3.2 Properties of Solids

Important Vocabulary/Terms

• Heat of fusion

• Heat of Vaporization

• Vapor Pressure

• Covalent Network Solids

• Biomolecules and Synthetic Polymers

• Metals

Properties of Ionic Solids

• Solubility and Conductivity:

  • Most are soluble in polar solvents.

  • They conduct electricity when molten or dissolved in a polar solvent, as the charged particles are free to move.

    • The higher the concentration of ions in a solution, the higher the electrical conductivity.

• Strong bonds:

  • Very strong Coulombic forces of attraction.

    • High melting points

    • Very hard

    • Low volatility

• Cleave Along Planes:

  • Brittle 3D structure

  • Ions line up in a repetitive pattern that maximizes attractive forces and minimizes repulsive forces.

  • Not malleable or ductile.

Properties of Molecular Solids

• Most molecular solids do not conduct electricity when molten or dissolved in water.

  • The individual molecules have no net charge, as their valence electrons are tightly held within covalent bonds and lone pairs.

  • Acid molecules that can ionize and conduct electricity.

• Most molecular solids are held together by intermolecular forces, which are much weaker than ionic or covalent bonds.

  • They have much bigger higher vapor pressures than ionic solids.

  • They have much lower melting and boiling points than ionic bonds.

Molecular Solids

• Molecular shape plays a role in their physical state.

• In a solid, molecular are held together in a regular pattern by intermolecular forces that attempt to maximize attractions and minimize repulsions.

Change of State: Heating Curve

Heat of Fusion (^H fus)

• ^H fus - The heat absorbed 1 mole of a solid liquefies.

• Effect Energy is required to expand/seber

• his is why molar heat of fusion, ^H fus, values are always positive.

• molting is always an endothermic process.

^H fus For Ionic Compounds

• as ionic bonds are much stronger than intermolecular forces, the ^Hfus values for ionic compounds are very large.

• The melting and boiling temperatures for ionic compounds are very high.

Molecular Liquids

• In a Liquid, intermolecular forces also attempt to maximize attractions and minimize repulsions. These forces are strong, but not as strong as they are in a solid, so the molecules have more freedom to move.

Heat of Vaporization (^Hvap)

• ^Hvap - the heat absorbed as 1 mole of a liquid becomes gaseous.

• Energy is required to sever the intermolecular forces of attraction, as a molecule move from the liquid to the gas phase.

• Vaporization is always endothermic, so ^Hvap values are positive.

• Ideally, there are no intermolecular forces of attraction between as particles.

Vapor Pressure

• When molecules leave the surface of a liquid to enter the gas phase, they exert the pressure.

• The vapor pressure exerted depends on the rate of evaporation per unit area of the liquid’s surface.

• Rate of evaporation and vapor pressure increases as temperature increases.

• When two substances are at the same temperature, the rate of evaporation and vapor pressure will be higher in the substance that has weaker intermolecular forces.

Boiling Points

• A liquid boils when its vapor pressure equals atmospheric pressure.

• Evaporation occurs inside the liquid when the vapor pressure equals the atmospheric pressure.

• Bubbles are water vapor, not ‘air’.

Boiling Points of Water

• Boiling points decrease as elevation increases

Boiling Points of Different Liquids

• The vapor pressure on the surface of a liquid depends on the strength of its intermolecular forces.

  • A molecule restrained by strong intermolecular forces requires more energy to break free from the liquid state.

  • When a system requires more energy to cause its molecules to enter the gas phase, it will also require more energy to cause its vapor pressure to equal the atmospheric pressure.

Sublimation

• Solids can evaporate and have a vapor pressure

• As intermolecular forces are stronger in solids, the vapor pressures of solids are normally low.

• Solids with high vapor pressures, have relatively weak intermolecular forces.

Vapor Pressures of Ionic Solids

• Ionic compounds have very low vapor pressures and very high boiling points.

  • Strong Coulombic interactions between cations and anions.

Covalent Network Solids

• Always composed of one two nonmetals held together by networks of covalent bonds.

• Carbon group elements often form covalent network solids, as they can form four covalent bonds.

• Very high melting points and normally very hard, as atoms are covalently bonded with fixed bond angles.

  • e.g. A diamond is one molecule

    • Many carbon atoms bound together with sp³ hybrid orbitals.

    • Each carbon makes a single covalent bond with 4 other carbon atoms.

    • Very hard and very high melting point (3550 C)

Graphite

• Each carbon forms three sp² hybrid orbitals that bond with three other carbon atoms.

• These sheets sit on top of one another.

• Delocalized pi bonds between sheets.

• Weak pi bonds and london dispersion forces allow sheets to slide over one another (pencils)

• If hooked up to a potential difference, electrons will flow.

• High melting point, as covalent bonds between carbon in each layer are relatively strong.

SiO_2 (The empirical formula for quartz)

• Network of SiO_4 tetrahedra

• Every silicon atom is covalently bonded to four oxygen atoms.

• Every oxygen atom is covalently bonded to two silicon atoms.

Si SiC

• A 3-D network with a geometry that is similar to that of a diamond.

SiC (The empirical formula for Quartz)

• A covalent network of SiO_4 tetrahedra.

• Every silicon atom is covalently bonded to four oxygen atoms.

• Every oxygen atom is covalently bonded to two silicon atoms.

Si (Forms a covalent network with itself)

• A 3-D network with a geometry that is similar to that of a diamond.

Biomolecules - Protein Structures

• H-Bonds form between oxygens and the hydrogens that are bonded to nitrogens, within the same chain.

• The following secondary structures can form:

  • The a-helix structure is held together by H-Bonds.

  • The B-pleated sheet structure is also held together by H-bonds.

• Tertiary structures are caused by intermolecular interactions between R groups.

• Quaternary structures are caused by intermolecular interactions between different chains.

Protein Function

• The overall surface shape determines function.

• A groove creates the opportunity for a protein to interact with other molecules.

• Enzymes break down other molecules through this type of interaction.

• Vocab:

  • Active Site - Groove

  • Substrate - Molecule to be broken down by enzyme

  • Enzyme - Substrate complex

• Water soluable proteins have polar ‘R’ groups that face out and non-polar ‘R’ groups that face in.

  • H-bonds form with water.

Synthetic Polymers - Polyethylene

• Plastics are non-polar, so they are held together by london dispersion forces.

• London dispersion forces hold chains together.

Properties of Synthetic Polymers

• Plastics are generally flexible solids or viscous liquids.

• Heating plastics increase flexibility and allows them to be molded.

  • Molecular vibrations increase

  • London Dispersion forces weaken

• properties of synthetic polymers can be modified by manipulating their structures.

Metallic Solids

• Bonding is not covalent

  • Not enough electrons to fill octets

• Bonding results from attractions between nuclei and delocalized valence electrons moving throughout the structure.

• Bond strength increases as the number of bonding electrons increases.

Metallic Bonding

Electron Sea Model

• Nuclei and inner core electrons are localized, while valence electrons are free to move around the solid.

  • Conducts electricity

  • Conducts heat

  • Malleable and Ductile

    • Lacks directional bonds

Steel - An Interstitial Alloy

• Carbon fills some spaces between iron atoms.

• Interstitial carbon atoms make the lattice more rigid, less malleable, and less ductile.

• Retains a ‘sea of electrons’ so it can conduct electricity.

Brass - A Substitutional Alloy

• zinc atoms are substitued for some copper atoms.

• Substitutional alloys remain malleable and ductile.

• Retains a ‘sea of electrons’ so it can conduct electricity.