Intermolecular Forces

What are intermolecular forces?

Forces that exist between molecules.

What are the four main types of intermolecular forces (from weakest to strongest)?

1⃣ Dispersion forces (London dispersion)
2⃣ Dipole–dipole forces
3⃣ Hydrogen bonding (special dipole–dipole)
4⃣ Ion–dipole forces

What type of intermolecular force occurs between an ion and a nearby polar molecule?

Ion-Dipole Force

What happens when an ion and a nearby polar molecule attract each other?

It results in an ion-dipole interaction.

Give an example of an ion-dipole interaction.

NaCl + H₂O

What type of molecules are involved in an ion-dipole force?

An ion and a polar molecule (dipole).

What property of ionic compounds is explained by ion-dipole forces?

The solubility of ionic compounds in water (a polar solvent).

In the example NaCl + H₂O, what role does water play?

Water acts as the polar molecule that interacts with the ions of NaCl.

What are dipole–dipole forces?

Forces of attraction between oppositely charged ends of polar molecules.

What does the strength of a dipole–dipole force depend on?

The polarity of the molecule.

When do dipole–dipole interactions occur?

When polar molecules lie near one another and one end of a dipole attracts the oppositely charged end of another dipole.

What type of molecules experience dipole–dipole forces?

Polar molecules.

What is hydrogen bonding?

A special type of dipole–dipole interaction between a hydrogen atom bonded to a highly electronegative nonmetal atom.

Which elements can form hydrogen bonds?

Nitrogen (N), Oxygen (O), and Fluorine (F).

Why is hydrogen bonding important?

It greatly affects the properties of water, proteins, and the structure of DNA.

How does hydrogen bonding differ from a regular dipole–dipole interaction?

It is stronger and occurs only when hydrogen is bonded to highly electronegative atoms like N, O, or F.

What are London dispersion forces?

Weak attractions between instantaneous dipoles and induced dipoles in nonpolar molecules

When do London dispersion forces occur?

When temporary dipoles are formed due to the random movement of electrons in atoms or molecules.

What types of molecules experience London dispersion forces?

All molecules, but they are dominant in nonpolar molecules such as O₂, N₂, and Cl₂.

What causes the formation of an instantaneous dipole?

The uneven distribution of electrons at a given moment in an atom or molecule.

How strong are London dispersion forces compared to other intermolecular forces?

They are the weakest type of intermolecular force.

What are Van der Waals forces also known as?

London forces.

What causes Van der Waals forces?

Momentary changes in electron density in a molecule.

What type of compounds have only Van der Waals forces as their attractive forces?

Nonpolar compounds.

How can a nonpolar molecule like CH₄ exhibit Van der Waals forces?

At any instant, its electron density may become unsymmetrical, creating a temporary dipole that induces a dipole in another molecule.

What do these temporary dipoles result in?

Weak interactions known as Van der Waals forces.

What are examples of substances where Van der Waals interactions occur?

Geckos, proteins, graphite sheets, water, and polymers.

What types of forces are included under Van der Waals forces?

Forces of dispersion and dipole–dipole interactions.

Why do real gases not follow the ideal gas law?

Because real gases are affected by intermolecular forces and have molecules that occupy volume

What are crystalline solids?

Solids with particles arranged in a well-defined, organized pattern.

What is an example of a crystalline solid?

Table salt

What are allotropes?

Different structural forms in which some crystalline solids can exist.

What are amorphous solids?

Non-crystalline solids whose particles are not arranged in a regular pattern.

Give examples of amorphous solids.

Charcoal, rubber, glass, cotton candy

How is amorphous carbon (charcoal) produced?

By the destructive distillation of wood.

What is carbon black?

A powdery soot formed from the incomplete combustion of natural gas.

What are the common uses of carbon black?

As pigment in printing inks, carbon paper, and photocopying machines

What are liquid crystals?

A phase of matter whose order is between that of a liquid and a crystalline solid.

What is the shape of the molecules that make up liquid crystals?

Long or cylindrical.

What type of intermolecular forces exist in liquid crystals?
 

Dispersion forces, dipole–dipole forces, and hydrogen bonding.

What are the three main types of liquid crystals?

1. Nematic phase

2. Cholesteric phase

3. Smectic phase.

What is the nematic phase?

It is the least ordered type of liquid crystal where molecules lie in the same direction but their ends are not aligned.

What is the molecular arrangement in the nematic phase?

Molecules are parallel in orientation but not positioned in a specific order.

What is the cholesteric phase?

A type of liquid crystal where molecules are arranged in layers, with each layer rotated at a fixed angle relative to the next one.

How is the cholesteric phase similar to the nematic phase?

Both have molecules aligned in the same general direction, but the cholesteric phase has an additional layered rotation.

What is a unique property of cholesteric crystals?

Their color changes with temperature

What are cholesteric crystals commonly used for?

In applications where color changes with temperature.

What is the smectic phase?

A type of liquid crystal close to the solid phase where molecules are arranged in layers.

How do molecules behave in the smectic phase?

They are ordered in layers but can float or move freely between these layers.

Arrange the phases of liquid crystals in order of increasing opacity.

Nematic → Smectic → Cholesteric

What are examples of devices that use liquid crystals?

LCD screens, televisions, cellphones, computers, and watches.

How do liquid crystals respond to temperature changes?

They turn solid when cold and become liquid when hot.