Liquids and Solids: Kinetic Molecular Model and Intermolecular Forces

General Chemistry II

Kinetic Molecular Model

• describes matter as made up of tiny particles in constant motion

• explains the physical properties of matter, particularly gases, in terms of the motion and behavior of tiny particles

• Particles are in constant, random motion, and their collisions and energy levels determine macroscopic properties like temperature and pressure.

 Kinetic Molecular Model: SOLIDS

• solid particles are closely packed in a fixed, ordered structure and experience strong intermolecular forces, causing them to vibrate in place but not move around freely.

• solids have a definite shape and volume due to these strong forces and limited particle movement.

Kinetic Molecular Model: LIQUIDS

• particles are in constant, random motion, and their kinetic energy is related to the liquid’s temperature.

• the intermolecular forces are strong enough to keep the particles close together, but not so strong as to fix them in a specific position, allowing them to move past each other and giving liquids their fluidity  

Characteristic Properties of Gases, Liquids, and Solids

Intermolecular Forces

• the attractive or repulsive forces that exist between molecules, influencing their physical properties

• They are weaker than intramolecular forces (like covalent bounds) that holds atoms together within a molecule.

London Dispersion Forces

• also known as dispersion forces or instantaneous dipole-induced dipole force

• type of intermolecular force - that is, a force that occurs between molecules

• the weakest of all the van der Waals forces, but they are present in all molecules, whether polar or nonpolar

What Causes London Dispersion Forces?

• Electrons are constantly moving around the nucleus of atoms and molecules

• At any instant, the distribution of these electrons might be uneven - this creates a temporary (instantaneous) dipole.

• This temporary dipole can influence nearby atoms or molecules, causing them to also form induced dipoles.

• These two temporary dipoles can then attract each other, and that attraction is the London dispersion force.

Key Characteristics

• Weak, but universal: Even noble gases like helium and argon, and nonpolar molecules like O2 OR ch4, exhibit London dispersion forces.

• Strength increases with:

  Molecular size/mass: Larger atoms/molecule have more electrons, so their electron cloud is more polarizable, which increases dispersion force

  Surface area: Long, straight molecules have more contact area and thus more opportunity for dispersion interactions compared to compact or branched molecules. 

Dipole-Dipole Forces

• attractive force that occur between the positive end of one polar molecule and the negative end of another polar molecule

• exist only in polar-molecules - that is, molecules that have a permanent dipole

How Are Dipole-Dipole Forces Formed?

• Molecules have polar bonds due to differences in electronegativity 

• These bonds cause a separation of charge, making one end of the molecule partially positive (symbol) and the other partially negative (symbol)

• In a group of such polar molecules, the positive end of one molecule is electrostatically attracted to the negative end of a neighboring molecule

• This attraction between opposite charges is the dipole-dipole force.

Ion-Dipole Forces

• Attractive forces that occur between a fully charged ion (either a cation or anion) and the partially charged end of a polar molecule (dipole).

How do Ion-Dipole Forces form?

1. An ion( from an iconic compound like  NA + or CI -) is present in a polar substance (usually a liquid like water).

2. Water molecules (or any polar molecules) have partial positive and partial negative changes due to uneven sharing of electrons.

3. The ion is attracted to the oppositely charged side of the polar molecule

4. 1. Cations (eg -, NA +) are attracted to the negative (symbol) side of the dipole (like oxygen in H20).

5. 2. Anions (e.g., CI -) are attracted to the positive (symbol) side (like hydrogen in H20)

The interaction is called an ion-dipole force.

Strength of Ion-Dipole Forces

• Stronger than dipole-dipole and dispersion forces because:

• Ions carry full charges (+1, -1, etc.)

• Dipoles only have partial charges (symbol)

• More polar solvents (like water) lead to stronger ion-dipole forces.

Hydrogen Bonding

A strong dipole-dipole attraction that occurs when a hydrogen atom, covalently bonded to a highly electronegative atom ( such as nitrogen (N), oxygen (O), or fluorine (F)), is attracted to another electronegative atom with a lone pair of electrons.

Conditions for Hydrogen Bonding

Hydrogen bonding can only occur when three conditions are met:

1. Hydrogen (H) is directly bonded to N, O, or F.

2. The N, O, or F atom is small and highly electronegative, which makes the H-X bond very polar.

3. There is a lone pair of electrons on another nearby N, O, or F atom that attracts the hydrogen.

How Does it Form?

Let’s look at water (H20) as an example: Each water molecule has two polar O-H bonds. Oxygen is more electronegative, so it pulls electron density toward itself, making

H -> partially positive (symbol)

O -> partially negative (symbol)

The hydrogen (symbol) of one water molecule is attracted to the oxygen (symbol) of another

Properties of Liquids

Liquids -> 

Liquids

Solids

• Particles in solids are not moving in the same manner as those in liquids or gases

• Particles simply vibrate and rotate in place rather than move about

• Solids generally held together by iconic or strong covalent bonding, and the attractive forces between the atoms, ions, or molecules in solids are very strong

Solids are classified as:

1. Amorphous solids

2. Crystalline solids

Amorphous SOLID

• Do not have much order in their structures

An amorphous solid does not possess a well-defined arrangement and long-range molecular order.

An amorphous solid Melts Over a Ranger. They soften and melt over a range of temperatures rather than at a specific point.

Irregular Fracture When broken they fracture coincidentally 

Regular Edgers and forces

When broken

Crystalline SOLID

• hat make up the solid exist in a regular, well-defined arrangement

A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions

A crystalline solid have Sharp melting point. They melt at a specific, defined temperature

Isotropic:

Their properties are generally the same in all directions.

Anisotropic.

Their properties can vary depending on the direction of measurement due to the ordered structure

Crystalline solids

• Processes rigid and long-range order

• Unit cell is the basic repeating structural unit

• Lattice is an ordered array of points describing the arrangement of particles that forn a cyrstal

Types of crystals:

1. Ionic crystals

2. Molecular

3. Covalent

4. Metalic

At lattice points:

• Atoms

• Molecules

• Ions

Ionic crystals

• Lattice points occupied by cations and anions

• Held together by electrostatic attraction

• Hard, brittle, high melting point

• Poor conductor of heat and electricity

Covalent crystals

• Lattice points occupied by atoms

• Held together by covalent bonds

• Hard, high melting point

• Poor conductor of heat and electricity

Molecular crystals

• Lattice points occupied by molecules

• Held together by intermolecular forces

• Soft, low melting point

• Poor conductor of heat and electricity

Metallic crystals

• Lattice points occupied by metal atoms

• Held together by metallic bonds

• Soft to hard, low to high melting point

• Good conductors of heat and electricity