Crystalline solids Same lattice can be used to describe many different designs For designs based on the fcc unit cell: lattice points, empty spaces, edge lengths The only requirement is that the same design element must be associated with each lattice point Chloride ions in NaCl are associated with the lattice points of a fcc unit cell. The sodium ions are placed between the chloride ions.

Types of Crystals 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 CsClZnSCaF

Types of Crystals Covalent Crystals Lattice points occupied by atoms Held together by covalent bonds Hard, high melting point Poor conductor of heat and electricity Graphite is good conductor of electricity 12.6 diamondgraphite Diamond.

Types of Crystals Covalent Crystals Lattice points occupied by atoms Held together by covalent bonds Hard, high melting point Poor conductor of heat and electricity Graphite is good conductor of electricity 12.6 diamondgraphite Graphite.

Types of Crystals Molecular Crystals Lattice points occupied by molecules Held together by intermolecular forces Soft, low melting point Poor conductor of heat and electricity 12.6 Sulfur.S8S8

Types of Crystals 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 12.6 nucleus & inner shell e - mobile “sea” of e - Copper.

Types of Crystals 12.6

An amorphous solid does not possess a well-defined arrangement and long-range molecular order. A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing. The word glass is often used as a general term to refer to any amorphous solid. Melted material that is cooled very slowly has a better chance at forming a crystalline solid. Crystalline quartz (SiO 2 ) Non-crystalline SiO 2 glass 12.7

Phase Changes

Energy Changes Associated with Changes of State Heat of Fusion: Energy required to change a solid at its melting point to a liquid. Heat of Vaporization: Energy required to change a liquid at its boiling point to a gas.

Energy Changes Associated with Changes of State Heat of Fusion: Energy required to change a solid at its melting point to a liquid. Heat of Vaporization: Energy required to change a liquid at its boiling point to a gas. Why are the heat of vaporization (ΔH vap ) values generally larger? In the transition from the liquid to the vapor state, all intermolecular forces must be broken. In melting, many of these interactions remain.

Energy Changes Associated with Changes of State The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other. The temperature of the substance does not rise during the phase change. 1.Put energy in to get to transition temperature 2.Put more energy in to make the phase transition

Vapor Pressure At any temperature, some molecules in a liquid have enough energy to escape. As the temperature rises, the fraction of molecules that have enough energy to escape increases. As more molecules escape the liquid, the pressure they exert increases.

The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. Vapor pressure increases with temperature. The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. The normal boiling point is the temperature at which its vapor pressure is 760 torr. At the top of Mt. Everest, where the atmospheric pressure is roughly 1/3 that at sea level, the boiling point of water is roughly 70 o C and the normal boiling point of water is 100 o C.

At standard atmospheric pressure, ethylene glycol boils at a much higher temperature than diethyl ether. Why?

Molar heat of vaporization (  H vap ) is the energy required to vaporize 1 mole of a liquid; a measure of how strongly molecules are held in a liquid. ΔH vap is determined by measuring the vapor pressure of a liquid at different temperatures. ln P = -  H vap RT + C Clausius-Clapeyron Equation P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/Kmol) 12.8

Molar heat of vaporization (  H vap ) is the energy required to vaporize 1 mole of a liquid; a measure of how strongly molecules are held in a liquid. ΔH vap is determined by measuring the vapor pressure of a liquid at different temperatures. ln P = -  H vap RT + C Clausius-Clapeyron Equation P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/Kmol) 12.8 ln =  H vap R (1/T 2 – 1/T 1 ) P2P2 P1P1

Diethyl ether is a volatile, highly flammable organic liquid that is used mainly as a solvent. The vapor pressure of diethyl ether is 401 mmHg at 18 o C. Calculate its vapor pressure at 32 o C. Convert temperatures from Celsius to Kelvin because the gas constant has units of J mol -1 K -1. Plug and chug. ln =  H vap R (1/T 2 – 1/T 1 ) P2P2 P1P1 ln 26,000 J mol J mol -1 K -1 (1/305 K – 1/291 K) P2P2 401 mmHg = P2P2 e = P 2 = 657 mmHg

12.8

The critical temperature (T c ) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure (P c ) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature Increasing the pressure of a gas can cause it to condense (liquify); decreasing the temperature can also cause condensation.

12.8 H 2 O (s) H 2 O (l) The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium

Molar heat of fusion (  H fus ) is the energy required to melt 1 mole of a solid substance. 12.8

H 2 O (s) H 2 O (g) Molar heat of sublimation (  H sub ) is the energy required to sublime 1 mole of a solid.  H sub =  H fus +  H vap (Hess’s Law)

Phase Diagrams Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases.

Phase Diagrams The AB line is the liquid-vapor interface. It starts at the triple point (A), the point at which all three states are in equilibrium.

Phase Diagrams Each point along this line is the boiling point of the substance at that pressure.

Phase Diagrams The AD line is the interface between liquid and solid. The melting point at each pressure can be found along this line.

Phase Diagrams Below A the substance cannot exist in the liquid state. Along the AC line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line.