polar than the trans isomer due to the arrangement of the chlorine atoms, which affects the overall dipole moment and, consequently, the strength of dipole-dipole interactions.
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Related Practice
Textbook Question

Suppose you have two colorless molecular liquids, one boiling at - 84 °C, the other at 34 °C, and both at atmospheric 6 pressure. Which of the following statements is correct? For each statement that is not correct, modify the statement so that it is correct. (d) The two liquids have identical vapor pressures at their normal boiling points.

Textbook Question

Suppose you have two colorless molecular liquids, one boiling at - 84 °C, the other at 34 °C, and both at atmospheric 6 pressure. Which of the following statements is correct? For each statement that is not correct, modify the statement so that it is correct. (e) At - 84 °C both liquids have vapor pressures of 760 mm Hg.

Textbook Question

The table below shows the normal boiling points of benzene and benzene derivatives.

(a) How many of these compounds exhibit dispersion interactions?

Textbook Question

The table below shows the normal boiling points of benzene and benzene derivatives. (e) Why is the boiling point of phenol the highest of all?

Textbook Question

Use the normal boiling points propane (C3H8) -42.1 °C butane (C4H10) -0.5 °C pentane (C5H12) 36.1 °C hexane (C6H14) 68.7 °C heptane (C7H16) 98.4 °C to estimate the normal boiling point of octane (C8H18). Explain the trend in the boiling points.