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Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and StructureProblem 34c,d
Chapter 4, Problem 34c,d

Explain why
c. H2O (100 °C) has a higher boiling point than HF 120 °C2.
d. HF 120 °C2 has a higher boiling point than NH3 (-33 °C).

Verified step by step guidance
1
Step 1: Begin by understanding the concept of boiling point. The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure, allowing the substance to transition from liquid to gas. Boiling points are influenced by intermolecular forces such as hydrogen bonding, dipole-dipole interactions, and London dispersion forces.
Step 2: Analyze part (c) regarding H2O and HF. Both molecules exhibit hydrogen bonding, which is a strong intermolecular force. However, H2O can form up to two hydrogen bonds per molecule due to its two hydrogen atoms and two lone pairs on oxygen, whereas HF can only form one hydrogen bond per molecule because it has one hydrogen atom and three lone pairs on fluorine. This ability to form more hydrogen bonds in H2O results in stronger intermolecular forces, leading to a higher boiling point for H2O (100 °C) compared to HF.
Step 3: Examine part (d) regarding HF and NH3. HF has a higher boiling point than NH3 because HF exhibits stronger hydrogen bonding. Fluorine is the most electronegative element, which makes the hydrogen bond in HF extremely strong. In contrast, NH3 forms hydrogen bonds through its nitrogen atom, which is less electronegative than fluorine, resulting in weaker hydrogen bonds. This difference in hydrogen bond strength accounts for HF's higher boiling point (120 °C) compared to NH3 (-33 °C).
Step 4: Consider the role of molecular structure and electronegativity in determining boiling points. The electronegativity of the atoms involved in hydrogen bonding directly affects the strength of the bond. Fluorine's high electronegativity in HF leads to stronger hydrogen bonds than nitrogen's in NH3, while oxygen's ability to form multiple hydrogen bonds in H2O further enhances its boiling point.
Step 5: Summarize the findings: H2O has a higher boiling point than HF due to its ability to form more hydrogen bonds per molecule, and HF has a higher boiling point than NH3 due to the stronger hydrogen bonds formed by fluorine compared to nitrogen. These differences in intermolecular forces are key to understanding the boiling point trends.

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Key Concepts

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

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. In the case of H2O, each water molecule can form up to four hydrogen bonds, leading to a higher boiling point due to the increased energy required to break these interactions.
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Molecular Polarity

Molecular polarity refers to the distribution of electrical charge across a molecule, which affects its physical properties, including boiling point. HF is a polar molecule, but its boiling point is influenced by the strength and number of hydrogen bonds it can form compared to H2O. Despite HF having a higher molecular weight, the extensive hydrogen bonding in water results in a higher boiling point.
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Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles (atoms, molecules, or ions). These forces include hydrogen bonds, dipole-dipole interactions, and London dispersion forces. The boiling point of a substance is directly related to the strength of these forces; stronger intermolecular forces result in higher boiling points, as seen when comparing HF and NH3.
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Related Practice
Textbook Question

Explain why

a. H2O (100 °C) has a higher boiling point than CH3OH 165 °C2.

b. H2O (100 °C) has a higher boiling point than NH3 (-33 °C).

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Textbook Question

The effectiveness of a barbiturate as a sedative is related to its ability to penetrate the nonpolar membrane of a cell. Which of the following barbiturates would you expect to be the more effective sedative?

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Textbook Question

Which of the preceding compounds forms hydrogen bonds with a solvent such as ethanol?

1. CH3CH2OCH2CH2OH

2. CH3CH2N(CH3)2

3. CH3CH2CH2CH2Br

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Textbook Question

Which of the preceding compounds forms hydrogen bonds with a solvent such as ethanol?

4. CH3CH2CH2NHCH3 

5. CH3CH2CH2COOH

6. CH3CH2CH2CH2F

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Textbook Question

Rank the following compounds from highest boiling to lowest boiling:

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Textbook Question

In which solvent would cyclohexane have the lowest solubility: 1-pentanol, diethyl ether, ethanol, or hexane?

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