Transcript Unit 7
Properties of gases & gas laws. Properties of Gases Molecules have lots of space between them Low density Compressible – why are gases compressible but solids & liquids are not? Essentially no attractive force between gas molecules, this allows them to move freely. Kinetic molecular theory http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm http://www.colorado.edu/UCB/AcademicAffairs/ArtsSciences/physics/Physics Initiative/Physics2000/bec/temperature.html Gas molecules very small in relation to distance between them, so volume of gas molecules is negligible. No attractive force between molecules this allows gases expand to fill the space of a container Molecules in constant random motion – move independently of each other & collisions are perfectly elastic so no energy is lost Energy of a gas is proportional to it’s temperature Diffusion & effusion The kinetic theory accounts for the ease of which gases diffuse & effuse. Diffusion – mixing of gases to make a homogeneous mixture. Effusion – passage of gas through a hole How do we measure gases How have we been measuring moles of substances? Why won’t this work for gases? What unit of measure can we use to measure the amount of gas? What other factors can be measured regarding gases? Units used for gases Volume – L Temperature – Kelvin 273K = 0oC All temperatures must be in K. Add 273 to change oC to K. Pressure – Pascals, mmHg, torr, atm 1 torr = 1 mmHg 760 torr = 1 atm 101.3 kPa = 1 atm Zumdahl Ch. 5 # 35, 36c Barometers measure pressure The atmospheric gas pressure pushes down on the mercury. This in turn pushes mercury up the empty tube. The height of mercury in mm is a measure of the pressure The air is exerting. Why does atmospheric pressure decrease as your altitude increases? Zumdahl, Zumdahl, DeCoste World of Chemistry 2002, pg401 Sea level vs. Mount Everest Prentice Hall, Inc. 2002 From high to low •How do aerosol cans work? •How does pressure inside change as hold sprayer? •When you shake the can you can hear liquid, but it will not come out? •Soda can demo •Ear popping (air plane or scuba) Kinetic energy vs. velocity If the Hulk & I run into you with the same amount of energy (force) what has to be true? KE = ½ mv2 Temperature is a measure of the kinetic energy of molecules. If 2 gases have the same temp they have the same KE. Which has a larger velocity at the same temp, H2 or Xe? Zumdahl Ch. 5 #99-102 Computer animation of Gas laws http://www.mhhe.com/physsci/chemistry/essentialch emistry/flash/gasesv6.swf w/sound http://www.marymount.k12.ny.us/marynet/06stwbwrk /06gaslaws.html student generated flash animations # of molecules vs. volume Avogadro’s Law: n is proportional to V, as n increases so does volume Copyright ©2007 Pearson Benjamin Cummings. All rights reserved. # of molecules vs. pressure n is proportional to P, as n increases so does pressure Application of n vs. V & P V vs. n – as you blow up a balloon you add more gas molecules & the balloon expands in volume P vs. n – as you add more gas to a fixed volume container, the container will explode once you exceed a certain P Prentice Hall, Inc., 2002 T vs. V – Charles Law In the lab as the temperature of the water around the closed syringe increased, the volume inside the syringe also increased. T is proportional to V, as one increases so does the other. As the T increases the molecules increase in kinetic energy & thus hit the walls with greater force which results in expansion of the container (the force pushes the sides of the container out). T vs. V problems Eg. 1 A balloon inflated in a room at 24oC has a volume of 4.00L. The balloon is then heated to a T of 58oC. What is the new volume if the pressure remains constant? Eg. 2 A container with 5.00 L of air at –50oC is warmed to 100.0oC. What is the new volume if P stays constant? T vs. P – Guy Lussac’s Law In the lab T is proportional to P, as one increases so does the other. As temperature increases the molecules have more energy & thus hit the walls harder thereby increasing the temp. T vs. P problems Eg. 3 The pressure in an auto tier is 198 kPa at 27oC. At the end of a trip on a hot sunny day, the pressure has risen to 225 kPa. What is the temperature of air in the tire? Eg. 4 The gas left in a used aerosol can is at a pressure of 103 kPa at 25oC. If this can is thrown onto a fire, what will the pressure be at 928oC? P vs. V – Boyle’s Law In the lab, as you increased the pressure on the end of the closed syringe the volume decreased. P & V are inversely related. As one goes up the other goes down. It’s like a see saw. As the volume increases the gases hit the walls of the container LESS OFTEN, thus there is less overall pressure on the container. P vs. V problems Eg. 5 A high-altitude balloon contains 30.0 L of helium gas at 103 kPa. What is the volume when the balloon rises to an altitude where the pressure is only 25.0 kPa Eg. 6 The volume of anesthetic gas changes from 2.5 L to 1.5L. If the original pressure was 40.5 kPa, what will be the new pressure? Combined Gas Law PV/T Eg. 7 A sample of argon is trapped in a gas bulb at a pressure of 760 torr when the volume is 100mL and the temperature is 35.0oC. What is the temperature if its pressure becomes 720 torr and volume is 200mL? Ideal Gas Law PV = nRT R = 0.08206 L*atm/K*mol Eg. 8 A balloon with 0.81 mol of gas at 300K has a pressure of 1.02 atm. What is the volume of the balloon? Eg. 9 A sample of oxygen at 24.0oC and 745 torr was found to have a volume of 455mL. How many grams of O2 were in the sample? Gas Laws Practice Problems Zumdahl Ch5 42-46, 49-51, 59,61, Application questions Real life application of gases document Who cares besides AP chem students? http://www.aquaholic.com/gasses/laws.htm STP for gases Standard temperature & pressure For gases STP means T = 273K of 0oC P = 1 atm Avogadro’s Law: At STP 1 mol of any gas has a volume of 22.4 L. At STP 1 mol = 22.4L Gas stoichiometry What mass of He is required to fill a 2.5 L balloon? At STP, how many liters of oxygen at are needed to combine with exactly 1.50L of hydrogen to produce water? More gas stoichiometry A student uses the decomposition of CaCO3 to prepare CO2. How large should the flask be in order to hold all the CO2 if 1.25g of CaCO3 is used. The pressure of the CO2 will be 740 torr & final temperature will be 25.0oC. Na2CO3 + 2HCl 2NaCl + CO2 + H2O Zumdahl 63-65, 68 Molar mass of a gas A student collected a sample of gas in a 0.220 L gas collection tube until its pressure reached 0.757 atm at a temp of 25.0oC. The sample had a mass of 0.299 g. Calculate the molecular mass of the gas. Density, MM & ideal gas law A gaseous compound of phosphorus and fluorine was found to have a density of 3.50g/L at a temp of 25.0oC and pressure of 740 torr. Determine the molar mass of the gas. Zumdahl Ch. 5 #75-77 Partial pressure The pressure of an individual gas does NOT change w/ the addition of other gases. If the pressure of each gas in a container is known, then the sum of these pressures is the total pressure. Each gas adds a part to the pressure i.e. has a partial pressure. Analogy: People pounding on a wall. How do we determine the partial pressure of my pounding on the desk? Suppose you want to fill a pressurized tank having a volume of 4.00L with oxygen enriched air for use in diving, and you want the tank to contain 50.0 g of O2 and 150g of N2. What will the total pressure have to be at 25.0oC? Mole fractions & partial pressure Remember that P is proportional to n. The partial pressure of any gas can be determined using it’s percentage of the make up of all the gases in the container otherwise known as mole fraction. Doing the same example again, 1.56 mol O2 & 5.36 mol N2. Partial pressure many ways Now compare their pressure ratios. PO2 = 9.54atm PN2 = 32.8atm More Partial stuff A mixture of gases has a PAr of 0.725 atm, PO2 of 0.210atm. What is the mole fraction of each gas? If the flask has a volume of 2.0L container & is at 32oC, then calculate the mass of each gas. Zumdahl Ch.5# 79-82, 85, 86, 88 Additional problems Zumdahl 5th ed. Ch. 5 handout #95,97-99,102,103 Resources The following can be found in the pick up folder. Gas laws animation.doc