Gases. Chapter 12. Properties of Gases Pressure. Expand to completely fill their container Take the shape of their container Low density

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1 Copyright 2004 by Houghton Mifflin Company. Gases Chapter 12 All rights reserved. 1 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 are always homogeneous Fluid Pressure Pressure = total force applied to a certain area larger force = larger pressure smaller area = larger pressure Gas pressure caused by gas molecules colliding with container or surface More forceful collisions or more frequent collisions mean higher gas pressure 3

2 Pressure Air Pressure Constantly present when air present Decreases with altitude due to less air Varies with weather conditions Measured using a barometer Column of Hg supported by air pressure Longer Hg column supported = higher pressure Force of air on Hg surface balanced by pull of gravity on Hg column 12.1 Pressure Manometer Measures pressure of gas sample relative to atmospheric pressure Pressure Units atmosphere (atm) mm Hg, inches Hg torr Pascal (Pa) pounds per square inch (psi, lbs./in 2 ) atm = mm Hg = torr = in Hg = 101,325 Pa = psi 6

3 Kinetic Molecular Theory Postulates of KMT of Gases 1. Gases consist of tiny particles (atoms/molecules) 2. The volume of the particles is neglegible in comparison to the empty space between them 3. The constant random motion of the particles results in collisions with the walls which is felt as pressure 4. The particle do not attract nor repel each other 5. The average kinetic energy of the gas particles is directly proportional to the temperature of the gas The Ideal Gas Law Ideal Gas Law: PV = nrt Ideal Gas Constant: R = L atm / mole K What volume would 25.0 g of carbon dioxide gas occupy at 25.0 C & 775 torr? The Ideal Gas Law Ideal Gas Law: PV = nrt Ideal Gas Constant: R = L atm / mole K Put all of the variables on one side of the equation: PV nt = R The ratio of the variables will always be constant If you change one variable, the others will adjust so that the ratio remains constant P 1 V 1 = R = P 2V 2 n 1 T 1 n 2 T 2 9

4 Boyle s Law Pressure is inversely proportional to volume constant T and amount of gas if P is increased, V decrease 12.2 Boyle s Law Pressure is inversely proportional to volume constant T and amount of gas if P is increased, V decrease graph P vs V is curve Boyle s Law Pressure is inversely proportional to volume constant T and amount of gas if P is increased, V decrease graph P vs V is curve graph P vs 1/V is straight line 12

5 Boyle s Law Pressure is inversely proportional to volume constant T and amount of gas if P is increased, V decrease At 1.05 atm a balloon has a volume of 1.25 L. If it is taken to the bottom of a lake, where the pressure is 1452 torr, what will be the new volume of the balloon? P 1 V 1 n 1 T 1 = R = P 2 V 2 n 2 T Charles Law Volume is directly proportional to temperature at constant P & n as T increases, so does V graph of V vs T is straight line Charles Law Volume is directly proportional to temperature at constant P & n as T increases, so does V graph of V vs T is straight line If a balloon has a volume of 1.25 L at 25.0 C, what would its volume be at if it was put into a freezer at 0.00 C? P 1 V 1 n 1 T 1 = R = P 2V 2 n 2 T 2 15

6 Avogadro s Law Volume directly proportional to the number of gas molecules at constant P & T more gas molecules occupy larger volume 12.4 Avogadro s Law Volume directly proportional to the number of gas molecules at constant P & T more gas molecules occupy larger volume If 2.55 mol of helium occupies 59.5 L, calculate the mass of helium that must be added to increase the volume to 183 L, at constant temperature & pressure P 1 V 1 n 1 T 1 = R = P 2 V 2 n 2 T Avogadro s Law Volume directly proportional to the number of gas molecules Standard conditions (STP): 1 atm & 273 K Calculate the molar volume of a gas at STP Molar volume: 18

7 Dalton s Law The total pressure of a mixture of gases equals the sum of the partial pressures of each gas Partial pressures is the pressure a gas in a mixture would exert if it were alone in the container P total = P gas A + P gas B Dalton s Law The total pressure of a mixture of gases equals the sum of the partial pressures of each gas Particularly useful for determining the pressure a dry gas would have after it is collected over water P air = P wet gas = P dry gas + P water vapor P water vapor depends on the temperature, look up in table Dalton s Law The total pressure of a mixture of gases equals the sum of the partial pressures of each gas Particularly useful for determining the pressure a dry gas would have after it is collected over water Calculate the partial pressure of an oxygen sample that s been collected over water at 30 C. The total pressure is measured to be 753 torr and the partial pressure of water at 30 C is torr. 21

8 Gas Stoichiometry The ideal gas law, or the combined gas law, can be used to convert data of a gas into moles, or from moles into other information. Stoichiometry always uses mole ratios in the calculations Gas Stoichiometry What volume of ammonia could be collected from 3.00 moles of nitrogen gas reacting with 12.0 moles of hydrogen gas, at 15 C and 1263 torr? 23