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It’s a Gas

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Air is a gas It is a mixture of several gases.It is a mixture of several gases. It surrounds you all of the time.It surrounds you all of the time. It inflates tiresIt inflates tires It provides cushioning in an air mattressIt provides cushioning in an air mattress It transmits sound waves so you can hearIt transmits sound waves so you can hear So, how do you know it exists???So, how do you know it exists???

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Gases What gases are important for each of the following: O 2, CO 2 and/or He? A. B. C. D. A. B. C. D.

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Gases What gases are important for each of the following: O 2, CO 2 and/or He?What gases are important for each of the following: O 2, CO 2 and/or He? A. CO 2 B. O 2 /CO 2 C. O 2 D. HeA. CO 2 B. O 2 /CO 2 C. O 2 D. He

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The Nature of Gases The systematic study of gases began 300 years ago. What did they learn? What did my high school chemistry teacher say?

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1. Gases have mass Proof: Weigh a basketball deflated and inflated. The mass increases. The increase is due to air molecules.Proof: Weigh a basketball deflated and inflated. The mass increases. The increase is due to air molecules. Mass: 2567 g + Mass: 2571 g What was the mass of gas added?

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2. It is easy to compress a gas Why can you put more air in a tire, but can’t add more water to a glass full of water?Why can you put more air in a tire, but can’t add more water to a glass full of water? If you squeeze a gas, its volume can be reduced considerably.If you squeeze a gas, its volume can be reduced considerably. This is why gases are used as shock absorbers and in air bags.This is why gases are used as shock absorbers and in air bags.

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3. Gases completely fill their containers This property explains why nowhere around you is there an absence of air.This property explains why nowhere around you is there an absence of air. Air in a balloon is distributed evenly throughout the balloon not just on the bottomAir in a balloon is distributed evenly throughout the balloon not just on the bottom

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4. Different gases can move through each other quite rapidly The movement of one substance through another is called DIFFUSION.The movement of one substance through another is called DIFFUSION. Therefore, gases diffuse easily through each other.Therefore, gases diffuse easily through each other. You observe diffusion when smell popcorn at the theater or when a skunk is nearby.You observe diffusion when smell popcorn at the theater or when a skunk is nearby.

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5. Gases exert pressure You have experienced the effects of changing air pressure when your ears “pop”You have experienced the effects of changing air pressure when your ears “pop” You have observed air pressure when you inflate a balloon.You have observed air pressure when you inflate a balloon.

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Units of Pressure 1 atm = 760 mm Hg 1 atm = 760 torr 1 atm = 101.325 kPa Barometer Pressure = Force Area

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Sea level1 atm 4 miles0.5 atm 10 miles0.2 atm

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6. The pressure of a gas depends on its temperature. The higher the temperature of a gas, the higher the pressure.The higher the temperature of a gas, the higher the pressure. Your tire pressure can become dangerously high on hot southwest summer days.Your tire pressure can become dangerously high on hot southwest summer days.

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Summary of Gas Properties Gases have mass.Gases have mass. It is easy to compress a gas.It is easy to compress a gas. Gases completely fill their containers.Gases completely fill their containers. Different gases can move rapidly through each other.Different gases can move rapidly through each other. Gases exert pressure.Gases exert pressure. The pressure of a gas depends on temperature.The pressure of a gas depends on temperature.

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Kinetic-Molecular Theory All of the gas properties covered are explained by the kinetic-molecular theory.All of the gas properties covered are explained by the kinetic-molecular theory. Kinetic means motionKinetic means motion Molecular means moleculesMolecular means molecules Therefore, Kinetic-Molecular means motion of molecules.Therefore, Kinetic-Molecular means motion of molecules.

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K-M Theory Gases consist of discrete molecules that have mass. Every molecules is independent of other gas molecules.

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K-M Theory Individual molecules are small and far apart compared to their size. This assumption explains why gases can be so easily compressed.

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K-M Theory Gas molecules are in continuous, random, straight line motion with varying velocities. This explains why gases immediately fill their containers.

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K-M Theory Gases exert pressure because their particles frequently collide with the wall of the container in which they are held.

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K-M Theory Collisions are elastic. Collisions occur without any loss of energy (speed)

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K-M Theory Gas molecules exert no attraction or repulsion force on one another. Attraction Repulsion

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K-M Theory: Temperature Temperature is a measure of the amount of the average kinetic energy of the particles in matter. The more kinetic energy the particles have, the higher the temperature. The temperature of particles are typically recorded in one of three ways: 1. Fahrenheit (ºF) 2. Celsius (ºC) 3. Kelvin (K) Do you remember which is the standard unit????

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In this analogy each billiard ball represent different gas molecules moving in random motion. Pressure result from collision of each between ball and the boundary. Each collision is perfectly elastic with each ball exhibiting no attractive or repulsive force between each other. Motion (energy) Temperature (K) Collision (impact) Pressure (atm) Boundary (container size) Volume (L) KMT: Billiard analogy

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Summary of KMT Postulates Gas particles are in constant random motion.Gas particles are in constant random motion. Gas particles occupy no volume.Gas particles occupy no volume. Collisions between gas particles are perfectly elastic: there is no loss of kinetic energy.Collisions between gas particles are perfectly elastic: there is no loss of kinetic energy. There are neither attractive nor repulsive forces between gas particles.There are neither attractive nor repulsive forces between gas particles. The higher the absolute temperature, the higher the average kinetic energy of the gas.The higher the absolute temperature, the higher the average kinetic energy of the gas.

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Boyle’s Law At constant temperature, the volume of a gas varies inversely with its pressure.At constant temperature, the volume of a gas varies inversely with its pressure. In other words, as the pressure increases, the volume of the gas decreases.In other words, as the pressure increases, the volume of the gas decreases. As the pressure decreases, the volume of the gas increases.As the pressure decreases, the volume of the gas increases. UP → DOWN and DOWN → UPUP → DOWN and DOWN → UP

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The Math of Boyle’s Law P 1 V 1 = P 2 V 2P 1 V 1 = P 2 V 2 10 liters of air at 1 atm is compressed to a pressure of 4 atm. What is the volume of the compressed air.10 liters of air at 1 atm is compressed to a pressure of 4 atm. What is the volume of the compressed air. (1 atm)(10 L) = (4 atm)V 2 (1 atm)(10 L) = (4 atm)V 2 (1 atm)(10 L) = (4 atm)V 2 (4 atm) (4 atm)(1 atm)(10 L) = (4 atm)V 2 (4 atm) (4 atm)

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The Math of Boyle’s Law (continued) P 1 V 1 = P 2 V 2P 1 V 1 = P 2 V 2 (1 atm)(10 L) = (4 atm)V 2 (4 atm) (4 atm)(1 atm)(10 L) = (4 atm)V 2 (4 atm) (4 atm) V 2 =(1 atm)(10 l) (4 atm)V 2 =(1 atm)(10 l) (4 atm) V 2 =2.5 LV 2 =2.5 L

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Kinetic Molecular Theory and Boyle’s Law In a smaller volume, the number of collisions between the particles and the walls of the container is concentrated on a smaller area (think high heels), so the pressure is greater.In a smaller volume, the number of collisions between the particles and the walls of the container is concentrated on a smaller area (think high heels), so the pressure is greater. In a larger volume, the number of collisions between the particles and the walls of the container is spread out over a larger area (think snowshoes), so the pressure is less.In a larger volume, the number of collisions between the particles and the walls of the container is spread out over a larger area (think snowshoes), so the pressure is less.

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Avogadro’s Hypothesis At constant temperature and pressure, equal volumes of gases contain equal numbers of particles.At constant temperature and pressure, equal volumes of gases contain equal numbers of particles. Or restated in mole-speak, at constant temperature and pressure, equal volumes of gases contain equal numbers of moles.Or restated in mole-speak, at constant temperature and pressure, equal volumes of gases contain equal numbers of moles.

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The Math of Avogadro’s Hypothesis V 1 = V 2 n 1 n 2 Where V is Volume and n is moles.V 1 = V 2 n 1 n 2 Where V is Volume and n is moles. One mole of ozone gas (O 3 ) occupies 22.4 L. The ozone decomposes to 1.5 moles of molecular oxygen (O 2 ). What is the volume of the resulting molecular oxygen?One mole of ozone gas (O 3 ) occupies 22.4 L. The ozone decomposes to 1.5 moles of molecular oxygen (O 2 ). What is the volume of the resulting molecular oxygen? 22.4 L= V 2 ___ 1 mol 1.5 mol22.4 L= V 2 ___ 1 mol 1.5 mol

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The Math of Avogadro’s Hypothesis (continued) 22.4 L= V 2 ___ 1 mol 1.5 mol22.4 L= V 2 ___ 1 mol 1.5 mol 22.4 L ( 1.5 mol )=V 2 1.0 mol22.4 L ( 1.5 mol )=V 2 1.0 mol V 2 = 33.6 LV 2 = 33.6 L

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Kinetic Molecular Theory and Avogadro’s Hypothesis As the number of gas particles increases, the frequency of collisions with the walls of the container must increase. This, in turn, leads to an increase in the pressure of the gas. Flexible containers, such as a balloon, will expand until the pressure of the gas inside the balloon once again balances the pressure of the gas outside. Thus, the volume of the gas is proportional to the number of gas particles.As the number of gas particles increases, the frequency of collisions with the walls of the container must increase. This, in turn, leads to an increase in the pressure of the gas. Flexible containers, such as a balloon, will expand until the pressure of the gas inside the balloon once again balances the pressure of the gas outside. Thus, the volume of the gas is proportional to the number of gas particles.

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Summary Boyle’s Law: P 1 V I = P 2 V 2Boyle’s Law: P 1 V I = P 2 V 2 Avogadro’s Hypothesis: V 1 = V 2 n 1 n 2Avogadro’s Hypothesis: V 1 = V 2 n 1 n 2 Put the right numbers in the right places.Put the right numbers in the right places. Isolate the unknown variable and solve.Isolate the unknown variable and solve.

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