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CHM 2045

Phases Thursday, July 25, 2013 2:42 PM 1. Solids have a definite shape and a definite volume a. The moleculesdo not change positions, resulting in a fixed structure, whose moleculesare in contact with neighboring moleculesat all times 2. Liquids have a definite volume but no definite shape a. The moleculeschange position (translational motion)resulting in continuous random motion,and molecules that remain in contact with neighboring molecules at all times 3. Propertiesof solids a. The moleculesof a solid are arranged in a fixed lattice structure, on the macroscopiclevel, they have repeating structure subunits called unit cells b. Thus, the structure of a crystalline solid is defined by the size and shape of the unit cell and the locations of atoms within the unit cell c. The three most commonunit cells are the simple cubic with one atom per repeating unit cell, the body-centered cubic with two atoms per repeating unit cell, and the face centered cubic, with four atomsper repeating unit cell i. ii. d. Alkaline metals often pack to form the simple cubic e. Gold and silver pack to form a face-centeredcubic f. Solids have density 4. Propertiesof liquids a. No definite shape, but they do have a definite volume b. Because their moleculesare in motion, liquids can flow, which defines them as a fluid c. Liquids typically have the highest heat capacity of the three commonphases, and liquids are compressible d. The density of a liquid decreases with increasing temperature (because the volume increases but the mass remains the same), with the EXCEPTION OF WATER FROM 0C to 4C e. IF A COMPOUND IS LIQUID AT ROOM TEMPERATURE,IT HAS A MELTING POINT LESS THAN ROOM TEMPERATUERAND A BOILING POINT GREATER THAN ROOM TEMPERATURE f. Surface tension generally increases as the intermolecularforces increase i. It is defined as the energy required to increase the surface area of a liquid by a unit amount 5. Phase Change Processes a. A phase change process is a physical process, not a chemical process, by which the state of matter changes b. In a phase change, the moleculesthemselvesdo not change, but the interactions between moleculesdo c. d. e. Isothermal - Conditions where the temperatureof the system does not change f. Isobaric - pressure is constant g. Isochoric - volumedoes not change h. Adiabatic - system is perfectly insulated, so that heat neither enters nor exits the system (qconstant) 6. Phase Diagrams a. A phase diagram summarizes the different states of matterfor a given compound or a. A phase diagram summarizes the different states of matterfor a given compound or element with respect to temp and pressure b. Water has an atypical phase diagram i. The difference is the negative slope associatedwith the line separating liquid from solid ii. Most compounds can be compressedfrom a liquid into a solid at constant temperature iii. The unusual thing about water is that an isothermal increase in pressure compresses the solid (ice) into liquid water, resulting in the liquid being denser than the solid 7. Water facts a. Its liquid from is denser than its solid form b. Its densest at 4 degrees Celsius c. The solid can be compressed into a liquid under relatively mild conditions d. Water has the densest hydrogen bonding of any compound 8. Supercritical fluids a. A supercritical fluid exists when the conditions exceed the critical point b. A gas normally liquefies at some point when pressure is applied i. A cylinder containing water vapor at 100 degrees celcious will form liquid water if 760 torr of pressure is applied ii. If the temperatureis 110 degrees, the liquid phase does not form unti the pressure is 1075 iii. At 374 degrees, the liquid phase only forms at 1.655e5 torr iv. Aboce this temperature,no amount of pressure causes a distinct liquid phase to form v. Instead, as pressure increases, the gas becomessteadily more comperssed c. The highest temperatureat which a distinct liquid phase can form is called the critical temperature d. The critical pressure is the pressure required to bring about liquefaction at this critical temp e. When the temp exceeds the crit temp, and the pressure exceeds the crit pressure, the liquid and gas phase are indistinguishable from each other, and the substance is in a state called a supercritical fluid 9. Heating Curve a. b. A material is heated or cooled at a constant rate under isobaric conditions over a broad temperaturerange c. Shows the SAME features as a horizontal line in a standard phase diagram, but in significantly more detail d. If we heat an ice cube initially at -25C at 1 atm, the temperature of the ice increases e. As long as the temp is below 0, the ice cube remains in the solid state f. When the temp reaches 0, the ice begins to melt g. Because melting is an endothermic process, the head we add at 0C is used to convert ice to liquid water, and the temperatureremains constant until all the ice has melted liquid water, and the temperatureremains constant until all the ice has melted i. The enthalpy of vaporization is greater in magnitude than the enthalpy of fusion (length of the blue lines) because more energy is necessary to break the intermolecularforces (going from a liquid to gas) than is necessary to weak the intermolecularforces (solid to liquid) h. We can calculate the enthalpy change of the system for each segment of the heating curve i. The amount of heat needed to raise the temperatureof segment AB, CD, and EF is calculated by the product of the specific heat, mass and temperaturechange ii. The greater the specific heat of a substance, the more heat we must add to accomplish a certain temp change iii. The slope of the line is inversely proportional to the heat capacity 1) The flatter the line, the greater the heat capacity i. The slope for each of the three phases are different, implying that the heat capacities for each of the three phases are different j. The heat capacity of the liquid Is the greatest of the three phases 10. Vapor Pressure a. About every liquid and solid (more so liquid) there is a finite amount of vapor formed when moleculesat the surface layer escape b. A vapor is composedof gas particles that are in in equilibrium with the liquid phase, so they temporarilyexist as gas, until they condense back into their more favorable phase c. "Force per unit area above the surface of a liquid exerted by molecules formed upon evaporationof the liquid" i. The vapor pressure of a liquid is simply the partial pressure exerted by the gas moleculesformed by evaporationwhen it is in equilibrium with the gas molecules condensing back into the liquid d. The vapor pressure above a pure liquid depends on the temperatureof the liquid and the Δh vaporizationthe liquid e. f. It is difficult to measure the vapor pressure of an open system so we generally consider vapor pressure in an open system from a theoretical perspective,and apply values that have been determined
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