Introductory Chemistry , 3 rd Edition Nivaldo Tro

1. Introductory Chemistry, 3rd EditionNivaldo Tro Chapter 11 Gases Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2009, Prentice Hall

2. Properties of Gases • Expand to completely fill their container. • Take the shape of their container. • Low density. • Much less than solid or liquid state. • Compressible. • Mixtures of gases are always homogeneous. • Fluid. Tro's Introductory Chemistry, Chapter 11

3. The Structure of a Gas • Gases are composed of particles that are flying around very fast in their container(s). • They move in straight lines until they encounter either the container wall or another particle, then they bounce off. • If you were able to take a snapshot of the particles in a gas, you would find that there is a lot of empty space in there. Tro's Introductory Chemistry, Chapter 11

4. Kinetic Molecular Theory • The particles of the gas (either atoms or molecules) are constantly moving. • The attraction between particles is negligible. • When the moving particles hit another particle or the container, they do not stick, but they bounce off and continue moving in another direction. • Like billiard balls. Tro's Introductory Chemistry, Chapter 11

5. Kinetic Molecular Theory of Gases • There is a lot of empty space between the particles in a gas. • Compared to the size of the particles. • The average kinetic energy of the particles is directly proportional to the Kelvin temperature. • As you raise the temperature of the gas, the average speed of the particles increases. • But don’t be fooled into thinking all the particles are moving at the same speed!! Tro's Introductory Chemistry, Chapter 11

6. Kinetic Molecular Theory Tro's Introductory Chemistry, Chapter 11

7. Gas Particles Pushing • Gas molecules are constantly in motion. • As they move and strike a surface, they push on that surface. • Push = force. • If we could measure the total amount of force exerted by gas molecules hitting the entire surface at any one instant, we would know the pressure the gas is exerting. • Pressure = force per unit area. Tro's Introductory Chemistry, Chapter 11

8. The Effect of Gas Pressure • The pressure exerted by a gas can cause some amazing and startling effects. • Whenever there is a pressure difference, a gas will flow from area of high pressure to low pressure. • The bigger the difference in pressure, the stronger the flow of the gas. • If there is something in the gas’ path, the gas will try to push it along as the gas flows. Tro's Introductory Chemistry, Chapter 11

9. Which Way Would Air Flow? Two filled balloons are connected with a long pipe. One of the balloons is plunged down into the water. Which way will the air flow? Will air flow from the lower balloon toward the top balloon; or will it flow from the top balloon to the bottom one? Tro's Introductory Chemistry, Chapter 11 9

10. Is This Possible at a Depth of 20 m? Tro's Introductory Chemistry, Chapter 11 10

11. Soda Straws and Gas Pressure The pressure of the air inside the straw is the same as the pressure of the air outside the straw—so liquid levels are the same on both sides. The pressure of the air inside the straw is lower than the pressure of the air outside the straw—so liquid is pushed up the straw by the outside air. Tro's Introductory Chemistry, Chapter 11

12. Gas Properties Explained • Gases have taken the shape and volume of their container(s) because the particles don’t stick together, allowing them to move and fill the container(s) they’re in. • In solids and liquids, the particles are attracted to each other strongly enough so they stick together. • Gases are compressible and have low density because of the large amount of unoccupied space between the particles. Tro's Introductory Chemistry, Chapter 11

13. Properties—Indefinite Shape and Indefinite Volume Because the gas molecules have enough kinetic energy to overcome attractions, they keep moving around and spreading out until they fill the container. As a result, gases take the shape and the volume of the container they are in. Tro's Introductory Chemistry, Chapter 11

14. Properties—Compressibility Because there is a lot of unoccupied space in the structure of a gas, the gas molecules can be squeezed closer together. Tro's Introductory Chemistry, Chapter 11

15. Gas Properties Explained—Low Density Because there is a lot of unoccupied space in the structure of a gas, gases do not have a lot of mass in a given volume, the result is that they have low density. Tro's Introductory Chemistry, Chapter 11

16. The Pressure of a Gas • Pressure is the result of the constant movement of the gas molecules and their collisions with the surfaces around them. • The pressure of a gas depends on several factors: • Number of gas particles in a given volume. • Volume of the container. • Average speed of the gas particles. Tro's Introductory Chemistry, Chapter 11

17. Density and Pressure • Pressure is the result of the constant movement of the gas molecules and their collisions with the surfaces around them. • When more molecules are added, more molecules hit the container at any one instant, resulting in higher pressure. • Also higher density. Tro's Introductory Chemistry, Chapter 11

18. Air Pressure • The atmosphere exerts a pressure on everything it contacts. • On average 14.7 psi. • The atmosphere goes up about 370 miles, but 80% is in the first 10 miles from Earth’s surface. • This is the same pressure that a column of water would exert if it were about 10.3 m high. Tro's Introductory Chemistry, Chapter 11

19. Measuring Air Pressure • Use a barometer. • Column of mercury supported by air pressure. • Force of the air on the surface of the mercury balanced by the pull of gravity on the column of mercury. gravity Tro's Introductory Chemistry, Chapter 11

20. Atmospheric Pressure and Altitude • The higher up in the atmosphere you go, the lower the atmospheric pressure is around you. • At the surface, the atmospheric pressure is 14.7 psi, but at 10,000 ft is is only 10.0 psi. • Rapid changes in atmospheric pressure may cause your ears to “pop” due to an imbalance in pressure on either side of your ear drum. Tro's Introductory Chemistry, Chapter 11

21. Pressure Imbalance in Ear If there is a difference in pressure across the eardrum membrane, the membrane will be pushed out—what we commonly call a “popped eardrum.” Tro's Introductory Chemistry, Chapter 11

22. Common Units of Pressure Tro's Introductory Chemistry, Chapter 11

23. psi atm mmHg Example 11.1—A High-Performance Bicycle Tire Has a Pressure of 125 psi. What Is the Pressure in mmHg? Given: Find: 125 psi mmHg Solution Map: Relationships: 1 atm = 14.7 psi, 1 atm = 760 mmHg Solution: Check: Since mmHg are smaller than psi, the answer makes sense.

24. Practice—Convert 45.5 psi into kPa. Tro's Introductory Chemistry, Chapter 11

25. psi atm kPa Practice—Convert 45.5 psi into kPa, Continued Given: Find: 45.5 psi kPa Concept Plan: Relationships: 1 atm = 14.7 psi, 1 atm = 101.325 kPa Solution: Check: Since kPa are smaller than psi, the answer makes sense.

26. Boyle’s Law • Pressure of a gas is inversely proportional to its volume. • Constant T and amount of gas. • Graph P vs. V is curved. • Graph P vs. 1/V is in a straight line. • As P increases, V decreases by the same factor. • P x V = constant. • P1 x V1 = P2 x V2. Tro's Introductory Chemistry, Chapter 11

27. Boyle’s Experiment • Added Hg to a J-tube with air trapped inside. • Used length of air column as a measure of volume. Tro's Introductory Chemistry, Chapter 11

28. Tro's Introductory Chemistry, Chapter 11

29. Tro's Introductory Chemistry, Chapter 11

30. Boyle’s Experiment, P x V Tro's Introductory Chemistry, Chapter 11

31. When you double the pressure on a gas, the volume is cut in half (as long as the temperature and amount of gas do not change). Tro's Introductory Chemistry, Chapter 11

32. Gas Laws Explained— Boyle’s Law • Boyle’s law says that the volume of a gas is inversely proportional to the pressure. • Decreasing the volume forces the molecules into a smaller space. • More molecules will collide with the container at any one instant, increasing the pressure. Tro's Introductory Chemistry, Chapter 11

33. Boyle’s Law and Diving Scuba tanks have a regulator so that the air from the tank is delivered at the same pressure as the water surrounding you. This allows you to take in air even when the outside pressure is large. • Since water is more dense than air, for each 10 m you dive below the surface, the pressure on your lungs increases 1 atm. • At 20 m the total pressure is 3 atm. • If your tank contained air at 1 atm of pressure, you would not be able to inhale it into your lungs. • You can only generate enough force to overcome about 1.06 atm. Tro's Introductory Chemistry, Chapter 11

34. Boyle’s Law and Diving, Continued • If a diver holds her breath and rises to the surface quickly, the outside pressure drops to 1 atm. • According to Boyle’s law, what should happen to the volume of air in the lungs? • Since the pressure is decreasing by a factor of 3, the volume will expand by a factor of 3, causing damage to internal organs. Always Exhale When Rising!! Tro's Introductory Chemistry, Chapter 11

35. V1, P1, P2 V2 Example 11.2—A Cylinder with a Movable Piston Has a Volume of 6.0 L at 4.0 atm. What Is the Volume at 1.0 atm? Given: Find: V1 =6.0 L, P1 = 4.0 atm, P2 = 1.0 atm V2, L Solution Map: Relationships: P1∙ V1= P2∙ V2 Solution: Check: Since P and V are inversely proportional, when the pressure decreases ~4x, the volume should increase ~4x, and it does.

36. Practice—A Balloon Is Put in a Bell Jar and the Pressure Is Reduced from 782 torr to 0.500 atm. If the Volume of the Balloon Is Now 2780 mL, What Was It Originally?(1 atm = 760 torr) Tro's Introductory Chemistry, Chapter 11

37. V2, P1, P2 V1 Practice—A Balloon Is Put in a Bell Jar and the Pressure Is Reduced from 782 torr to 0.500 atm. If the Volume of the Balloon Is Now 2780 mL, What Was It Originally?, Continued Given: Find: V2 =2780 mL, P1 = 762 torr, P2 = 0.500 atm V1, mL Solution Map: Relationships: P1∙ V1= P2∙ V2 , 1 atm = 760 torr (exactly) Solution: Check: Since P and V are inversely proportional, when the pressure decreases ~2x, the volume should increase ~2x, and it does.

38. Temperature Scales 100°C 373 K 212°F 671 R BP Water 0°C 273 K 32°F 459 R MP Ice -38.9°C 234.1 K -38°F 421 R BP Mercury -183°C 90 K -297°F 162 R BP Oxygen BP Helium -269°C 4 K -452°F 7 R -273°C 0 K -459 °F 0 R Absolute Zero Celsius Kelvin Fahrenheit Rankine

39. Gas Laws and Temperature • Gases expand when heated and contract when cooled, so there is a relationship between volume and temperature. • Gas molecules move faster when heated, causing them to strike surfaces with more force, so there is a relationship between pressure and temperature. • In order for the relationships to be proportional, the temperature must be measured on an absolute scale. • When doing gas problems, always convert your temperatures to kelvins. K = °C + 273 & °C = K - 273 °F = 1.8 °C + 32 & °C = 0.556(°F-32) Tro's Introductory Chemistry, Chapter 11

40. Standard Conditions • Common reference points for comparing. • Standard pressure = 1.00 atm. • Standard temperature = 0 °C. • 273 K. • STP. Tro's Introductory Chemistry, Chapter 11

41. Volume and Temperature • In a rigid container, raising the temperature increases the pressure. • For a cylinder with a piston, the pressure outside and inside stay the same. • To keep the pressure from rising, the piston moves out increasing the volume of the cylinder. • As volume increases, pressure decreases. Tro's Introductory Chemistry, Chapter 11

42. Volume and Temperature, Continued As a gas is heated, it expands. This causes the density of the gas to decrease. Because the hot air in the balloon is less dense than the surrounding air, it rises. Tro's Introductory Chemistry, Chapter 11

43. Charles’s Law • Volume is directly proportional to temperature. • Constant P and amount of gas. • Graph of V vs. T is a straight line. • As T increases, V also increases. • Kelvin T = Celsius T + 273. • V = constant x T. • If T is measured in kelvin. Tro's Introductory Chemistry, Chapter 11

45. We’re losing altitude. Quick, Professor, give your lecture on Charles’s law!

46. Absolute Zero • Theoretical temperature at which a gas would have zero volume and no pressure. • Kelvin calculated by extrapolation. • 0 K = -273.15 °C = -459 °F = 0 R. • Never attainable. • Though we’ve gotten real close! • All gas law problems use the Kelvin temperature scale. Tro's Introductory Chemistry, Chapter 11

47. Determining Absolute Zero William Thomson, the Lord of Kelvin, extrapolated the line graphs of volume vs. temp- erature to determine the theoretical temperature that a gas would have given a volume of 0. Tro's Introductory Chemistry, Chapter 11

48. T(K) = t(°C) + 273, V1, V2, T2 T1 Example 11.3—A Gas Has a Volume of 2.57 L at 0 °C. What Was the Temperature at 2.80 L? Given: Find: V1 =2.80 L, V2 = 2.57 L, t2 = 0°C t1, K and °C Solution Map: Relationships: Solution: Check: Since T and V are directly proportional, when the volume decreases, the temperature should decrease, and it does.

49. Practice—The Temperature Inside a Balloon Is Raised from 25.0 °C to 250.0 °C. If the Volume of Cold Air Was 10.0 L, What Is the Volume of Hot Air? Tro's Introductory Chemistry, Chapter 11

50. T(K) = t(°C) + 273.15, V1, T1, T2 V2 Practice—The Temperature Inside a Balloon Is Raised from 25.0 °C to 250.0 °C. If the Volume of Cold Air Was 10.0 L, What Is the Volume of Hot Air?, Continued Given: Find: V1 =10.0 L, t1 = 25.0 °C L, t2 = 250.0 °C V2, L Solution Map: Relationships: Solution: Check: Since T and V are directly proportional, when the temperature increases, the volume should increase, and it does.