Even at temperatures well below its melting point of 114 C, solid iodine exhibits an appreciable sublimation pressure. Here, purple iodine vapor is

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1 Let s take a closer look at the behavior of gases, according to the Kinetic Theory. Part Three GHS Honors Chem 1 A gas is composed of small spheres, that have insignificant volume, and are far apart from one another. The spheres move rapidly in constant motion, traveling in straight lines until they strike another sphere, or the wall of the container. As such, they fill their container, and assume the shape/volume of the container. The spheres collide in perfectly elastic collisions; the K.E. from one sphere is transferred w/o loss from one particle to another. GHS Honors Chem 2 An Elastic Collision Gas Particles have different energies with respect to one another. Some particles move slower/faster than others. Do all of the particles in a Gas have the Same Energy? Remember, the Kinetic Energy is a measure of the Average Energy of the gas particles within a sample. GHS Honors Chem 3 The Kinetic energies are distributed around an Average KE, which corresponds to the temperature of the gas The higher the Average KE, the higher the temperature of the gas. GHS Honors Chem 4 The pressure that the colliding gas spheres have on the container is Gas Pressure. The Standard Unit of Pressure: pascal (Pa) Standard Atmospheric Pressure at Sea Level & 25 o C: Atmospheric Pressure: is the pressure that air gases (N 2, O 2, Argon, CO 2 ) exert on the Earth s surface due to gravity. GHS Honors Chem 5 Atmospheric Pressure, How Does it Change at Varying Altitudes? At Higher altitudes, what happens to the weight of air? The "weight" of air at higher altitudes is less, so the atmospheric pressure is lower the Air thins out at higher altitudes. the air at low altitudes is dense; higher weight of air corresponds to higher atmospheric pressure Standard Atmospheric Pressure at Sea Level & 25 o C: 1 Atm = 760 mm Hg = kpa GHS Honors Chem 6 1

2 A Torricelli Barometer: measures Atmospheric Pressure As the Atmospheric Pressure increases, the level of Mercury in the Barometer increases. What would happen to the level of Mercury at the top of Mount Everest? The Manometer: Used to measure Gas Pressure Both Tubes are open to the Atmosphere Pressure is exerted to the left tube The Pressure exerted is read as the difference between the liquid levels = 4 atm GHS Honors Chem 7 GHS Honors Chem 8 Vaporization: the conversion of a liquid to a gas/vapor below the Boiling of the liquid. Evaporation: vaporization of an UNCONTAINED liquid (not in a closed container), such as water in a lake Vapor Pressure: can be thought of as the degree to which the liquid molecules are escaping into the vapor phase. The vapor pressure increases with temperature, because at higher temperature the molecules are moving faster and more able to overcome the attractive intermolecular forces that tend to bind them together. Has higher Kinetic Energy and is able to escape from the attractive forces within liquid water Liquid evaporates until nothing is left. GHS Honors Chem 9 GHS Honors Chem 10 Boiling occurs when the vapor pressure reaches or exceeds the surrounding pressure from the atmosphere or whatever else is in contact with the liquid. The Vapor Pressure EQUALS the Atmospheric Pressure at the Boiling Vapor Pressure Curves At standard atmospheric pressure (1 atmosphere = Pa), water boils at approximately 100 degrees Celsius. That is simply another way of saying that the vapor pressure of water at that temperature is 1 atmosphere. At higher pressures (such as the pressure generated in a pressure cooker), the temperature must be higher before the vapor pressure reaches the surrounding pressure, so water under pressure boils at a higher temperature. GHS Honors Chem 11 GHS Honors Chem 12 2

3 The high surface tension of water (due to Cohesion) causes the vapor pressure of water to be low. Water molecules are quite happy to remain attracted to neighboring water molecules, and do not want to push into the gaseous phase. Water s Low Vapor Pressure GHS Honors Chem 13 Denver has an Atmospheric Pressure of 640 mm Hg (torr). Water will boil at ~ 95 o C. At the top of Mount Everest, Water will Boil at 70 o C. What would the Boiling of Water be at the top of Mount Everest; higher or lower than 100 o C? GHS Honors Chem 14 What happens when the liquid is sealed, in a confined space? In a closed container, the liquid and vapor are in equilibrium. Evaporation & Condensation are in Equilibrium. This represents the Equilibrium Vapor Pressure. GHS Honors Chem 15 Equilibrium Vapor Pressure is a function of temperature Some animations: Animation 10 of ChemASAP: D:\ChemASAP.exe GHS Honors Chem 16 Vapor Pressure Suppose we have a closed container into which we pour some water. As soon as we add the water we check a pressure gauge connected to the container. We let the container sit for a while and then we check the pressure again. What might the pressure gauge indicate? What would happen if we increased the temperature of the solution at t=later? GHS Honors Chem 17 Sublimation The passing of a substance from a solid to a vapor without passing through the liquid state. Sublimation occurs in solids when the vapor pressure of the solid exceeds the atmospheric pressure at or near room temperature. Deposition The condensation of a vapor to a solid (the reverse of sublimation) GHS Honors Chem 18 3

4 Sublimation & Deposition of Iodine (I 2 ) Even at temperatures well below its melting point of 114 C, solid iodine exhibits an appreciable sublimation pressure. Here, purple iodine vapor is produced at about 70 C. Deposition of the vapor to solid iodine occurs on the colder walls of the flask. GHS Honors Chem 19 GHS Honors Chem 20 Describe the relationship between solid, liquid, and gas phases of a substance in a sealed container. A Phase Diagram of Water gives the conditions of temperature and pressure at which the substance exists as a solid, liquid, & gas. Normal Normal Melting Boiling The Triple Triple : Describes the only set of Conditions (Temp & Pressure) where all three phases can exist in equilibrium with one another. GHS Honors Chem 21 GHS Honors Chem 22 The Triple Triple : Describes the only set of Conditions (Temp & Pressure) where all three phases can exist in Triple equilibrium with one another. The Critical The vapor pressure curve ends at the critical point, which is the temperature above which the gas cannot be liquefied no matter how much pressure is applied (the kinetic energy simply is too great for attractive forces to overcome, regardless of the applied pressure). GHS Honors Chem 23 GHS Honors Chem 24 4

5 The Critical The of Water and CO 2 How are they Different? GHS Honors Chem 25 GHS Honors Chem 26 Let s Find out what you have learned about The phase diagram of a substance is given below. What occurs when the substance is heated from 100 C to 120 C C at 3 atm pressure? A typical phase diagram for a substance is given below. At what point on the diagram do gas and liquid exist at equilibrium? Let s Find out what you have learned about 1. Referring to the figure shown here, what is the phase of water at point 1? 2. How many phases are there at points 2, 4, or 8? 3. What changes are indicated by the line from point 1, 2, 3, 4, and 5? 4. What changes are indicated by the line from 6, 4, 7, 8, and 9? 5. At the conditions indicated by point 3 and 6, what is the stable phase of water? GHS Honors Chem 27 GHS Honors Chem 28 Let s Find out what you have learned about 1. In this sketch, what is the temperature of the triple point of water with respect to 0 C? 2. What is the melting point of ice at exactly 1 atm? 3. When it snows, the pressure is usually less than atm. If the sketch of the phase diagram is true, what should be the melting point of ice? 4. On the planet X, the atmosphere pressure is 10 atm, and the normal temperature is 250 K. What is the stable phase of carbon dioxide? 5. What is the melting point of solid carbon dioxide on the planet X whose atmosphere pressure is 10 atm? GHS Honors Chem 29 5