IB Chemistry Topic S2.2.9 Intermolecular Forces and Physical Properties Revision Notes (SL/HL)

IB Chemistry Topic S2.2.9 Intermolecular Forces and Physical Properties Revision Notes (SL/HL) | RevisionDojo

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Relative Strengths of Intermolecular Forces and Their Impact on Properties

Types of Intermolecular Forces

Definition

Intermolecular forces

Intermolecular forces are the electrostatic attractions between molecules.

While they are much weaker than covalent or ionic bonds, they strongly influence a substance’s physical properties.
Recapping from S2.2.8 Intermolecular forces, the main types, ranked by increasing strength, are:
1. London Dispersion Forces (LDFs): Found in all molecules, these forces arise from temporary dipoles caused by the random movement of electrons.
2. Dipole-Dipole Forces: Occur between polar molecules with permanent dipoles.
3. Hydrogen Bonding: A stronger type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F).
The relative strengths can be summarized as:
$$
\text{London Dispersion Forces}< \text{Dipole-Dipole Forces} < \text{Hydrogen Bonding}
$$

Tip

Hydrogen bonds are not true chemical bonds but are significantly stronger than other intermolecular forces.
This strength explains why substances like water have unusually high boiling points.

Why Does Strength Vary?

The strength of intermolecular forces depends on:

Molecular Size and Shape (LDFs): Larger molecules with more electrons have stronger LDFs because their electron clouds are more easily polarized.
Polarity (Dipole-Dipole): Molecules with a greater difference in electronegativity between bonded atoms exhibit stronger dipole-dipole interactions.
Hydrogen Bonding: The strength depends on how electronegative the atom bonded to hydrogen is, as well as how many hydrogen bonds can form.

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Relative Strengths of Intermolecular Forces and Their Impact on Properties

Types of Intermolecular Forces

Definition

Intermolecular forces

Intermolecular forces are the electrostatic attractions between molecules.

While they are much weaker than covalent or ionic bonds, they strongly influence a substance’s physical properties.
Recapping from S2.2.8 Intermolecular forces, the main types, ranked by increasing strength, are:
1. London Dispersion Forces (LDFs): Found in all molecules, these forces arise from temporary dipoles caused by the random movement of electrons.
2. Dipole-Dipole Forces: Occur between polar molecules with permanent dipoles.
3. Hydrogen Bonding: A stronger type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F).
The relative strengths can be summarized as:
$$
\text{London Dispersion Forces}< \text{Dipole-Dipole Forces} < \text{Hydrogen Bonding}
$$

Tip

Hydrogen bonds are not true chemical bonds but are significantly stronger than other intermolecular forces.
This strength explains why substances like water have unusually high boiling points.

Why Does Strength Vary?

The strength of intermolecular forces depends on:

Molecular Size and Shape (LDFs): Larger molecules with more electrons have stronger LDFs because their electron clouds are more easily polarized.
Polarity (Dipole-Dipole): Molecules with a greater difference in electronegativity between bonded atoms exhibit stronger dipole-dipole interactions.
Hydrogen Bonding: The strength depends on how electronegative the atom bonded to hydrogen is, as well as how many hydrogen bonds can form.

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry S2.2.9 Intermolecular Forces and Physical Properties Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Relative Strengths of Intermolecular Forces and Their Impact on Properties

Types of Intermolecular Forces

Why Does Strength Vary?

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Relative Strengths of Intermolecular Forces and Their Impact on Properties

Types of Intermolecular Forces

Why Does Strength Vary?

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