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3.3 Kinetic Model of an Ideal Gas Definition of Properties Pressure Volume

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DEFINITION A state of matter where particles are so separated, that intermolecular forces are almost negligible. Particles have a higher kinetic energy than liquid, or solid.

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Gas particles move randomly.

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Kinetic Model Ideal Gas Assumptions 1)Molecules are spheres 2) Molecules identical 3) Perfectly elastic collisions (no loss KE). 4) No intermolecular forces – constant v between collisions – straight line. 5) No molecular volume.

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Ideal Gasses Obey ideal Gas Law PV = nRT P Pressure Pascals V in m 3. n – number of moles. R – gas constant 8.31 J/K mol. T – Kelvin.

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Pressure Force/Area. Pascal = N/m 2. 1 Atm = 101 kPa So if you have 1 m 2 window, it has 101,000 N pressing on it.

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Ex 1: A 360 N child sits on a stool which weighs 41 N. The bottom of the stools legs touch the ground over an area of 19.3 cm 2 :

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Gas Pressure Gas Molecules Exert Pressure on Container Collisions between gas molecules with each other & the container. Pressure from p / impulse of particle = Ft when they bounce & changes p.

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Low P, less force (KE), lower collision rate. The p (mv) causes impulse, J on walls. F t = J. F = p/t is a rate. High P, more force (KE), higher collision rate.

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Relationships Pressure Volume P increases, Volume decreases (fixed T) PV = nRT P = nRT 1 V

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Pressure & Volume Decrease volume - collision rate increases. P goes up.

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Temperature Volume T increases, V increases (Fixed P) PV = nRT V = nR T P

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Temperature & Volume When you push the piston in to reduce the volume, some molecules are swatted giving them E. Work is done on gas. When a gas pushes the piston out, increasing the volume, it does work & the E of the gas goes down.

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PV = nRT P = nR T V Pressure, Temperature (fixed V)

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Pressure, Temperature As T goes up molecular KE goes up, P goes up. Frequency of collisions go up. p increases with v. Impulse on wall increases.

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When gas P reaches zero, T = absolute zero.

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1. Why does blowing into a balloon increase its volume? 1. Blowing air into the balloon increases the # of air molecules, increasing the rate of collision inside the balloon, and increasing the pressure on the balloon wall.

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Particulate Nature of Matter and Changes of State 4 min. https://www.youtube.com/watch?v=ndw9X YA4iF0https://www.youtube.com/watch?v=ndw9X YA4iF0

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Homework: Read Hamper Chap 3.3. Look at purple box pg 62. For each assessment statement – write a few sentences to address the learning goal. In Class finish IB packet in groups.

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Deleted from IB 2009

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Problems

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Pressure Calc’s Ex 1. A 360 N child sits on a three legged stool which weighs 41 N. The bottom of the stools legs touch the ground over an area of 19.3 cm 2. What is the average pressure exerted by the stool on the ground? (hint: change cm 2 to m 2 ) How does the pressure change the student balances the stool on two legs?

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2.1 x 10 5 Pa

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Gas Laws

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Ideal Gas Laws For given sample of gas, the following factors are related: Pressure (Pa) Volume (m 3 ) Temperature (K) (mass – number of moles)

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Pressure Law At constant volume, P T The ratioP is a constant T

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What happens to the volume of a gas when the temperature decreases or increases?

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What happens to the volume of a gas when the pressure is increased or decreased?

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Pressure & Volume are inversely related.

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How do we account for the mass or the amount of substance present?

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If temperature and pressure remain constant... What would happen to the volume of a gas if the number of moles (amount of molecules) is increased?

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Effect of changing mass (# moles) on volume. Density ratio of mass to volume is constant. Direct Relationship.

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What would happen to pressure of a gas as mass/moles increased?

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Pressure vs Mass is direct. The density (mass/volume) increases as the volume is held fixed by the piston and the temperature is fixed.

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The mass changed by injecting molecules. The density (mass/volume) changes with the injection of the mass. This would be a very difficult experiment to perform in reality, because both P and V must be held constant.

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Avogadro: Equal volumes of gas at STP have equal #of molecules. Mole is an amount. At STP 1 mol has Avogadro’s number N A of particles (atoms or molecules). N A = 6.02 x molc/mol.

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To find # moles, n, in substance given the mass of substance. n = mm = mass in g MM = molar mass #g/mol from periodic table. 1 mole of any gas has fixed volume. At STP the volume is 22.4 dm 3 or 22.4 L.

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Ex: A weather balloon volume 1.0 m 3 contains helium at a pressure of 1.01 x 10 5 N/m 2 and a temperature of 35 o C. What is the mass of the helium in the balloon if one mole of helium has a mass of x kg? Using the ideal Gas Law

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