IB Chemistry Topic S2.2.8 Intermolecular Forces Revision Notes (SL/HL)

IB Chemistry Topic S2.2.8 Intermolecular Forces Revision Notes (SL/HL) | RevisionDojo

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London Dispersion Forces: The Universal Attraction

Did you know that even noble gases like helium and nonpolar molecules such as nitrogen gas $N_2$ experience forces of attraction?
These forces, called London dispersion forces (LDFs), arise from temporary instantaneous dipoles.

Definition

London dispersion forces

London dispersion forces are weak intermolecular forces caused by temporary dipoles that arise due to momentary fluctuations in electron distribution within atoms or molecules.

What Causes London Dispersion Forces?

Electrons within atoms or molecules are always in motion.
At any instant, this motion can create an uneven distribution of electron density, forming a temporary dipole, a region of partial positive charge $δ^+$ and partial negative charge $δ^-$.
This temporary dipole can induce a similar dipole in a neighboring molecule, leading to a weak electrostatic attraction between the two.

Analogy

Think of London dispersion forces as a fleeting handshake between molecules.
Even if they aren’t "friends" (polar), they can still briefly connect due to these temporary charges.

Factors Affecting London Dispersion Forces

Number of Electrons:
1. Molecules with more electrons have greater polarizability, meaning their electron clouds are more easily distorted, resulting in stronger LDFs.
2. For instance, iodine $I_2$ has stronger dispersion forces than fluorine $F_2$, leading to a higher boiling point.
Molecular Shape:
1. Elongated molecules like pentane have larger surface areas for contact, resulting in stronger LDFs compared to compact molecules like neopentane, even if their molecular masses are similar.

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IB Chemistry S2.2.8 Intermolecular Forces Notes

London Dispersion Forces: The Universal Attraction

What Causes London Dispersion Forces?

Factors Affecting London Dispersion Forces

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London Dispersion Forces: The Universal Attraction

What Causes London Dispersion Forces?

Factors Affecting London Dispersion Forces

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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 S2.2.8 Intermolecular Forces 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

London Dispersion Forces: The Universal Attraction

What Causes London Dispersion Forces?

Factors Affecting London Dispersion Forces

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Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

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London Dispersion Forces: The Universal Attraction

What Causes London Dispersion Forces?

Factors Affecting London Dispersion Forces

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

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