1_Gases(S)_Sept.13

# 1_Gases(S)_Sept.13 - Gases Petrucci Herring Madura and...

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CHEM 1000 3.0 Gases 1 Gases Petrucci, Herring Madura and Bissonnette : Chapter 6

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CHEM 1000 3.0 Gases 2 Problem set Chapter 6 Questions 5, 9, 13, 26, 39, 41, 50, 57, 69, 73, 74, 78, 82
CHEM 1000 3.0 Gases 3 We will be looking at Macroscopic and Microscopic properties: – Macroscopic Properties of bulk gases • Observable Pressure, volume, mass, temperature… – Microscopic Properties at the molecular level Not readily observable Mass of molecules, molecular speed, energy, collision frequency

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CHEM 1000 3.0 Gases 4 Macroscopic Properties Our aim is to look at the relationship between the macroscopic properties of a gas and end up with the gas laws
CHEM 1000 3.0 Gases 5 Pressure To contain a gas you must have a container capable of exerting a force on it (e.g. the walls of a balloon). This implies that the the gas is exerting a balancing force Normally we talk about the pressure (force/area) rather than force

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CHEM 1000 3.0 Gases 6 Measuring Pressure The simplest way to measure gas pressure is to have it balance a liquid pressure. Therefore we need to quantify the liquid pressure
CHEM 1000 3.0 Gases 7 A h Consider a cylinder of liquid with area A and height h The force exerted at the bottom of the cylinder is its weight F = m.g The pressure exerted is P = F/A = m.g/A The density of the liquid is d=m/V so m = V.d but V=A.h So P = m.g/A = g.V.d/A = g.A.h.d/A = g.h.d

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CHEM 1000 3.0 Gases 8 Barometer To measure Atmospheric Pressure On the left the tube is open On the right the tube is closed and a liquid column is supported by the atmospheric pressure: Air pressure equals the liquid pressure
CHEM 1000 3.0 Gases 9 Barometer (continued) So for a barometer P = g.h.d P = atmospheric pressure h = height of liquid column d = density of the liquid

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CHEM 1000 3.0 Gases 10 Barometer (continued) For a barometer P = g.h.d Note that if we have a constant “g” and constant “d” (like with a mercury barometer on earth) P h so we can use h as a way of describing pressure
CHEM 1000 3.0 Gases 11 Atmospheric Pressure The average pressure at sea level will support a column of 760 mm of mercury. This is the definition of the pressure unit of “atmospheres” 1 atm = 760 mm Hg Also the unit mm Hg has become known as Torr (after Torricelli) 1 atm = 760 mm Hg = 760 Torr

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CHEM 1000 3.0 Gases 12 Atmospheric Pressure What is 1 atm in pressure units? P = g.h.d g = 9.81 m.s -2 , h = 0.76 m, d Hg = 13.6 g.cm -3 = 13.6x10 3 kg.m -3 P = 9.81x0.76x13.6x10 3 = 1.013x10 5 Pa (N.m -2 )
CHEM 1000 3.0 Gases 13 If we made a barometer out of water, what would be the height of the water column if the pressure is 745 torr? The problem calls for the relationship between P and h P = g.h.d 2 - 3 - 3 3 - 5 s m 9.81 g m kg 10 00 . 1 cm g 1.00 d Pa 10 1.013 760 745 P = × = = × × = 3 5 10 1.00 h 81 . 9 10 013 . 1 760 745 g.h.d P × × × = × × = h = 10.1 m

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CHEM 1000 3.0 Gases 14 Measuring Gas Pressures Gas pressures can be measured with a manometer . This is similar to a barometer but measures pressure differences using a liquid.
CHEM 1000 3.0 Gases 15 When one side of the manometer is open to the atmosphere

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CHEM 1000 3.0 Gases 16 Gas Laws The aim is to determine the relationship between the gas observables (pressure, volume, mass, temperature).
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