Problem 18 - Chemistry: An Atoms-Focused Approach

Question

Why do the strengths of London dispersion forces increase with increasing molecular size?

Step-by-Step Solution

Answer

Answer: The strength of London dispersion forces increases with increasing molecular size because larger molecules have more electrons and larger electron clouds, making them more susceptible to temporary fluctuations in electron distribution. These fluctuations produce stronger temporary dipoles, which in turn lead to stronger London dispersion forces between molecules.

Step 1: Defining London dispersion forces

London dispersion forces, also known as van der Waals forces or instantaneous dipole-induced dipole forces, are weak intermolecular forces present between all molecules, regardless of their polarity. These forces arise due to temporary fluctuations in electron distribution within the electron cloud of a molecule, which can create transient dipoles. These transient dipoles can then induce dipoles in neighboring molecules, resulting in attractive forces between them.

Step 2: Explaining the role of electron distribution

The strength of London dispersion forces is proportional to the number of electrons a molecule has since a larger electron cloud is more susceptible to temporary fluctuations in electron distribution. This means that larger molecules with more electrons will have more significant fluctuations in their electron clouds, which can create stronger temporary dipoles, leading to stronger dispersion forces.

Step 3: Connecting molecular size to the strength of London dispersion forces

As molecular size increases, the number of electrons within the molecule usually increases as well. This means that molecules with larger molecular sizes possess larger electron clouds and are more likely to experience fluctuations in their electron distribution. These fluctuations ultimately lead to stronger temporary dipoles and, in turn, stronger London dispersion forces between these larger molecules. In conclusion, the strength of London dispersion forces increases with increasing molecular size because larger molecules have more electrons and larger electron clouds, which are more susceptible to temporary fluctuations in electron distribution. These fluctuations produce stronger temporary dipoles and, as a result, stronger London dispersion forces between molecules.

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that hold molecules together in various states of matter such as solid, liquid, and gas. These forces are vital for understanding many physical properties of substances, like boiling and melting points. There are different types of intermolecular forces, including:

• Dispersion forces (London dispersion forces)
• Dipole-dipole interactions
• Hydrogen bonding

London dispersion forces are a specific type of intermolecular force present in all molecular interactions. They originate from temporary fluctuations in a molecule's electron distribution, resulting in transient dipoles, which induce dipoles in nearby molecules. Despite being weaker than other types of forces, their cumulative effect is significant, especially in large molecules.

Molecular Size

Molecular size refers to the volume or space occupied by a molecule. Larger molecules typically have more atoms and electrons. As molecular size increases, so does the complexity of its electron cloud. Molecules with larger sizes:

• Have bigger electron clouds
• Are more polarizable
• Experience stronger London dispersion forces

This increase in polarizability means that the electron cloud can be distorted more easily, leading to stronger temporary dipoles and thus stronger dispersion forces.

Electron Cloud

The electron cloud is the region around an atom or molecule where its electrons reside. These clouds are constantly shifting, as electrons move within them. The size and shape of an electron cloud affect how the molecule interacts with others. For instance:

• Large electron clouds are more likely to experience fluctuations
• Such fluctuations lead to temporary dipoles
• Hence, they contribute to stronger London dispersion forces

As the molecular size increases, the electron cloud becomes larger, making these fluctuations and resulting temporary dipoles more pronounced.

Temporary Dipoles

Temporary dipoles form when electrons within a molecule are unevenly distributed at a given time, causing a temporary shift in electrical charge. This phenomenon can induce neighbouring molecules to form dipoles in response, even if they are originally non-polar. The process involves:

• Fluctuating electron distribution within a molecule
• This creates an instantaneous dipole
• Which can induce dipoles in nearby molecules

These transient dipoles result in attractions called London dispersion forces. The presence and strength of these dipoles largely depend on the size and flexibility of the electron cloud, which is influenced by the molecular size.