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Published byMelvin Alexander Modified over 9 years ago
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The Behavior of Gases Kinetic Theory - “kinetic” = motion - kinetic energy – the energy an object has due to motion - kinetic theory – states that the tiny particles in all forms of matter are in constant motion
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Basic Assumptions of the Kinetic Theory: 1. A gas is composed of particles (atoms or molecules). The particles are small, hard and have a lot of space between them. 2. The particles in gas move constantly in a constant random motion. Motion is a straight line until they collide with something that changes the direction.
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3. All collisions are perfectly elastic. During collisions kinetic energy is transferred without loss from one particle to another, and total kinetic energy remains constant.
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Gas Pressure Gas pressure is the force exerted by a gas per unit surface area of an object. No particles = no pressure = vacuum Barometer – used to measure gas pressure Pascal – metric unit of pressure Atmospheric pressure at sea level is 101.3 kPa (kiloPascals) Increase in temp. = increase in kinetic energy = increase in pressure of gas in an enclosed volume
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Factors Affecting Gas Pressure Amount of Gas: more gas particles = increased gas pressure: 2 X n = 2 X P (n = number of moles of gas P = pressure in kPa) Gas particles flow from areas of high numbers (high pressure) to areas of low numbers of particles
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Volume: - reduced space (volume) = increased pressure: ½ V = 2 X P - increased space (volume) = reduced pressure: 2 X V = ½ P
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Temperature: - increase in temperature = increase in pressure - decrease in temperature – decrease in pressure
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Standard Pressure and Temperature Volume of gas varies as temp. and pressure change Volume of gas is determined at STP (standard temperature and pressure) standard temp. = 0o C 273 K standard pressure = 101.3 kPa or 1 atmosphere 1 atm = 0o at the beach = 14 pounds of pressure per square inch on your body.
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The Gas Laws! Boyle’s Law Charles’ Law Gay-Lussac’s Law Combined Gas Law
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Boyle’s Law (1662) “For a given mass of gas at constant temperature, the volume of the gas varies inversely with the pressure.” Pressure = Volume Volume = Pressure P 1 X V 1 = P 2 X V 2
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Example: A high altitude balloon contains 30.0 liters of helium at 103 kPa. What is the volume when the balloon rises to a volume where the pressure is only 25.0 kPa? (Assume that the temperature remains the same.) P 1 = 103 kPa V 1 = 30 L P 2 = 25.0 kPa V 2 = ? P 1 X V 1 = P 2 X V 2 V 2 =30.0 L X 103 kPa 25.0 kPa = 124 L V 2 = P 1 X V 1 P 2
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Pressure conversions: On a daily basis, pressure is measured in psi – “pounds per square inch” 1 psi = 6.895 kPa problems must be solved in kPa
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Try these: The pressure on 2.5 L of anesthetic gas changes from 105 kPa to 40.5 kPa. What will be the new volume if the temperature remains constant? A gas with a volume of 4.0 L at a pressure of 30 psi is allowed to expand to a volume of 12.0 L. What is the pressure in the container if the temperature remains constant? (Convert to kPa to solve, then back to psi.)
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Charles’ Law (1787) “The volume of a fixed mass of gas is directly proportional to its Kelvin temperature if the pressure is kept constant.” Temperature = volume V 1 = V 2 T 1 T 2
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Example: A balloon inflated in a room at 24 o C has a volume of 4.0 L. The balloon is then heated to a temperature of 58 o C. What is the new volume if the pressure remains constant? V 1 = 4.0 L T 1 = 24 o C T 2 = 58 o C V 2 = ?L Express the temp in kelvins: T 1 = 24 o C + 273 = 297 K T 2 = 58 o C + 273 = 331 K V 2 = V 1 X T 2 = 4.0L X 331 K = 4.46 L T 1 297 K
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Try These: If a sample of gas occupies 6.80 L at 325 o C, what will be its volume at 25 o C if the pressure does not change? Exactly 5.0 L of air at –50.0 o C is warmed to 100 o C. What is the new volume if the pressure remains the same?
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Gay-Lussac’s Law “The pressure of a gas is directly proportional to the Kelvin temperature if the volume remains constant.” temperature = pressure P 1 = P 2 T 1 T 2
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Example: The gas in a used aerosol can is at a pressure of 103 kPa at 25 o C. If this can is thrown onto a fire, what is the pressure of the gas when its temperature reaches 928 o C? P 1 = 103 kPa T 1 = 25 o C T 2 = 928 o C P 2 = ? kPa T 1 = 25 o C + 273 = 298 K T 2 = 928 o C + 273 = 1201 K P 2 = 103 kPa X 1201 K = 415 kPa 298 K
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Try this one! The pressure in an automobile tire is 198 kPa at 27 o C. At the end of the trip on a hot sunny day, the pressure has risen to 225 kPa. What is the temperature of the air in the tire? (Assume that the volume has not changed.)
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The Combined Gas Law The three gas laws can be combined: The other laws can be obtained by holding one quantity (P,V or T) constant: P 1 X V 1 = P 2 X V 2 T 1 T 2 P 1 X V 1 = P 2 X V 2 X T 1 T 2 (Temp. is constant) Equals Boyle’s Law!
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Try These! A gas at 155 kPa and 25 o C occupies a container with an initial volume of 1.0 L. By changing the volume, the pressure of the gas increases to 605 kPa as the temperature is raised to 125 o C. What is the new volume? A 5.0 L air sample at a temperature of –50 o C has a pressure of 107 kPa. What will be the new pressure if the temperature is raised to 102 o C and the volume expands to 7.0 L?
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