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Ch.8 - Covalent Compounds: Bonding Theories and Molecular Structure
All textbooksMcMurry 8th EditionCh.8 - Covalent Compounds: Bonding Theories and Molecular StructureProblem 125
Chapter 8, Problem 125

The water molecule has similar bond vibrations to carbon dioxide. Decide whether the symmetric, asymmetric, and bending vibrations in water will result in the absorption of IR radiation.

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1
Step 1: Understand the concept of IR absorption. Infrared (IR) radiation is absorbed by molecules when the radiation matches the energy of a vibrational mode of the molecule. This causes the molecule to vibrate more intensely. For a molecule to absorb IR radiation, the vibration must result in a change in the molecule's dipole moment.
Step 2: Analyze the water molecule. Water (H2O) is a bent molecule, with oxygen in the center and hydrogen atoms on the sides. It has a net dipole moment because of the difference in electronegativity between hydrogen and oxygen.
Step 3: Consider the types of vibrations. Water has three fundamental types of vibrations: symmetric stretching, asymmetric stretching, and bending. Symmetric stretching involves the two O-H bonds stretching or compressing in unison. Asymmetric stretching involves one O-H bond stretching while the other compresses. Bending involves the angle between the two O-H bonds changing.
Step 4: Determine if the vibrations change the dipole moment. Symmetric stretching does not change the dipole moment of water because the positive and negative charges remain balanced. However, asymmetric stretching and bending do change the dipole moment because they cause an imbalance in the charge distribution.
Step 5: Conclude which vibrations absorb IR radiation. Since IR radiation is absorbed when the vibration changes the dipole moment, only the asymmetric stretching and bending vibrations of water will result in the absorption of IR radiation. The symmetric stretching vibration will not.

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Key Concepts

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

Molecular Vibrations

Molecular vibrations refer to the oscillations of atoms within a molecule around their equilibrium positions. These vibrations can be classified into different types, such as symmetric stretching, asymmetric stretching, and bending. The specific vibrational modes of a molecule determine how it interacts with electromagnetic radiation, particularly infrared (IR) radiation.
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Molecular Polarity

Infrared Absorption

Infrared absorption occurs when a molecule absorbs IR radiation, leading to an increase in its vibrational energy. This process is crucial for understanding molecular interactions with light, as only certain vibrational modes that result in a change in the dipole moment of the molecule can absorb IR radiation. Thus, the ability of a molecule to absorb IR is directly linked to its vibrational characteristics.
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Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule, influencing its polarity. For a molecule to absorb IR radiation, its vibrational modes must result in a change in the dipole moment. Water, with its bent structure, exhibits changes in dipole moment during its vibrational modes, allowing it to absorb IR radiation effectively.
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Related Practice
Textbook Question

Cyclooctatetraene dianion, C8H82-, is an organic ion with the structure shown. Considering only the p bonds and not the s bonds, cyclooctatetraene dianion can be described by the following energy diagrams of its p molecular orbitals:

(a) What is the hybridization of the 8 carbon atoms?

Textbook Question

Cyclooctatetraene dianion, C8H82-, is an organic ion with the structure shown. Considering only the p bonds and not the s bonds, cyclooctatetraene dianion can be described by the following energy diagrams of its p molecular orbitals:

(b) Three of the p molecular orbitals are bonding, three are antibonding, and two are nonbonding, meaning that they have the same energy level as isolated p orbitals. Which is which?

Textbook Question

Cyclooctatetraene dianion, C8H82-, is an organic ion with the structure shown. Considering only the p bonds and not the s bonds, cyclooctatetraene dianion can be described by the following energy diagrams of its p molecular orbitals:

(c) Complete the MO energy diagram by assigning the appropriate numbers of p electrons to the various molecular orbitals, indicating the electrons using up/down arrows 1c T2.

Textbook Question
Bond vibrations for the symmetric and asymmetric stretch in methane are illustrated below. Decide whether each vibration will result in the absorption of IR radiation. Arrows indicate the movement of atoms during the vibration.

Textbook Question
Values of Ea = 6.3 kJ>mol and A = 6.0 * 108>1M # s2 have been measured for the bimolecular reaction: NO1g2 + F21g2S NOF1g2 + F1g2 (b) The product of the reaction is nitrosyl fluoride. Its formula is usually written as NOF, but its structure is actually ONF. Is the ONF molecule linear or bent?
Textbook Question
(b) When xenon absorbs 801 kJ/mol of energy, it is excited into a higher-energy state in which the outermost elec-tron has been promoted to the next available subshell. Write the electron configuration for this excited xenon.