Chem 122 - Intermolecular Forces

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Questions and Answers

Which substance has the highest boiling point due to stronger intermolecular forces?

HCl

MgCl2 (correct)

NaCl

PCl3

What type of intermolecular forces are primarily present in ethanol?

Hydrogen bonding (correct)

Ion-dipole

London dispersion

Dipole-dipole

Which of the following pairs has the compound with the stronger intermolecular forces listed first?

Hexane and 2,2-dimethylbutane

Diethyl ether and Butanol

CH3OH and CH3CH2OH

CH3NH2 and CH3F (correct)

Which intermolecular forces are chiefly responsible for the solubility of NaCl in water?

Ion-dipole forces Signup and view all the answers

Which substance would experience greater dispersion forces due to its molecular structure?

Hexane Signup and view all the answers

What effect does a longer carbon chain have on the boiling point of alcohols, such as CH3OH compared to CH3CH2OH?

Increases boiling point due to dispersion forces Signup and view all the answers

What type of bonding forces specifically result from the attraction between H and small electronegative atoms?

Hydrogen bonding forces Signup and view all the answers

How do dipole-dipole forces compare to hydrogen bonding in terms of strength?

Hydrogen bonding is much stronger than dipole-dipole forces Signup and view all the answers

What defines a saturated solution?

Has the maximum amount of solute dissolved in a given solvent Signup and view all the answers

Which factor does not influence the strength of dispersion forces?

Presence of double bonds Signup and view all the answers

What causes dipole-dipole forces between molecules?

Permanent polarization of the molecules Signup and view all the answers

How does an increase in temperature generally affect vapor pressure?

It increases the vapor pressure Signup and view all the answers

Which statement about polarizability is true?

Increased polarizability results in stronger attractions. Signup and view all the answers

When comparing intermolecular forces, which force is generally considered the weakest?

Dispersion (London) forces Signup and view all the answers

Why do larger molecules tend to have stronger dispersion forces?

They have a larger and more polarizable electron cloud. Signup and view all the answers

Which of the following substances would exhibit hydrogen bonding?

H2O Signup and view all the answers

What is the primary reason why salt dissolves in water?

Ionic attractions between salt ions are overcome by ion-dipole attractions. Signup and view all the answers

How do solute-solute and solvent-solvent forces affect solution formation?

They are replaced by solute-solvent attractions with similar magnitudes. Signup and view all the answers

What distinguishes the polarity of alcohols from other types of solutes?

Alcohols contain a polar -OH group that forms hydrogen bonds. Signup and view all the answers

In the context of intermolecular forces, what does the phrase 'like dissolves like' refer to?

Polar substances will dissolve other polar substances. Signup and view all the answers

Which factor enhances the solubility of smaller alcohols in water?

Hydrogen bonding between the solute and solvent. Signup and view all the answers

What type of intermolecular forces primarily exists between ions and water molecules in a salt solution?

Ion-dipole attractions Signup and view all the answers

What role do dispersion forces play in the interaction between alcohols and nonpolar substances like hexane?

They allow for weak interactions but do not promote solubility. Signup and view all the answers

What happens to the intermolecular forces in a solution when a soluble ionic compound is added to water?

Solvent forces become dominant and replace solute forces. Signup and view all the answers

Study Notes

Chem 122 - Fall 2024 - Dr. Rita Hatfield

Chem 122 Final Exam is Tuesday, December 10th, from 6-8 PM.
Discussion Worksheet 15 is posted.
Week 14 homework is due tonight.
Week 15 homework is due Friday, December 6th.
Chapter 12, Section 3 covers Intermolecular Forces.
Chapter 12 Sections (in the lecture):
- 12.1: Overview of Physical States and Phase Changes
- 12.2: Quantitative Aspects of Phase Changes
- 12.3: Types of Intermolecular Forces
- 12.4: Properties of the Liquid State

Nature of Intermolecular Forces

Intermolecular forces are attractions between molecules or between ions and molecules.
Intermolecular forces are weaker than bonding forces. Bonding forces involve smaller charges that are farther apart.
There are different types of intermolecular forces, each with varying strength and how they arise.

Intermolecular Attractions

The strength of attractions between particles determines the state of a substance (solid, liquid, or gas) at room temperature.
Moderate to strong attractive forces result in solids or liquids at room temperature.
Stronger attractive forces lead to higher boiling and melting points.

Comparison of Bonding Forces (intramolecular)

Ionic: Cation-anion attraction (400-4000 kJ/mol). Examples include NaCl.
Covalent: Nuclei-shared electron pair attraction (150-1100 kJ/mol). Examples include H2.
Metallic: Cation-delocalized electron attraction (75-1000 kJ/mol). Example includes Fe. Bonding forces are much greater than intermolecular forces (IMFs).

Comparison of Non-Bonding Forces (Intermolecular Forces- IMFs):

Overall strongest: Ion-dipole (40-600 kJ/mol). Examples include Na+ and H2O.
H-bond (10-40 kJ/mol): Polar bond to a highly electronegative atom (F, O, or N) with lone pairs. Examples include H2O, organic molecules with -OH, NH2, or -FH.
Dipole-dipole (5-25 kJ/mol): Permanent dipole-dipole attractions.
Ion-induced dipole (3-15 kJ/mol): Ionic charges distort polarizable electron clouds.
Dipole-induced dipole (2-10 kJ/mol): Polarizable electron clouds created through dipole interaction with a polar molecule.
Dispersion (London) (0.05-40 kJ/mol): Temporary fluctuations in electron distribution. All substances have dispersion forces.

Factors Affecting Strength of Induced Dipole

Polarizability: Larger molar mass = larger electron cloud = increased polarizability = stronger attractions.
Molecular Shape: More surface-to-surface contact = larger induced dipole = stronger attraction.

Effect of Molecular Size on Dispersion Force

Increasing molecular size tends to increase dispersion forces.

Effect of Molecular Shape on Dispersion Force

Linear shape (like n-pentane) allows for more contact points, resulting in stronger dispersion forces than branched shapes (like neopentane).

Dipole-Dipole Forces

Attractive forces due to permanent polarization in molecules caused by unequal electron distribution and molecular structure.

Hydrogen-Bonding Forces

A special type of dipole-dipole force; occurs when hydrogen is bonded to a small electronegative atom with lone pairs (N, O, or F).

Vapor Pressure and Boiling Point

Boiling point is the temperature when vapor pressure equals external pressure. Normal boiling point is at a pressure of 760 torr.
Higher external pressure increases the boiling point.
Vapor pressure increases with temperature.
Weaker intermolecular forces lead to higher vapor pressures.

Summarizing VP Trends in Graph – 3 different molecules

Vapor pressure decreases with increasing intermolecular forces strength.
Normal boiling points follow IMF trends (stronger IMF = higher boiling point).

Intermolecular Forces and Solubility

"Like dissolves like." Polar solvents dissolve polar solutes; nonpolar solvents dissolve nonpolar solutes.
Substances with similar IMF types will be soluble in each other. A solution forms when solute-solute and solvent-solvent attractions are replaced by solute-solvent attractions.
Factors affecting solubility include temperature etc.
A saturated solution is one where the maximum amount of solute has dissolved under given conditions.
Important intermolecular forces in cases of ionic or polar solutem: Ion-dipole
The strengths of intermolecular forces affect how well molecules or substances dissolve into each others.

Dual Polarity and Effects on Solubility

Alcohols have a polar -OH group and a nonpolar hydrocarbon chain.
The -OH group interacts with water through hydrogen bonding.
The hydrocarbon chain interacts with nonpolar solvents through dispersion forces.

"Like Dissolves Like": Solubility of Methanol in Water

Methanol solubility in water is due to hydrogen bonding between the polar methanol molecule and water molecules. Small changes to the structure of the molecule such as an additional CH2 grouping, can dramatically alter the solubility, even with similar molar masses.

Solubility (mol alcohol/1000 g solvent) of a Series of Alcohols in Water and Hexane

Solubility trend in water increases as the number of CH3 grows.
Solubility in hexane increases as the number of CH3 groups grows.

Examples of IMFs in Biological Macromolecules

Intermolecular forces are important for the structure and function of biological macromolecules like DNA and proteins. A knowledge of the forces involved provides information on the three dimensional structures of DNA, RNA, proteins, etc.

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Description

This quiz covers Chapter 12, Section 3 of Chem 122, focusing on intermolecular forces. Understand the various types of intermolecular attractions and their implications on physical states and phase changes. Prepare effectively for the final exam by testing your knowledge on these crucial concepts.