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GASES Chemistry 2011

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Properties of Gases

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Characteristics of Gases Fill their containers completely Have mass Can be compressed Exert pressure Mix rapidly

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Kinetic Molecular Theory

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A simple model to explain the properties of gases Assumptions: 1.The volume of the individual gas particles is insignificant (zero). 2.The particles are in constant random motion. Perfectly elastic collisions. (no loss of energy) 3.The particles exert no forces (repulsion or attraction) on each other 4.The average kinetic energy is directly proportional to it’s temperature in Kelvin.

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KMT summary Very small particles (atoms or molecules) Very far apart Constant rapid, random motion

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Kinetic Energy Kinetic energy depends on temperature KE = 3/2 RT Kinetic energy is also dependent on mass and velocity of particles, KE = mv 2 /2

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Measuring Gases

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Four Variables to describe a gas Pressure Volume Temperature Number of particles

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Pressure The amount of force exerted by gas molecules hitting the walls of their container Force per unit area Typical units atm, mm Hg, torr, kPa, psi Conversions 1 atm = 760 mm Hg = 760 torr = kPa = 14.7 psi

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Barometers

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Manometer and Barometers Manometers measure gas pressure. Barometers measure atmospheric pressure.

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1. If atmospheric pressure is 753 mm Hg, what is the pressure of the gas in this manometer? Express your answer in atmospheres.

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If atmospheric pressure is atm, what is the pressure of the gas in this manometer? Express your answer in atmospheres.

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Homework – Copy and Complete Express your answers in atm.

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Do Now: Turn in your homework Complete pg 390 #25-28

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Manometer answers atm atm atm atm atm atm

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Temperature and Kinetic Theory Temperature is the measure of the average kinetic energy of the particles in a substance. KE = 1 mv 2 or KE = 3 RT 22

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Heat ≠ Temperature Heat is the amount of energy transferred from a hotter object to a cooler object. Heat is measured in Joules

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Boyle’s Law

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At constant temperature, pressure and volume of a gas are inversely related. Mathematically, P 1 V 1 = P 2 V 2 Where, P 1 = Pressure at one state (initial), V 1 = Volume at one state (initial), P 2 = Pressure at another state (final), V 2 = Volume at another state (final)

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Charles’ Law

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If the amount and pressure of a gas are constant, the volume of the gas is directly proportional to temperature in Kelvin. Mathematically, V 1 = V 2 T 1 T 2

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Check your Boyle’s Law and Charles’ Law Worksheet Boyle’s Law, pg mL kPa L mL 5.88mL mL L mL Charles’ Law, pg mL K or 610 o C 3.98 mL 4.50 mL L K or -93 o C L K or 57 o C

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Guy – Lussac’s Law If the amount and volume of gas are constant, pressure is directly proportional to temperature. Mathematically, P 1 = P 2 T 1 T 2

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Hmwk Pg 472 #32-34, 42, 44

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Check your hmwk answers 48. The pressure increases by a factor of four. 50. The volume decreases. They have less kinetic energy, which causes less pressure on the inside of the balloon kPa L L K

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Combined Gas Law

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Avogadro’s Law

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If pressure and temperature are constant, volume is directly proportional to the amount (moles) of gas. Mathematically, V 1 = V 2 n 1 n 2

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New combined gas law Combines all four gas laws. Anything that is held constant when changing conditions gets crossed out.

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Ideal Gas Law

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Combines all four variables of the gas, pressure, temperature, amount, and volume. PV = nRT Where, P = pressure in kPa or atm V = volume in Liters n = moles of gas R = Universal Gas constant ( L·atm/K·mol or 8.31 L·kPa/K·mol) T = Temperature in Kelvin

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Using the ideal gas law When a particular gas is at one state. No changes in any of the conditions When you know three of the four variables, you can solve for the 4 th. Example, Calculate the volume of 3.00 mol H 2 at 24 o C and kPa L H 2

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Density and ideal gas

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What is the fault in the logic? I noticed my tires were a bit low and went to the gas station. As I was filling the tires, I thought about the kinetic molecular theory (KMT). I realized that I was increasing both the pressure and volume of the tires. “Hmmm,” I thought, “that goes against what I learned in chemistry, where I was told pressure and volume are inversely proportional.”

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Dalton’s Law of Partial Pressure

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The total pressure of a system is the sum of the partial pressures of all the gases that make up the system. P tot = P 1 + P 2 + P 3 + … –Where P tot is the total pressure –P 1 is the pressure of gas 1 –P 2 is the pressure of gas 2 –P 3 is the pressure of gas 3

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Example L of a gas is collected over water at 23.0°C with a total pressure of kPa. What is the pressure of the dry gas? The vapor pressure of water at 23.0 o C is 2.81 kPa

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