Prentice Hall Chemistry (c) 2005Section Assessment Answers Chapter 13 By Daniel R. Barnes Init: 2/10/2010 WARNING: some images and content in this presentation may have been taken without permission from the world wide web. It is intended for use only by Mr. Barnes and his students. It is not meant to be copied or distributed. 13.1 Instructional Goals SWBAT . . . . . . explain gas pressure, diffusion, temperature, and heat flow in terms of molecular motion. 13.1 Section Assessment 3. Briefly describe the assumptions of kinetic theory as applied to gases. Kinetic theory imagines that gases are made of tiny particles (atoms &/or molecules) that move perpetually, randomly, and quickly. The theory assumes that collisions between particles are perfectly elastic. The book uses the word ³constant´. I don¶t like it. I prefer the word ³perpetual´. Any idea why? Anybody? Anybody? Well, I¶ll tell ya . . . Gas molecules never stop moving, but the word ³constant´ sort of implies that their motion does not change. Gas molecules change speed every time they collide. Collisions with other molecules or with other objects cause not only changes in direction (³random´ motion), but also changes in speed. According to kinetic theory. Use kinetic theory to explain what causes gas pressure.1 Section Assessment 4. . gases are made of molecules (or lonely atoms) that are always flying around. bashing into things.13. It is all these little tiny collisions that results in gas pressure. . Some are traveling considerably faster than average.1 Section Assessment 5. KE = ½ mv2 In this equation. but there will still be a variety of speeds. Imagine a cup of water. Even if all the water in the cup is at the same temperature. some are traveling considerably slower than average. KE = kinetic energy. while most of the molecules are traveling very close to average. and. the molecules will speed up. m = mass. on the average. How is the Kelvin temperature of a substance related to the average kinetic energy of its particles? Kelvin temperature is a direct measurement of the average molecular kinetic energy of a material. and v = speed. let¶s say 294 K (21oC). that doesn¶t mean that the molecules are all traveling at the same speed. Heat the water up to a higher temperature (let¶s say 300 K).13. 1 Section Assessment 5.13. How is the Kelvin temperature of a substance related to the average kinetic energy of its particles? . Convert the following pressures to kilopascals. As always. .13.3 kPa 1 atm = 96 kPa This is a unit conversion problem. so you¶re going to need to put this initial amount into the form of a fraction. . you¶ll need to multiply it by a conversion fraction. a.3 kPa.1 Section Assessment 6. so . We want to get rid of the atm and replace it with kPa. . so . . What is the relationship between atmospheres and kilopascals? Your CST reference sheet says that 1 atm = 101. 0. .95 atm 1 x 101. 0 kPa By pretty much the same method that we used with 6a . Convert the following pressures to kilopascals. 0.13.3 kPa 760 mm Hg = 96 kPa = 6.3 kPa 1 atm 101. 45 mm Hg 1 x x 101.1 Section Assessment 6. .95 atm 1 b. . a. . A cylinder of oxygen gas is cooled from 300 K (27oC) down to 150 K (-123oC). is only divided by the square root of 2 (about 1. v2 is cut in half. CAUTION! Although KE is cut in half. not by 2 itself. . is a direct measurement of the average molecular kinetic energy of a body of matter. the average kinetic energy of the oxygen molecules is also cut in half. That means that if KE is cut in half. not v. By what factor does the average kinetic energy of the oxygen molecules decrease? Temperature. if the Kelvin temperature gets cut in half . then v.13. . that does not mean that average speed is cut in half. KE = ½ mv2 Note that the speed (v) is squared. If v2 is cut in half. and KE does depend upon speed. .41 or so). when measured in Kelvins. Therefore.1 Section Assessment 7. . . which is the square root of v2. . explain how intermolecular forces and molecular motion influence states of matter and the energy changes that result from phase changes.13.2 Instructional Goals SWBAT . . . . . . . boiling point.2 Section Assessment 8. . are determined largely by . freezing point. viscosity. What factors help determine the physical properties of liquids? Such physical properties as density. . so if there are significant charges on molecules. the forces between molecules. and others. + + One key example of this is the electrical attraction between the positive regions of one molecule and the negative regions of another molecule. . Opposites attract. . - .13. resulting in higher freezing and boiling points. . this will result in stronger intermolecular forces. it can break free of the bonds that make it stick to neighboring molecules. Evaporation is endothermic.13. This is hard work. If a molecule¶s neighbors sacrifice their motion energy. and requires energy. In terms of kinetic energy. the material gets colder. In other words. When molecules in a material slow down. Liquid molecules can¶t hold onto each other if they¶re moving too fast.2 Section Assessment 9. Since the main kind of energy molecules have is their motion (kinetic energy). allowing it to fly free into the air. its neighbors must smack it hard enough for it to overcome the forces that attract the molecule to its neighbors. explain how a molecule in a liquid evaporates. they slow down. . a molecule¶s neighbors have to sacrifice some of their motion to provide the energy required to bat their buddy out of the park. if a liquid molecule has enough kinetic energy. In order for a molecule to evaporate. then the rate of evaporation must equal the rate of condensation. What is the relationship between the rate of evaporation of the liquid and the rate of condensation of the vapor in the container? If vapor pressure is constant. If opposite processes are equal. . and nothing changes. then all opposite processes must be equal. The two processes that add or subtract vapor from the airspace in a container are evaporation (add) and condensation (subtract). then their effects cancel each other out. If vapor pressure is constant.13. A liquid is in a closed container and has a constant vapor pressure. its opposite.2 Section Assessment 10. 2 Section Assessment 11. What condition must exist for a liquid to boil? The teacher¶s edition of the textbook says. so even in boiling water.´ Who has two thumbs and doesn¶t like that answer? The book¶s answer sort of insinuates that every molecule in the liquid has enough energy to vaporize. ³Particles throughout the liquid must have enough energy to vaporize. there are some slow molecules. . Molecules have to have a certain minimum speed to break away from each other. I beg to disagree. Even in a hot liquid.13. there are some molecules that are traveling too slow to escape from their attractive neighbors. 3 kPa when its temperature is 100oC. instead of somewhat watery air. air pressure = 101. is the temperature at which the vapor pressure of the liquid equals atmospheric pressure. so that¶s how hot water needs to get to boil (at 101. ³Particles throughout the liquid must have enough energy to vaporize. What condition must exist for a liquid to boil? The teacher¶s edition of the textbook says.2 Section Assessment 11. .3 kPa). At sea level on Earth on a typical day. the water vapor could still only condense just as fast as the water vaporized. Therefore. Water has a vapor pressure of 101.13.´ The definition of ³boiling point´. as I understand it. a liquid must be so hot that its vapor pressure equals local air pressure. even if the boiling water were surrounded by pure water vapor.3 kPa. I would say that in order to boil. Under those conditions. ethanol at 50 kPa boils at 62oC .13.2 Section Assessment 12. Use Figure 13.9 to determine the boiling point of each liquid. ethanoic acid at 27 kPa boils at 76oC b. a. chloroform at 80 kPa boils at 52oC c. Just imagine that if the molecules flying out of a body of water were to hit a surface on the way out. a liquid must be hotter. but it is done. they¶d exert pressure on the surface.13. and.´ To have a higher vapor pressure. It may seem odd to use pressure units as a way to say how quickly molecules are vaporizing. In other words. If molecules are flying into the air faster. more pressure. heating a liquid makes its molecules fly into the air faster. . Explain why the boiling point of a liquid varies with atmospheric pressure. there will be more collisions. If the atmospheric pressure changes. the boiling point will change. ³Boiling occurs when the vapor pressure of a liquid equals the external pressure.2 Section Assessment 13. therefore. just as a thrown ball slows down as it rises into the sky. this causes the average temperature of the remaining liquid molecules to decrease. in turn. There is also the very important consideration that whenever an object moves away from something it¶s attracted to (like its fellow liquid molecules). Explain how evaporation lowers the temperature of a liquid. Therefore. it¶s going to slow down as it moves away. makes the temperature go down.) Temperature is average molecular kinetic energy.2 Section Assessment 14.13. This. when the fastest molecules leave. (The slower ones can¶t break away from their attractive neighbors. The first molecules to leave a liquid and fly into the air are the fastest ones. . . . explain the orderly structure of solids in terms of intermolecular forces. .13.3 Instructional Goals SWBAT . and how molecular motion can disrupt a solid in spite of these forces. . . . The locations of particles are fixed.13. how are the particles arranged in solids? Particles in solids are packed tightly together in an orderly arrangement.3 Section Assessment 15. . In general. 13. . What does the shape of a crystal tell you about the structure of a crystal? The shape of a crystal reflects the arrangement of the particles in the solid.3 Section Assessment 16. 13.3 Section Assessment 17. . How do allotropes of an element differ? Allotropes are different molecular forms of the same element in the same physical state. . the liquid and solid states are in equilibrium. What phases are in equilibrium at a substance¶s melting point? At a substance¶s melting point.3 Section Assessment 18.13. How do the melting points of ionic solids generally compare with those of molecular solids? Ionic solids generally have higher melting points than do molecular solids. .3 Section Assessment 19.13. What is the difference between a crystal lattice and a unit cell? A crystal lattice is a repeating array of unit cells. .3 Section Assessment 20.13. That¶s why it¶s extra credit. . just in case you did 13.13. I¶m providing the answers anyway. .4 Instructional Goals There are no SWBAT]S for section 13.4. . because I¶m sure you¶re dying to see if you got them right .4. . What properties must a solid have to undergo sublimation? Sublimation occurs in solids that have vapor pressures that exceed atmospheric pressure at or near room temperature.4 Section Assessment 21.13. 13. . What do the curved lines on a phase diagram represent? The curved lines in a phase diagram show the conditions of temperature and pressure at which two phases exist in equilibrium.4 Section Assessment 22. Some practical uses of sublimation are: Freeze-dried coffee. dry ice as a coolant. air fresheners. .13. Describe one practical use of sublimation.4 Section Assessment 23. separating mixtures. and purifying substances. and gas) can be in equilibrium with each other. liquid. What does the triple point on a phase diagram describe? The triple point in a phase diagram describes the only combination of temperature and pressure at which all three phases (solid. .4 Section Assessment 24.13. 15. the boiling point of water would be about 60oC.4 Section Assessment 25. . estimate the boiling point of water at a pressure of 50 kPa.13.15. at 50 kPa. Using Figure 13. According to Figure 13.