QHEM Chapter 14: Liquids and Solids

Solids vs Liquids vs Gases

Solids=high density, slightly compressible, rigid (keeps its shape)

Liquid= Water molecules close together but can move around to some extent

Gas=low density, highly compressible, fill container

Density for ice vs water

Water=1.00g/ml

Ice=0.917 g/ml

Changes of state

Changes of state are physical changes so no chemical bonds are broken.

Phase Changes

When a substance changes from solid to liquid to gas, molecules remain intact. The changes in state are due to changes in the forces among molecules rather than in those within the molecule. This is bc it has to do with the speed and proximity of the water molecules.

Solid to liquid: As energy is added, motions of molecules increase and achieve a liquid’s greater movement and disorder characteristic.

Liquid to Gas: As more energy is added, the gaseous state is eventually reached with individual molecules far apart and interacting relatively little.

Intramolecular forces

WITHIN the molecule. Molecules are formed by sharing electrons between the atoms. Holds the atoms of a molecule together.

Intermolecular forces

forces that occur BETWEEN molecules (bonds between 2 molecules). Intermolecular bonds are weaker than intramolecular forces.

Possible phase changes

Gas—> liquid=condensation

Liquid—> gas =vaporization

Liquid—> solid =freezing

Solid—>liquid = melting (fusion)

Solid—> gas = sublimation

Gas—> solid = deposition

Phase changes have associated energy changes

Molar heat of fusion—enthalpy required to melt 1 mol of a solid substance

Molar heat of vaporization—Enthalpy required to change 1 mol of a liquid to its vapor.

Molar heat of sublimation—enthalpy required to change 1 mol of a solid to its vapor.

UNITS=kJ/mol of heat

Intermolecular forces Three types

Forces that occur between molecules, there are three major forces number one dipole dipole forces number two hydrogen bonding which is a dipole dipole subgroup, and London dispersion forces, a.k.a. induced dipole or Vanderwal’s forces Remember, these are not chemical bonds it is just a force that attracts molecules together

Dipole dipole forces

Affects molecules with polar bonds. dipole moment: Molecules with polar bonds often behave in an electric field as if they had a center of positive charge and a center of negative charge. Molecules with dipole moments can attract each other electrostatically. They lineup so that the positive and negative ends are close to each other. About one percent as strong as covalent or ionic bonds because forces are less stronger than chemical bonds.

Hydrogen bonding

This is the strongest Intermolecular force, but still weak in comparison to chemical bonding. Strong Dipole-dipole forces. Hydrogen is bound or bonded to a highly electronegative atom, nitrogen, oxygen or fluorine. Requires a molecule with OH, NH, or FH bond. It is generally the strongest of the three intermolecular forces. One to 5% of the strength of a normal Covalent bond

London dispersion forces

Non-polar molecules. The forces that exist typically among noble gas atoms, and non-polar molecules The weakest of the three intermolecular forces. Important and non-polar molecules because it is the only forces that occur there. Can appear in all molecules, but we are focusing on non-polar molecules. The greater the surface area, the higher, the boiling point straight bonds, have a higher boiling point than branches In electron models.

Melting and boiling points

In general, the stronger, the intermolecular forces, the higher, the melting and boiling points.

1. Hydrogen bonding

2. Dipole-dipole

3. Dipole-induced dipole (between a polar and nonpolar molecule in which the nonpolar molecule has an induced dipole that attracts to the polar molecule)

4. LDF

Solids

Solids play an important role in our life. Most solids are mixtures of compounds like wood or glass. Some solids are pure substances or compound/elements for example, diamonds (Carbon allotrope). Many substances form, crystalline solids

Crystalline solids

Regular arrangement of their components, producing beautiful regularly shaped crystals. There are three types: ionic solid, Molecular solids, and atomic solids.

Ionic solids

Stable substances with high melting/boiling points that are also hard and brittle. They are held together by strong Electrostatic forces between ions.

Molecular solids

Fundamental particle is a molecule. They melt at relatively low temperatures. They are held together by weak intermolecular forces such as dipole-dipole or LDF’s. They are soft and insulators.

Atomic solids

Fundamental particles is the atom. They’re held together by weak London dispersion forces. Properties very greatly. For example group 8 has low melting points (ex:xenon and argon) while diamonds have very high melting points.

Bonding In metals

Metallic solids represent another type of solids. They are durable have high melting points and are held together by non-directional metal covalent bonds (Call the electron sea model) among the closely packed atoms. An example is alloys

Alloys

A mixture of two elements that have metallic properties. Metallic bonding occurs when a group of metal atoms shares a cloud of valence Electrons. There are two types: substitutional and interstitial

1) Substitutional: different atoms are substituted for the “host” metal atoms (brass)

2) Interstitial: small atoms (Metals or nonmetals) Are introduced into the “ Holes” In the metallic structure