Chemistry Chapter on Solids and Liquids

Kinetic Molecular Model of Solids and Liquids

• The kinetic molecular theory explains the states of matter. It posits that matter is composed of tiny particles that are always in motion. This theory explains properties and behaviors of solids, liquids, and gases.
• The kinetic molecular model explains the properties of solids and liquids in terms of intermolecular forces of attraction and the kinetic energy of individual particles.

Intermolecular Forces

• Intermolecular forces are attractive forces between neighboring particles of one or more substances. They pull particles together.

Kinetic Energy

• Kinetic energy is the energy that comes from motion. It keeps particles at a distance, moving around. Kinetic energy depends on the temperature of the substance.

Kinetic Molecular Model - Perspective

• A few drops of blue food coloring added to water will spread throughout the water, eventually coloring it. This happens because the particles of water and food coloring are constantly moving due to their kinetic energy.

Kinetic Molecular Model of the Liquid State

• This model explains how liquids behave.

Liquid Properties

• Definite Volume: Fixed volume, does not vary. Liquids cannot expand to fill a container.
• Fluidity: Liquids can flow and adjust to the shape of their containers because their molecules are free to move.
• Density: The molecules of a liquid are relatively close together, making liquids denser than gases.
• Slightly Compressible: Liquids are much less compressible than gases because their particles are closer together.
• Dissolving Ability: Liquids can dissolve solids, liquids, and gases.
• Ability to Diffuse: Liquids diffuse more slowly than gases due to the constant motion of particles.
• Tendency to Evaporate and Boil: Vaporization is the liquid to gas phase change. Evaporation and boiling are the two classifications of vaporization.
• Tendency to Solidify: Freezing is the liquid to solid phase change.

Questions on Liquids

• Why are liquids denser than gases? Molecules are closer together, resulting in more molecules in a given area.
• Why are liquids harder to compress than gases? Molecules are closer together.
• Why do liquids diffuse slower than gases? Molecules are not moving as fast.
• Can a liquid boil without increasing the temperature? Yes, by lowering the atmospheric pressure.

Kinetic Molecular Model of the Solid State

• This model explains how solids behave.

Solid Properties

• Definite Shape and Volume: Solids maintain a definite shape and volume, regardless of the container. This is due to tightly packed particles.
• Non-fluid: Solid particles cannot flow because they are held in relatively fixed positions.
• High Density: Solids are packed more closely than liquids or gases.
• Incompressible: Solid particles are packed so tightly together that there is virtually no space between them.
• Intermolecular Force: Intermolecular forces between particles in solids are stronger than in liquids or gases. This keeps the solid rigid.

Intermolecular Forces of Attraction

• Intermolecular forces (IMF) of attraction are the forces between molecules. These include dipole-dipole, hydrogen bonding, ion-dipole, and London dispersion forces.
• These forces are weaker than covalent bonds that occur within a molecule.

Dipole-Dipole Forces

• Dipole-dipole forces exist between polar molecules.
• Polar molecules have unequal electron densities which creates a dipole (partial positive and partial negative ends).
• Polar molecules attract each other when unlike charges are close together and repel when like charges are close together.

Hydrogen Bonds

• Hydrogen bonds are special dipole-dipole interactions.
• They occur in molecules that contain hydrogen bonded to a small, highly electronegative atom (e.g., N, O, F).
• The electronegative atom attracts electrons toward itself, creating partial negative charges. The hydrogen then acquires a partial positive charge, which strongly attracts the partial negative charge of a neighboring electronegative atom.
• Unlike covalent bonds, hydrogen bonds do not involve sharing of electrons. They are intermolecular forces.

Ion-Dipole Forces

• Ion-dipole forces occur between an ion and a polar molecule.
• The degree of ion-dipole interaction depends on the size and charge of the ion. Cations interact more strongly than anions with dipoles.
• Cations (positive ions) interact more strongly with dipoles.

London Dispersion Forces

• London dispersion forces are intermolecular forces that exist between all atoms and molecules.
• They are the weakest of the intermolecular forces.
• They occur due to temporary variations in electron distribution within a molecule, creating temporary dipoles.
• The size of a molecule affects the London dispersion force between two molecules. Larger molecules have greater London dispersion forces.