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Prentice Hall Chemistry (c) 2005 Section 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

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Prentice Hall Chemistry (c) 2005Section Assessment Answers Chapter

13

By Daniel R. Barnes Init: 2/10/2010WARNING: 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 Assessment3. 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 constant. I dont like it. I prefer the word perpetual. Any idea why? Anybody? Anybody? Well, Ill tell ya . . . Gas molecules never stop moving, but the word constant 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 (random motion), but also changes in speed.

13.1 Section Assessment4. Use kinetic theory to explain what causes gas pressure.

According to kinetic theory, gases are made of molecules (or lonely atoms) that are always flying around, bashing into things. It is all these little tiny collisions that results in gas pressure.

13.1 Section Assessment5. 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. KE = mv2 In this equation, KE = kinetic energy, m = mass, and v = speed.

Imagine a cup of water. Even if all the water in the cup is at the same temperature, lets say 294 K (21oC), that doesnt mean that the molecules are all traveling at the same speed. Some are traveling considerably faster than average, some are traveling considerably slower than average, while most of the molecules are traveling very close to average. Heat the water up to a higher temperature (lets say 300 K), and, on the average, the molecules will speed up, but there will still be a variety of speeds.

13.1 Section Assessment5. How is the Kelvin temperature of a substance related to the average kinetic energy of its particles?

13.1 Section Assessment6. Convert the following pressures to kilopascals. a. 0.95 atm 1 x 101.3 kPa 1 atm = 96 kPa

This is a unit conversion problem, so youre going to need to put this initial amount into the form of a fraction. As always, youll need to multiply it by a conversion fraction. 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 = 101.3 kPa, so . . .

13.1 Section Assessment6. Convert the following pressures to kilopascals. a. 0.95 atm 1 b. 45 mm Hg 1 x x 101.3 kPa 1 atm 101.3 kPa 760 mm Hg = 96 kPa

=

6.0 kPa

By pretty much the same method that we used with 6a . . .

13.1 Section Assessment7. A cylinder of oxygen gas is cooled from 300 K (27oC) down to 150 K (-123oC). By what factor does the average kinetic energy of the oxygen molecules decrease? Temperature, when measured in Kelvins, is a direct measurement of the average molecular kinetic energy of a body of matter. Therefore, if the Kelvin temperature gets cut in half . . . . . . the average kinetic energy of the oxygen molecules is also cut in half. CAUTION! Although KE is cut in half, and KE does depend upon speed, that does not mean that average speed is cut in half. KE = mv2 Note that the speed (v) is squared. That means that if KE is cut in half, v2 is cut in half, not v. If v2 is cut in half, then v, which is the square root of v2, is only divided by the square root of 2 (about 1.41 or so), not by 2 itself.

13.2 Instructional Goals

SWBAT . . .. . . explain how intermolecular forces and molecular motion influence states of matter and the energy changes that result from phase changes.

13.2 Section Assessment8. What factors help determine the physical properties of liquids? Such physical properties as density, freezing point, boiling point, viscosity, and others, are determined largely by . . . . . . the forces between molecules.

+

+

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, so if there are significant charges on molecules, this will result in stronger intermolecular forces, resulting in higher freezing and boiling points.

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13.2 Section Assessment9. In terms of kinetic energy, explain how a molecule in a liquid evaporates. Liquid molecules cant hold onto each other if theyre moving too fast. In other words, if a liquid molecule has enough kinetic energy, it can break free of the bonds that make it stick to neighboring molecules, allowing it to fly free into the air. In order for a molecule to evaporate, its neighbors must smack it hard enough for it to overcome the forces that attract the molecule to its neighbors. This is hard work, and requires energy. Since the main kind of energy molecules have is their motion (kinetic energy), a molecules neighbors have to sacrifice some of their motion to provide the energy required to bat their buddy out of the park. If a molecules neighbors sacrifice their motion energy, they slow down. When molecules in a material slow down, the material gets colder. Evaporation is endothermic.

13.2 Section Assessment10. A liquid is in a closed container and has a constant vapor pressure. 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, then all opposite processes must be equal. If opposite processes are equal, then their effects cancel each other out, and nothing changes. The two processes that add or subtract vapor from the airspace in a container are evaporation (add) and condensation (subtract). If vapor pressure is constant, then the rate of evaporation must equal the rate of condensation, its opposite.

13.2 Section Assessment11. What condition must exist for a liquid to boil? The teachers edition of the textbook says, Particles throughout the liquid must have enough energy to vaporize. Who has two thumbs and doesnt like that answer? The books answer sort of insinuates that every molecule in the liquid has enough energy to vaporize. I beg to disagree. Even in a hot liquid, there are some slow molecules. Molecules have to have a certain minimum speed to break away from each other, so even in boiling water, there are some molecules that are traveling too slow to escape from their attractive neighbors.

13.2 Section Assessment11. What condition must exist for a liquid to boil? The teachers edition of the textbook says, Particles throughout the liquid must have enough energy to vaporize. The definition of boiling point, as I understand it, is the temperature at which the vapor pressure of the liquid equals atmospheric pressure. Therefore, I would say that in order to boil, a liquid must be so hot that its vapor pressure equals local air pressure. At sea level on Earth on a typical day, air pressure = 101.3 kPa. Water has a vapor pressure of 101.3 kPa when its temperature is 100oC, so thats how hot water needs to get to boil (at 101.3 kPa). Under those conditions, even if the boiling water were surrounded by pure water vapor, instead of somewhat watery air, the water vapor could still only condense just as fast as the water vaporized.

13.2 Section Assessment12. Use Figure 13.9 to determine the boiling point of each liquid. a. ethanoic acid at 27 kPa boils at 76oC b. chloroform at 80 kPa boils at 52oC c. ethanol at 50 kPa boils at 62oC

13.2 Section Assessment13. Explain why the boiling point of a liquid varies with atmospheric pressure. Boiling occurs when the vapor pressure of a liquid equals the external pressure. If the atmospheric pressure changes, the boiling point will change. To have a higher vapor pressure, a liquid must be hotter. In other words, heating a liquid makes its molecules fly into the air faster. It may seem odd to use pressure units as a way to say how quickly molecules are vaporizing, but it is done. Just imagine that if the molecules flying out of a body of water were to hit a surface on the way out, theyd exert pressure on the surface. If molecules are flying into the air faster, there will be more collisions, and, therefore, more pressure.

13.2 Section Assessment14. Explain how evaporation lowers the temperature of a liquid. The first molecules to leave a liquid and fly into the air are the fastest ones. (The slower ones cant break away from their attractive neighbors.) Temperature is average molecular kinetic energy. Therefore, when the fastest molecules leave, this causes the average temperature of the remaining liquid molecules to decrease. This, in turn, makes the temperature go down. There is also the very important consideration that whenever an object moves away from something its attracted to (like its fellow liquid molecules), its going to slow down as it moves away, just as a thrown ball slows down as it rises into the sky.

13.3 Instructional Goals

SWBAT . . .. . . explain the orderly structure of solids in terms of intermolecular forces, and how molecular motion can disrupt a solid in spite of these forces.

13.3 Section Assessment15. In general, how are the particles arranged in solids? Particles in solids are packed tightly together in an orderly arrangement. The locations of particles are fixed.

13.3 Section Assessment16. 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.

13.3 Section Assessment17. How do allotropes of an element differ? Allotropes are different molecular forms of the same element in the same physical state.

13.3 Section Assessment18. What phases are in equilibrium at a substances melting point? At a substances melting point, the liquid and solid states are in equilibrium.

13.3 Section Assessment19. 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.

13.3 Section Assessment20. What is the difference between a crystal lattice and a unit cell? A crystal lattice is a repeating array of unit cells.

13.4 Instructional Goals

There are no

SWBAT]Sfor section 13.4. Thats why its extra credit. Im providing the answers anyway, just in case you did 13.4, because Im sure youre dying to see if you got them right . . .

13.4 Section Assessment21. 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.

13.4 Section Assessment22. 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.

13.4 Section Assessment23. Describe one practical use of sublimation. Some practical uses of sublimation are: Freeze-dried coffee, dry ice as a coolant, air fresheners, separating mixtures, and purifying substances.

13.4 Section Assessment24. 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, liquid, and gas) can be in equilibrium with each other.

13.4 Section Assessment25. Using Figure 13.15, estimate the boiling point of water at a pressure of 50 kPa. According to Figure 13.15, at 50 kPa, the boiling point of water would be about 60oC.