Chapter 6 Gases 1

6.1 Properties of Gases 6.2 Gas Pressure Kinetic Theory of Gases A gas consists of small particles that move rapidly in straight lines. have essentially no attractive (or repulsive) forces. are very far apart. have very small volumes compared to the volume of the container they occupy. have kinetic energies that increase with an increase in temperature. 2

Properties That Describe a Gas Gases are described in terms of four properties: pressure (P), volume (V), temperature (T), and amount (n). 3

Gas Pressure Gas pressure is a force acting on a specific area. Pressure (P) = force area has units of atm, mmHg, torr, lb/in. 2, and kilopascals(kPa). 1 atm = 760 mm Hg (exact) 1 atm = 760 torr 1 atm = 14.7 lb/in. 2 1 atm = Pa 1 atm = kPa 4

Examples A. What is 475 mmHg expressed in atm? 1) 475 atm 2) atm 3) 3.61 x 10 5 atm B. The pressure in a tire is 2.00 atm. What is this pressure in mmHg? 1) 2.00 mmHg 2) 1520 mmHg 3) mmHg 5

Atmospheric Pressure Atmospheric pressure is the pressure exerted by a column of air from the top of the atmosphere to the surface of the Earth. 6

Altitude and Atmospheric Pressure Atmospheric pressure is about 1 atmosphere at sea level. depends on the altitude and the weather. is lower at higher altitudes, where the density of air is less. is higher on a rainy day than on a sunny day. 7

Barometer A barometer measures the pressure exerted by the gases in the atmosphere. indicates atmospheric pressure as the height in mm of the mercury column. 8

6.3Pressure and Volume (Boyle’s Law) Boyle’s law states that the pressure of a gas is inversely related to its volume when T and n are constant. if volume decreases, the pressure increases. 9

PV Constant in Boyle’s Law In Boyle’s law, the product P x V is constant as long as T and n do not change. P 1 V 1 = 8.0 atm x 2.0 L = 16 atm L P 2 V 2 = 4.0 atm x 4.0 L = 16 atm L P 3 V 3 = 2.0 atm x 8.0 L = 16 atm L Boyle’s law can be stated as P 1 V 1 = P 2 V 2 (T, n constant) 10

Boyles’ Law and Breathing During an inhalation, the lungs expand. the pressure in the lungs decreases. air flows towards the lower pressure in the lungs. 11

Boyles’ Law and Breathing During an exhalation, lung volume decreases. pressure within the lungs increases. air flows from the higher pressure in the lungs to the outside. 12

Examples For a cylinder containing helium gas, indicate if cylinder A or cylinder B represents the new volume for the following changes (n and T are constant). 1) pressure decreases 2) pressure increases 13

Calculation with Boyle’s Law Freon-12, CCl 2 F 2, is used in refrigeration systems. What is the new volume (L) of a 8.0 L sample of Freon gas initially at 550 mmHg after its pressure is changed to 2200 mmHg at constant T and n? 1. Set up a data table: Conditions 1Conditions 2 P 1 = 550 mmHgP 2 = 2200 mmHg V 1 = 8.0 LV 2 = 14

Examples If a sample of helium gas has a volume of 120 mL and a pressure of 850 mmHg, what is the new volume if the pressure is changed to 425 mmHg? 1) 60 mL 2) 120 mL3) 240 mL 15

6.4Temperature and Volume (Charles’s Law) In Charles’s Law, the Kelvin temperature of a gas is directly related to the volume. P and n are constant. when the temperature of a gas increases, its volume increases. For two conditions, Charles’s law is written V 1 = V 2 ( P and n constant) T 1 T 2 16

Calculations Using Charles’s Law A balloon has a volume of 785 mL at 21 °C. If the temperature drops to 0 °C, what is the new volume of the balloon (P constant)? 1.Set up data table: Conditions 1Conditions 2 V 1 = 785 mLV 2 = ? T 1 = 21 °C = 294 KT 2 = 0 °C = 273 K Be sure to use the Kelvin (K) temperature in gas calculations. 17

Examples Use the gas laws to complete each sentence with 1) increases or 2) decreases. A. Pressure _______ when V decreases. B. When T decreases, V _______. C. Pressure _______ when V changes from 12 L to 24 L. D. Volume _______when T changes from 15 °C to 45 °C. 18

Examples A sample of oxygen gas has a volume of 420 mL at a temperature of 18 °C. At what temperature (in °C) will the volume of the oxygen be 640 mL (P and n constant)? 1) 443 °C 2) 170 °C 3) - 82 °C 19

6.5Temperature and Pressure (Gay-Lussac’s Law) In Gay-Lussac’s law the pressure exerted by a gas is directly related to the Kelvin temperature. V and n are constant. P 1 = P 2 T 1 T 2 20

Calculation with Gay-Lussac’s Law A gas has a pressure at 2.0 atm at 18 °C. What is the new pressure when the temperature is 62 °C? (V and n constant) 1. Set up a data table: Conditions 1Conditions 2 P 1 = 2.0 atmP 2 = T 1 = 18 °C T 2 = 62 °C = 291 K = 335 K 21 ?

Example A gas has a pressure of 645 torr at 128 °C. What is the temperature in Celsius if the pressure increases to 824 torr? (n and V remain constant) 22

6.6 The Combined Gas Law The combined gas law uses Boyle’s law, Charles’s law, and Gay- Lussac’s law (n is constant). P 1 V 1 =P 2 V 2 T 1 T 2 23

Combined Gas Law Calculation A sample of helium gas has a volume of L, a pressure of atm, and a temperature of 29 °C. At what temperature (°C) will the helium have a volume of 90.0 mL and a pressure of 3.20 atm? (n is constant) 1. Set up data table. Conditions 1Conditions 2 P 1 = atm P 2 = 3.20 atm V 1 = L (180 mL) V 2 = 90.0 mL T 1 = 29 °C = 302 KT 2 = ?? 24

Examples A gas has a volume of 675 mL at 35 °C and atm pressure. What is the volume (mL) of the gas at -95 °C and a pressure of 802 mmHg? (n constant) 25

6.7 Volume and Moles (Avogadro’s Law) In Avogadro’s law the volume of a gas is directly related to the number of moles (n) of gas. T and P are constant. V 1 = V 2 n 1 n 2 26

Example If 0.75 mole of helium gas occupies a volume of 1.5 L, what volume will 1.2 moles of helium occupy at the same temperature and pressure? 1) 0.94 L 2)1.8 L 3) 2.4 L 27 Copyright © 2009 by Pearson Education, Inc.

STP The volumes of gases can be compared at STP, Standard Temperature and Pressure, when they have the same temperature. standard temperature (T) 0 °C or 273 K the same pressure. standard pressure (P) 1 atm (760 mmHg) 28

Molar Volume At standard temperature and pressure (STP), 1 mole of a gas occupies a volume of 22.4 L, which is called its molar volume. 29 Copyright © 209 by Pearson Education, Inc. The molar volume at STP can be used to write conversion factors L and 1 mole 1 mole 22.4 L

Using Molar Volume What is the volume occupied by 2.75 moles of N 2 gas at STP? The molar volume is used to convert moles to liters. 30

Example A. What is the volume at STP of 4.00 g of CH 4 ? 1) 5.60 L2) 11.2 L3) 44.8 L B. How many g of He are present in 8.00 L of gas at STP? 1) 25.6 g2) g3) 1.43 g 31

STP and Gas Equations What volume (L) of O 2 gas at STP is needed to completely react with 15.0 g of aluminum? 4Al(s) + 3O 2 (g) 2Al 2 O 3 (s) Plan: g Al mole Al mole O 2 L O 2 (STP) 32

Example What mass of Fe will react with 5.50 L of O 2 at STP? 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) 33

6.8 Partial Pressures (Dalton’s Law) The partial pressure of a gas is the pressure of each gas in a mixture. is the pressure that gas would exert if it were by itself in the container. 34

Dalton’s Law of Partial Pressures Dalton’s law of partial pressures indicates that pressure depends on the total number of gas particles, not on the types of particles. the total pressure exerted by gases in a mixture is the sum of the partial pressures of those gases. P T = P 1 + P 2 + P

Total Pressure For example, at STP, 1 mole of a pure gas in a volume of 22.4 L will exert the same pressure as 1 mole of a gas mixture in 22.4 L. V = 22.4 L Gas mixtures mole O mole He 0.2 mole Ar 1.0 mole 1.0 mole N mole O mole He 1.0 mole 1.0 atm

Scuba Diving When a scuba diver dives, the increased pressure causes N 2 (g) to dissolve in the blood. If a diver rises too fast, the dissolved N 2 will form bubbles in the blood, a dangerous and painful condition called "the bends." Helium, which does not dissolve in the blood, is mixed with O 2 to prepare breathing mixtures for deep descents. 37

Gases We Breathe The air we breathe is a gas mixture. contains mostly N 2 and O 2, and small amounts of other gases. 38 7

Example A scuba tank contains O 2 with a pressure of atm and He at 855 mmHg. What is the total pressure in mmHg in the tank? 39

Example For a deep dive, a scuba diver uses a mixture of helium and oxygen with a pressure of 8.00 atm. If the oxygen has a partial pressure of 1280 mmHg, what is the partial pressure of the helium? 1) 520 mmHg 2) 2040 mmHg 3) 4800 mmHg 40

Examples A. If the atmospheric pressure today is 745 mmHg, what is the partial pressure (mmHg) of O 2 in the air? 1) ) 156 3) 760 B. At an atmospheric pressure of 714, what is the partial pressure (mmHg) N 2 in the air? 1) 557 2) 9.143)

Blood Gases In the lungs, O 2 enters the blood, while CO 2 from the blood is released. In the tissues, O 2 enters the cells, which releases CO 2 into the blood. 42

Blood Gases In the body, O 2 flows into the tissues because the partial pressure of O 2 is higher in blood, and lower in the tissues. CO 2 flows out of the tissues because the partial pressure of CO 2 is higher in the tissues, and lower in the blood. Partial Pressures in Blood and Tissue Oxygenated Deoxygenated Gas BloodBlood Tissues O or less CO or greater 43