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Chapter 13 continued…. Hope You Enjoyed Your Break!!. Review…. Answer the following questions on the paper I gave you… 1) Why does a cup of hot coffee become cold after sitting out on a table for a couple of hours? 2) What instrument is used to measure atmospheric pressure?
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Chapter 13 continued… Hope You Enjoyed Your Break!!
Review…. • Answer the following questions on the paper I gave you… • 1) Why does a cup of hot coffee become cold after sitting out on a table for a couple of hours? • 2) What instrument is used to measure atmospheric pressure? • 3) Convert 500 mm Hg to atm’s and Kpa • 4) What assumption of the KMT is incorrect? • 5) What factors determine the physical properties of a liquid?
Nature of Liquids: Evaporation • Closed vs. Open Container • Vapor Pressure: measure of the force exerted by a gas above a liquid.
Vapor Pressure • Dynamic Equilibrium: the rate of evaporation of liquid equals to rate of condensation of a vapor • -vapor pressure is constant at equilibrium • -condensation is a sign that equilibrium has occurred.
Animations Manometer and Pressure vs. Temp
Vapor Pressure and Temperature • As temperature increases, vapor pressure increases Increased temp more molecules with enough K.E escape liquid Higher frequency of evaporated molecules hitting wall of container Increased vapor Pressure
Boiling Point • Boiling occurs • At a specific temperature when the vapor pressure of the liquid equals atmospheric pressure. • Occurs when a liquid is heated and bubbles of vapor form. Evaporation can occur at any temperature, boiling point only occurs for a specific temperature for a liquid.
Boiling Point : Different Elevations Why does it take longer to cook food in water at high altitudes? Atmospheric pressure decreases with higher elevation; so water will boil at a lower temperature so food cooks slower Why does food take less time to cook in a pressure cooker The pressure in a pressure cooker can be greater than atmospheric pressure so the water can boiler at a higher temperature and food will cook faster.
Hand Boiler • How does the hand boiler shown to you in the front of the room work? • In a closed container, as the temperature goes up, so does the pressure. As the temperature increases, the molecules of gas in the container move faster, which increases the pressure. As the pressure increases in one of the chambers, the liquid will be pushed into the other one. alcohol vapor is forced up the tube, causing the liquid in the top bulb to appear to "boil."
Boiling Point: Different Liquids • Normal B.Point: The b.p. of a liquid at a pressure of 101.3kpa • Normal b.p. of water is 100 degrees Celsius
13.3 The Nature of Solids • WHAT IS A SOLID? • The general properties of solids reflect the orderly arrangement and the fixed locations of their particles. • Particles in a solid are packed tightly together • When you heat a solid, the particles vibrate more rapidly and KE increases. Eventually the solid melts. • Melting Point: is the temperature at which a solid changes into a liquid.
Solids • Most solids are in the form of a crystal, an orderly, repeating, three-dimensional pattern of atoms. • The shape of the crystal reflects the arrangement of particles within the solid • The smallest group of particles within a crystal that retains the geometric shape of the crystal is known as a unit cell.
Allotropes • What are a diamond, graphite and a buckyballall made from? • Allotrope: two or more different molecular forms of the same element in the same physical state.
Amorphous Solids • Amorphous solid: lacks an ordered internal structure. • Examples are rubber, plastic, asphalt, glass. When a crystalline solid breaks, the piece that broke off has same surface angles as original piece. When a non-crystalline solid breaks, the pieces that break off have irregular angles and jagged edges.
Changes of State • Sublimation: solid converts to a vapor. (skips liquid state) • Occurs in solids with vapor pressures that exceed atmospheric pressure at or near room temperature. • Sublimation can occur because solids can have a vapor pressure as well as liquids. Examples: Dry Ice (solid carbon dioxide) Air Fresheners
Freeze Drying • Sublimation is used for freeze drying: a method of removing water from food • Freeze-drying is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. (like a trip to the moon!) • Freeze-drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to gas. • This is how they make astronaut ice cream!
Any temperature and pressure combination not on a curve represents a single phase. • Phase diagram: plot of pressure vs. Temperature summarizing all equilibria between phases. • Given a temperature and pressure, phase diagrams tell us which phase will exist.
Phase Diagram • Features of a phase diagram: • Triple point: temperature and pressure at which all three phases (solid,liquid,gas) are in equilibrium. • Vapor-pressure curve: generally as pressure increases, temperature increases. • Melting point curve: as pressure increases, the solid phase is favored if the solid is more dense than the liquid. The curve will lean away from the more dense phase. • Normal melting point: melting point at 1 atm.
