Chapter 10 Gases Part 3 Real Gases Prepare for Recitation October 20th ALEKS Objective 8 due October 18th ANGEL Quiz 8 October 20th Lecture 22 October 12th Lecture 23 October 14th Lecture 24 October 17th Read Ch 10 1 10 9 Additional Preparation BLB 10 5 23 30 45 71 75 82 84 Packet 10 1 15 BLB 25 11 12 25 CH110 FA11 SAS 1 Chapter 10 Gases Part 3 Real Gases KEY QUESTIONS THE BIG PICTURE What does partial pressure mean What type of mixtures do gasses form What state equation can we use to describe mixtures of gasses Remember macroscopic properties come from microscopic origins How does the energy of a molecule in a gas compare with intermolecular forces How can we describe mixtures of gasses How do real gasses differ from ideal gasses By the end of Today s Lecture you should know Kinetic molecular theory Real gases van der Waals CH110 FA11 SAS 2 Effusion and Diffusion Effusion Diffusion Heavy molecules diffuse or effuse more than lighter ones r proportional to 1 M Graham s Law of Effusion r1 r2 M2 M1 Note Diffusion is more complicated due to collisions between gas molecules but it also obeys Graham s Law CH110 FA11 SAS 3 Practice Problem An unknown gas effuses at a rate 1 49 times faster than Cl2 What is the molecular weight of the gas A B C D E 16 0 g mol 31 9 g mol 47 6 g mol 106 g mol 157 g mol CH110 FA11 SAS 4 Collisions and Diffusion The rate of diffusion follows Graham s Law At STP molecules collide 1010 times per second N2 speed 500 m s but in 1 s it collides 1010 times Mean Free Path MFP What happens to the MFP as density and pressure decrease CH110 FA11 SAS 5 Review of Density and Molar Mass The Ideal Gas Law The Connection between moles and mass m mass n moles M molar mass The Ideal Gas Law becomes The definition of density m d V Density and the Ideal Gas Law CH110 FA11 SAS 6 Practice Problem Silicon tetrachloride is a starting material for the production of electronics grade silicon Calculate the density of pure silicon tetrachloride vapor at 85oC and 758 torr A B C D E 0 238 g L 5 77 g L 1 00 g L 24 3 g L 0 056 g L CH110 FA11 SAS 7 Real Gases Deviate from Ideal Behavior For an ideal gas PV 1 nRT PV nRT For a non ideal gas a real gas this is not true for pressure or temperature Reasons 1 Molecules have finite size PV 1 nRT 2 Molecules exert attractive forces IMF PV 1 nRT CH110 FA11 SAS 8 KMT and Non Ideal Gas Behavior Why does PV nRT Kinetic Molecular Theory has 5 key postulates 1 straight line motion in random directions 2 Molecules are small have no volume 3 No intermolecular interactions 4 Elastic collisions 5 Mean kinetic energy temperature E mv2 Deviations from ideal gas behavior are caused because gas molecules atoms have and have when they are close together CH110 FA11 SAS 9 Real Gases At High Pressure Postulate 2 in KMT is Not True For 1 mole of ideal gas PV RT 1 At low pressures deviation from ideal behavior is small At high pressures use of the ideal gas law leads to an appearance of larger n CH110 FA11 SAS 10 Real Gases At Low Temperature Postulate 3 in KMT is Not True For 1 mole of ideal gas PV RT 1 At low T or high P attractive forces lead to the appearance of a smaller n IMF cause molecules to stick together As temperature increases the behavior of real gases becomes more ideal CH110 FA11 SAS 11 How Do We Account for This Behavior Attractive forces lead to the appearance of a smaller volume or smaller n depending on which was given High Pressure Very High Pressure Finite molecular volume leads to repulsion and the appearance of a larger volume or larger n Low Pressure Attractive forces and finite molecular volume have minimal impact As Temperature increases the behavior of real gases becomes more ideal CH110 FA11 SAS high temps the kinetic energy overcomes the attractive forces 12 Non Ideal Behavior and Measured Pressure For any gas we can measure P V T But at higher P the measured P is too small because of attractive forces The amount of missing P is proportional to 1 the size of the attractive forces a 2 the frequency of collisions n V 2 To compensate use CH110 FA11 SAS 13 Non Ideal Behavior and Measured Volume For any gas we can measure P V T But at higher P the measured V is too large because of the finite molecular volume per mole b BLB Figure 10 25 Actual volume Vactual Vmeasured Vexcluded To compensate use CH110 FA11 SAS 14 van der Waals Equation The Equation of state for IDEAL gases PV nRT The Equation of state for REAL gases 2 n P 2 a V nb nRT V Table 10 3 lists van der Waals constants a b for various gas molecules CH110 FA11 SAS 15 Practice Problem What is the pressure P of 1 0 mol Cl2 in a 2 0 L vessel at 273 K Ideal Gas van der Waals a 6 49 L2 atm mol2 b 0 0562 L mol CH110 FA11 SAS 16 KMT and Non Ideal Gas Behavior Does This Make Sense 2 n P 2 a V nb nRT V Kinetic Molecular Theory has 5 key postulates and two have caused problems 2 Molecules are small have no volume 3 No intermolecular interactions CH110 FA11 SAS 17 KMT and Non Ideal Gas Behavior Does This Make Sense n2 P 2 a V nb nRT V Molecules are small have no volume At LOW Pressure The volume of the container is compared with the volume of the particles At HIGH pressures The volume of the particles becomes and the volume available to the gas is the container volume Gas Ne Ar Kr Xe b L mol 0 0171 0 0322 0 0398 0 0510 Size CH110 FA11 SAS 18 KMT and Non Ideal Gas Behavior Does This Make Sense n2 P 2 a V nb nRT V Molecules in a gas have no interactions At LOW Temperature Intermolecular forces tend to be and the KE is insufficient to overcome them At HIGH Temperature The KE of the particles becomes the intermolecular forces and the behavior of the gas becomes Gas a atm L2 mol2 IMF CH4 2 25 NH3 4 17 H2O 5 46 CH110 FA11 SAS Strength 19 What You Should Know Kinetic Molecular Theory How does KMT explain the pressure and temperature of a gas the gas laws and the rates of effusion and diffusion Real Gases What effect does the V of a gas particle have when we assume an ideal gas What effects do intermolecular forces have on the observed properties of gases when we assume an ideal gas How does the van der Waals Equation explain the properties …
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