Ch.11 - Liquids and Intermolecular Forces

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.11 - Liquids and Intermolecular Forces

Problem 19

Chapter 11, Problem 19

(a) List the following molecules in order of increasing polar- izability: GeCl4, CH4, SiCl4, SiH4, and GeBr4. (b) Predict the order of boiling points of the substances in part (a).

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Identify the number of electrons and the molecular size of each molecule. Molecules with more electrons and larger molecular sizes generally have higher polarizability.

Compare the electronegativity of the atoms within each molecule. Molecules with less electronegative substituents (like Br compared to Cl or H) will generally have higher polarizability due to weaker hold on the electron cloud.

Arrange the molecules based on the trends identified: molecules with heavier atoms and more electrons will be more polarizable.

For boiling points, recognize that molecules with higher polarizability will have stronger London dispersion forces, which are a type of intermolecular force.

Predict the order of boiling points based on the strength of the intermolecular forces, where stronger dispersion forces lead to higher boiling points.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Polarizability

Polarizability refers to the ability of an electron cloud around an atom or molecule to be distorted by an external electric field, leading to the formation of temporary dipoles. Larger atoms or molecules with more electrons generally exhibit higher polarizability due to their more diffuse electron clouds. This property is crucial for understanding intermolecular forces and how they influence physical properties like boiling points.

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that influence their physical properties, including boiling points. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonding. In the context of the given molecules, the strength of these forces, which is affected by polarizability, will determine the order of boiling points.

Boiling Point Trends

Boiling point trends can be predicted based on molecular size, shape, and the strength of intermolecular forces. Generally, larger and more polarizable molecules have higher boiling points due to stronger London dispersion forces. Understanding these trends allows for the comparison of different substances and helps predict their physical behavior under varying conditions.

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Related Practice

Textbook Question

Which type of intermolecular force accounts for each of these differences? (b) Xe is a liquid at atmospheric pressure and 120 K, whereas Ar is a gas under the same conditions. (c) Kr, atomic weight 84 amu, boils at 120.9 K, whereas Cl₂, molecular weight about 71 amu, boils at 238 K.

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Textbook Question

Which type of intermolecular force accounts for each of these differences? (a) CH₃OH boils at 65 °C; CH₃SH boils at 6 °C. (d) Acetone boils at 56 °C, whereas 2-methylpropane boils at -12 °C.

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Textbook Question

True or false: (b) For the noble gases the dispersion forces decrease while the boiling points increase as you go down the column in the periodic table.

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Textbook Question

Which member in each pair has the greater dispersion forces? (a) H₂O or H₂S,

Textbook Question

True or false: (e) The larger the atom, the more polarizable it is.

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Textbook Question

Describe the intermolecular forces that must be overcome to convert these substances from a liquid to a gas: (a) SO₂, (b) CH₃COOH, (c) H₂S.

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