Phase Changes (i.e., changes of state)

1. or “fuse” or condense Phase Changes (i.e., changes of state) -- melting, freezing, boiling (vaporization), condensation, sublimation, deposition (Evaporation?) passive process; room temp. -- energy changes required are related to IMFs heat of fusion (cf): energy per “something” required to melt a substance -- also called... latent heat heat of vaporization (cv): energy per “something” required to boil a substance

2. > How do magnitudes of cvand cf compare? KE must be increased enough to allow particles to slide, relative to each other (IMFs still in effect) IMFs must be completely overcome (i.e., “broken”)

3. Temp. Heat Added or Removed -- specific heat capacity: energy req’d to change temp. of 1 _____ of a substance 1oC (1 K) gram -- molar heat capacity: energy req’d to change temp. of 1 _____ of a substance 1oC (1 K) mole Heating curves are graphs of temperature v. heat added (or heat removed). Typical Heating Curve for a Pure Substance Within a phase: BP DH = q = m cpDT Between phases: MP/FP DH = q = +/– m cx

4. Phase Change Constants for Water cp,ice = 2.077 J/g-K cf = 333 J/g cp,water = 4.18 J/g-K cv = 2256 J/g cp,wv = 2.042 J/g-K Find the enthalpy change when 82.4 g of ice at –13.5oC turns to water at 72.8oC. warm ice to 0oC: q = m cpDT q = 82.4 (2.077) (0 – –13.5) q = 2310 J melt ice at 0oC: q = m cf q = 82.4 (333) = 27439 J heat water from 0oC to 72.8oC: q = m cpDT q = 82.4 (4.18) (72.8 – 0) = 25075 J qtot = DH = 54.8 kJ

5. supercooling: temporarily cooling a liquid below its freezing point without it forming a solid -- heat is removed so quickly that particles have no time to assume an ordered structure critical temperature: the highest temperature at which a substance can be a liquid -- as IMFs increase, crit. temp… increases critical pressure: the pressure required to bring about liquefaction at the critical temp. The intersection of the critical temperature and the critical pressure is called the critical point.

6. Pressure cookers raise the con- fining pressure on the water, so the water boils at a higher temp. A substance’s vapor pressure is the pressure exerted by a vapor in dynamic equilibrium with its liquid or solid phase. -- as IMFs increase, VP... -- as temperature increases, VP... -- liquids that evaporate easily are said to be ______ volatile (these have high VPs) external pressure (usually from atmos.) -- boiling occurs when…VP = -- normal boiling point (NBP): the boiling temp. of a liquid at 1 atm of pres.

7. Phase Diagram for a Typical Substance P SOLID LIQUID 1 atm GAS T Phase Diagrams -- graphs showing the conditions under which equilibria exist between different states of matter changes in FP/MP critical point (CT, CP) triple point v.p. curve of liquid v.p. curve of solid NFP/NMP NBP

8. s.c.f. Water is NOT a typical substance. Its phase diagram differs slightly, as shown below. Phase Diagram for Water P LIQUID SOLID 1 atm GAS T supercritical fluid: how we describe a substance at or beyond its critical point

9. Structures of Solids amorphous solid: the particles have no orderly structure -- e.g., rubber, glass -- IMFs are highly variable, so these solids have no specific… MP

10. The crystal lattice of sodium chloride (NaCl). crystalline solid: the particles are in well-defined arrangements -- e.g., ionic and elemental metallic solids -- these melt at VERY specific temps. crystal lattice: a 3-D array of points showing the crystal’s structure

11. 1 atom unit cell 2 atoms 4 atoms unit cell unit cell unit cells: the repeating units of a crystalline solid “continuous atomage” face-centered cubic primitive (or simple) cubic body-centered cubic