Chapter 10: Liquids and Solids

Intermolecular Forces Dipole Attraction - Positive/negative center of charge in polar molecules attract each other 1% of the strength of ionic/covalent bonds Hydrogen Bonding - Very strong dipole attraction between a hydrogen and very electronegative atom More electronegative = stronger bond London Dispersion Forces - A result of the “instantaneous dipole”, not all electrons are evenly distributed throughout the nucleus Affects electron dispersion in nearby atoms Stronger for larger atoms, due to more electrons Polarizability - How easy it is to change the electron distribution in atoms

Liquids Surface Tension - Resistance of a liquid to increasing surface area. Greater with molecules with larger intermolecular forces Capillary Action - Liquid’s tendency to rise in a narrow tube Common in polar molecules Molecules pulled up tube by charge differences between liquid and the tube The molecules above attract the ones below through cohesion, and the liquid is pulled up the tube Viscosity - A liquid’s resistance to flow Larger intermolecular forces = higher viscosity Large molecules can become entangled with each other

Structures and Types of Solids Two main categories of solids: Crystalline solids - Regular arrangement of atoms/molecules Amorphous solids - Lack of order in arrangement Lattice - Arrangement of components in a crystalline solid Unit cell - Smallest repeating component of a lattice Types of crystalline solids: Ionic solids - Ions present at lattice points Molecular solids - Covalently bonded molecules at lattice points Atomic solids - Single atoms at lattice points Metallic solids - Delocalized non-directional covalent bonding at lattice points Network solids - Strong directional covalent bonds making very large molecules Group 8A solids - Noble gas elements attracted to each other through dispersion forces

Network Atomic Solids Network solids - Atomic solids which contain directional covalent bonds Similar to a very large molecule Generally brittle, and do not conduct electricity Diamond - Hardest naturally occurring substance Tetrahedral unit cell, sp3 hybridized Graphite - Slippery and conductive Layers of carbon “sheets” in hexagonal rings, sp2 hybridized Remaining 2p orbital creates delocalized π bonds with sheets above/below Accounts for slipperiness, the sheets are able to easily move laterally to each other due to lack of strong bonding Silica - Main silicon/oxygen compound Silicon unable to form strong π bonds with oxygen due to its larger size Forms single bonds with 4 oxygen atoms Silicates - Have oxygen/silicon ratios greater than 2:1, contain silicon/oxygen anions Cations required to balance the negative charges Glass - Amorphous solid resulting from heating silica

Molecular/Ionic Solids Molecular solids - Solid in which the lattice positions are occupied by molecules Very strong bonding within molecules, but not as much between Ionic Solids - These solids have high melting points as a result of the strong attractions exist between negatively charged anions and positively charged cations (ionic bonds). The structure of ionic solids is represented by having the smaller cations contained within the spaces between anions. The possible structures are trigonal, tetrahedral, or octahedral.

Vapor Pressure Vapor Pressure - Pressure of vapor at equilibrium Determined by magnitude of intermolecular forces High vapor pressure liquids are generally volatile - evaporate quickly from open container Increases with temperature If the vapor pressure at one temperature is known, the vapor pressure at another temperature can be determined by the equation:

Changes of State Changes of state can be illustrated by a heating curve - a temperature vs. time graph: Melting point is when the liquid and solid forms of a substance have the same vapor pressure Boiling point is when the liquid’s vapor pressure is equal to the atmospheric pressure Supercooling - When a liquid is cooled below its freezing point, yet remains a liquid This is due to the molecules not necessarily forming into an organized structure Superheating - Liquid heated above boiling point, yet remains a liquid In order for bubbles to form, high-energy molecules must gather in the same area at once, which may not always occur

Phase Diagrams Phase Diagrams are used to represent the state of a substance at any given temperature and pressure. Most phase diagrams contain three lines, each line representing the boundary between two phases. An important point is known as the triple point. At this temperature and pressure, all three phases exist at the same time. Finally, another important point is the critical point. At the critical temperature, if the temperature were to be any higher, no amount of pressure would be able to cause the substance to liquify. Meanwhile, the critical pressure is the pressure required to liquify the substance at the critical temperature. Combining the two results in the critical point.

Review Questions 1. Why does the size of atoms/molecules affect the strength of London Dispersion Forces? 2. Describe the difference in structure between graphite and diamond 3. Sort the following from least to greatest melting point: Cl2 , NaCl, PCl3(polar)