04_-_Gases

04_-_Gases - ChE102: Chapter6Gases 2 When talking about...

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ChE 102: Chemistry for Engineers Chapter 6 – Gases
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04 - Gases 2 Definition of Pressure When talking about gases, four properties are often used: amount (moles), temperature, volume & pressure Pressure is the force per unit area = force/area Force resulting from the collision of the gas molecules against the walls of the container Area of the walls of the container holding the gas
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04 - Gases 3 Understanding Pressure Gas pressure is usually measured indirectly, by comparing it with liquid pressure Liquid pressure is directly proportional to the liquid density and the height of the liquid column P Hg = P air g h A g A h A g V A g m A F P Hg Hg Hg Hg Hg Hg Hg . . . . . . . . ρ = = = = = A g m A F P air air air . = = m Hg = m air Petrucci p181
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04 - Gases 4 Understanding Pressure Open-ended Manometer Petrucci p182
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04 - Gases 5 The Simple Gas Laws Boyle’s Law (1662):
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04 - Gases 6 The Simple Gas Laws Boyle’s Law: “For a fixed amount of gas at a constant temperature, the gas volume is inversely proportional to the gas pressure” PV = constant for a given temperature and given amount of substance V P 1
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04 - Gases 7 The Simple Gas Laws Charles’ Law (1787):
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04 - Gases 8 The Simple Gas Laws Charles’ Law: “The volume of a fixed amount of gas at constant pressure is directly proportional to the Kelvin (absolute) temperature” For all gases at small pressures, c = 273.15 T V + = c C T V V o ) ( 1 0 V 0 : volume of the gas at 0 t : temperature in o C c : a constant + = 15 . 273 ) ( 1 0 C T V V o
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04 - Gases 9 Aside  – Definition of Temperature from Charles’s Law Temperature is measured using the ratio of the volume of a gas at the measured temperature to the volume of the same gas at 0ºC (freezing point of water) If T = –273.15 ° C (absolute zero), then V = 0. Lowest temperature possible before V < 0, which has no physical meaning Kelvin temperature scale is based on this concept: T (K) = T (° C) + 273.15 Substituting this definition into Charles’ law gives: - = 1 15 . 273 ) ( 0 V V C T o ( 29 c K T V K T V V 0 0 15 . 273 15 . 273 ) ( 1 = - + =
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04 - Gases 10 The Simple Gas Laws Avogadro’s Law (1808): “At a fixed temperature and pressure, the volume of a gas is directly proportional to the amount of gas” STP: Standard Temperature and Pressure Temperature = 0ºC = 273.15 K Pressure = 1 atm = 760 mm Hg 1 mol = 22.4 L n V
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04 - Gases 11 The Ideal Gas Equation Combining all three gas relationships… Avogadro: Charles: Boyle: Combines to: Or the Ideal Gas Equation: PV = nRT Where R = C/c n V c T V V 0 = P nC V = P nC V = 0 c T P nC V =
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04 - Gases 12 Ideal Gas Constant At STP 1 mol of most gases occupies 22.4 L: R can also be given in units of J/(mol·K), since:
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04_-_Gases - ChE102: Chapter6Gases 2 When talking about...

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