Behavior of Gases Chapter 10 & 12
Pressure • What is pressure? A Force Exerted by a Gas over a Given Area • What causes pressure? Collisions of the Gas Particles with each other &the Walls of the Container That’s Pressure! Walls of the Container Gas Particle
Which shoes create the most pressure? Smaller the area of contact, larger the amount of pressure exerted by an object.
Units of Pressure atmospheres = atm millimeters of mercury = mmHg kilopascals = kPa pounds per square inch = psi 1 atm = 760mmHg = 101.3kPa = 14.7psi
Conversion Between Units of Pressure 1 atm = 760mmHg = 101.3kPa = 14.7psi
Sample Problem #1 How many kilopascals are equivalent to 880mmHg? 101.3 760 = 120 kPa
Sample Problem #2 Calculate the number of psi that are in 2.60atm. 14.7 1
K = ºC + 273 ºF -459 32 212 ºC -273 0 100 K 0 273 373 Temperature ALWAYS use absolute temperature (Kelvin) when working with gases.
Practice problems K = °C + 273 Ex) 32°C = ______ K K = 32 + 273 = 305 K Try one: How much is 75°C in Kelvin? K = 75 + 273 = 348 K
Behavior of Gases and the Kinetic Theory Kinetic refers to motion. The energy of an object has because of its motion is called kinetic energy. The Kinetic theory states that the tiny particles in all forms of matter are in constant motion.
Watch the video segment (Kinetic Molecular Theory – Standard Deviants School Chemistry: Molecular Geometry) and fill in the missing information in your workbook. http://app.discoveryeducation.com/search?Ntt=standard+deviants+gas
Postulates of the Kinetic Molecular Theory of Gases • A gas is composed of particles, usually molecules or atoms that are far apartfrom one another in comparison with their own dimensions. Particles are relatively far apart from one another and between them is empty space. • Gas molecules are in constant random motion. They travel in straight paths (unless they collide with a wall of a container) and move independently of each other.
Postulates of the Kinetic Molecular Theory of Gases • The molecules exert no force on each other or on the container until they collidewith each other or with the walls of the container. • The average kinetic energy of the molecules of a gas is proportionalto the temperature. • Every time a molecule collides with the wall, it exerts a force on it which we call pressure.
Applying this knowledge we know… • Gases fill their containers regardless of the shape and volume of the containers. • Because there is so much space between particles, gases are easily compressible. Because gases are compressible, they are used in automobile airbags and other safety devices designed to absorb the energy of an impact
All collisions are perfectly elastic. This means that during collisions kinetic energy is transferred without loss from one particle to another, and total kinetic energy remains constant. The average speed of oxygen molecules in air at 20oC is 1700 km/h. At these high speeds, the odor molecules from a hot pizza in Washington, D.C., should reach Mexico City in about 106 minutes. Why doesn’t this actually happen?
Answer Molecules are constantly striking molecules of air and rebounding in other directions. Their path of uninterrupted travel in a straight line is short.
Questions: Pressure increases with the additional gas particles. Pressure exerted by an enclosed gas is caused by collisions of gas particles with the walls of the container. If the number of gas particles is changed by any factor, the pressure changes by that same factor; until the container ruptures. What happens when a closed container is inflated?
A gas inside a bicycle tire exerts a pressure of 35 pounds per square inch (psi). How much air must be pumped into the tire to produce a pressure of 70 psi? Double the amount of air.
Note!!! The relationship between the amount of gas and pressure is proportional, assuming the volume & temperature stay the same. This means more gas = higher pressure and less gas = lower pressure.
C. What happens to pressure when a closed container is deflated? *Pressure of the gas is decreased because there are fewer gas particles and less collisions. *If the number of gas particles decreases by half, the pressure decreases by half. (Note: Gas particles move from region of higher pressure to lower pressure until equilibrium is reached.)
Homework • Complete pg. 4 in your booklet. • Bring an empty soda can to class tomorrow.
Bell ringer – 2/5/14 • Each of these flasks contains the same number of gas molecules. In which would the pressure be lowest? Explain your answer choice. • Which temperature is colder: 36°C or 278 K?
Combined Gas Law The Combined Gas Law helps us explain what happens to gases as the pressure, temperature, and volume changes in respect to moles of a substance. • = • = • =
Temperature must be in Kelvin . Remember: STP = 1.0atm and 273 K or 0.0°C
Guided Practice • 1. A hot air balloon has a volume of 7500L at 270K and a pressure of 1.2atm. What will be the volume of the balloon if the pressure changed to 0.90atm and the temperature decreases to 230K? • Givens and Unknowns: • P1 = 1.2 atm P2 = 0.90 atm • V1 = 7500 L V2= unknown • n1 = 1 mole n2 = 1 mole • T1 = 270 K T2 = 230 K
Substitute & Solve (Cross Multiply): (1.2 atm) (7500 L) = (0.90 atm) (V2) (1 mol) (270 K) (1 mol) (230 K) (1.2 atm)(7500 L)(1 mol)(230 K) = (0.90 atm)(V2)(1 mol)(270 K) 2070000 atm*L*mol*K = (V2)(243 atm*mol*K)
2070000 atm*L*mol*K = (V2) 243 atm*mol*K 8518.5 L = V2 Use 2 sig figs 8500 L = V2
Guided Practice • 2. The volume of a gas at STP is 22.4L. At 12oC, the volume of the balloon changes to 55.0L. What is the new pressure? • Givens and Unknowns: • P1 = 1.0 atm P2 = unknown • V1 = 22.4 L V2= 55.0 L • n1 = 1 mole n2 = 1 mole • T1 = 273 K T2 = 12oC + 273 = 285 K
Substitute & Solve (Cross Multiply): (1.0 atm) (22.4 L) = (P2) (55.0 L) (1 mol) (273 K) (1 mol) (285 K) (1.0 atm)(22.4 L)(1 mol)(273 K) = (P2)(55.0L)(1 mol)(285 K) 6115.2 atm*L*mol*K = (P2)(15675 L*mol*K)
6115.2 atm*L*mol*K = (P2) 15675 L*mol*K 0.390124 atm= P2 Use 2 sig figs 0.39 atm= P2
Homework Complete pg. 10 in your packet.
Soda Can Activity OBJECTIVES • Students will demonstrate the effects of air pressure. • Students will demonstrate that as a gas is heated it expands and as it cools it will contract. SAFETY • Be careful of hot water.
What caused the can to collapse? • As the water boils, the can becomes full of steam. • When the can is inverted into the cold water bath, the temperature of the gas inside the can drops and some of the water condenses. • Since the temperature drops and there are fewer gas particle collisions, the pressure inside the can decreases. • Since the pressure outside the can is now much greater, this higher pressure crushes the can.
The Effect of Changing Size of Container–Boyle’s Law WHAT IF…temperature and moles do not change and we just look at the relationship between pressure and volume. Our equation would look like this: P1 V1 = P2 V2Boyle’s Law Boyle’s Law states that at a constant temperature, the volume of a gas is inversely proportional to the pressure exerted by that gas.
The Effect of Changing Size of Container–Boyle’s Law Think of two kids (Paul Pressure and Victor Volume) on a see-saw. If Paul goes up, Victor goes down. If Victor goes up, Paul goes down. This is an inverse relationship.
http://www.grc.nasa.gov/WWW/k-12/airplane/boyle.html To show animation
Examples: • If a gas is compressed from 2L to 1L, the pressure will _____________by a factor of 2. • If a gas is expanded from 1L to 3L, the pressures will _______________ by a factor of 3. • Gases cool when they expand and heat when they compress. Why? increase decrease Thus, if you forget to wear your suit in space, you will_EXPLODE_!!! If the volume of a container decreases in size, the pressure of gas particles in the container increases.
Example Problems P1 V1 = P2 V2 • The pressure of a 3.5L balloon was determined to be 1.5atm. Assuming that the temperature remained constant, what would be the volume of the balloon if the pressure was decreased to 0.45atm? • At 45oC, a certain container of gas has the volume of 580mL and a pressure of 980mmHg. What would be the new volume of the gas at 250 mmHg and 45oC?
The Effect of Temperature changes on Volume – Charles’s Law • As the gas inside a balloon cools, the average KE of molecules decreases. • With fewer and less collisions, the gas molecules move closer together and occupy a smallervolume than they previously did. • The volume decreases, assuming no change in the amount of gas and pressure.
Charles’ Law Charles law states: At a constant pressure, the volume of a gas is directlyproportional to the temperature in Kelvin. As temperature increases, the volume increases As temperature decreases, the volume decreases.
Observe what happens to the balloon as liquid nitrogen is being poured on it.
Guided PracticeTemperature must be in Kelvins! • The temperature of a 0.65L sample of carbon dioxide gas is 580K. If the pressure remains constant, what is the new volume of the gas if the temperature increases to 1300K? • A balloon has a volume of 5.6L at a temperature of 98oC. If the volume of balloon increases to 9.5L, what will be the temperature of the gas in Celsius? Assume that the pressure remains constant.
Avogadro’s Law Avogadro's Law (Avogadro's theory;Avogadro's hypothesis) is a principle stated in 1811by the Italian chemist Amedeo Avogadro (1776-1856) that "equalvolumesof gases at the same temperature andpressure contain the same number ofmolecules regardless of their chemical nature and physical properties“. This number (Avogadro's number) is 6.02 X 1023. It is the number of molecules of any gas present in a volume of 22.41 L and is the same for the lightest gas (hydrogen) as for a heavy gas such as carbon dioxide or bromine.
Avogadro’s Law Or to put it another way, "the principle that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Thus, the molar volume of all ideal gases, at 0° C and a pressure of 1 atm., is 22.4 liters" V = the volume of the gas n = the amount of substanceof the gas
Avogadro’s Law states that that equal volumes of all gases at the same temperature and pressure contain the same number of molecules or moles.