LBS 172 Exam 1 Review Chapter 12 Gases I II III IV What is a gas a Properties i Non definite volume fills container can flow spread out can be compressed b Air is a gas composed of many gases i Relatively stable gases are O2 20 9 Ar 0 9 1 N2 78 1 Kr Ne He 0 0005 H2 0 00005 ii Gases whose composition varies are H2O 0 4 CO 1 CO2 0 035 CH4 0 0002 O3 0 000004 What kinds of substances are gases a Nonmetals small molecules i Binary molecules 1 H2 N2 O2 F2 Cl2 O3 ozone ii Noble gases 1 He Ne Ar Kr Xe Rn b Other gases i CO2 greenhouse gas 22x better than CH4 but decays slower ii CH4 greenhouse gas cow burps release 400 500 L day iii NH3 iv NO acid rain precursor v NO2 smog acid rain precursor vi N2O nitrous oxide is laughing gas used for dental work vii H2S smell of rotten eggs viii HCN hydrocyanic acid is a deadly poison that tastes like almonds ix SO2 acid rain precursor c Any aroma is due to gaseous molecules Microscopic characteristics of gases a Particles are randomly moving b Particles have elastic collisions i They bounce off of each other with no net loss of energy c Particles exert pressure by bouncing off of container wall Results in macroscopic world V VI a Simple relationship between moles pressure P volume V and temperature T b Gases are compressible c Gases have a much lower density than solids and liquids d Gases will mix evenly and completely when put in the same container i There is no such thing as immiscible gases Pressure a Gases exert pressure by bouncing off of the walls of a container i The more collisions the more pressure ii The higher the speed of the collisions the more pressure b Units of pressure i Atmospheric pressure at sea level is 1 01x105 Pa 101 kPa 1 atm atmosphere c How is pressure measured i The barometer was invented by Torricelli in the 1700s 1 It is a glass tube inverted in a dish of Hg and the atmospheric pressure keeps the Hg in the tube Hg is used because it is the densest liquid at room temperature 1 0 atm ii 1 0 atm 760 mm Hg 29 92 in Hg 760 torr 101 kPa 1 0x105 Pa Relationships between physical properties of gases a Relationship between pressure P and volume V i Boyle s Law was discovered in 1650 1 It states that V is inversely proportional to P a P increases V decreases 2 V 1 P V k 1 P PV k 3 P1V1 k P2V2 P1V1 P2V2 b Relationship between volume V and number of moles n i Avogadro s Law 1 It states that number of moles is directly proportional to volume under the conditions of constant pressure and temperature a V increases n increases 2 V n V kn V n k 3 V1 n1 k V2 n2 V1 n1 V2 n2 a Note that this law is not dependent on the type of gas only the number of moles of the gas c Relationship between volume V and temperature T VII VIII IX i Charles Law 1 It states that temperature is directly proportional to volume under the conditions of constant pressure and number of moles of gas a T increases V increases 2 V T V kT V T k 3 V1 T1 k V2 T2 V1 T1 V2 T2 a Temperature must be converted to the Kelvin scale because it is necessary that the temperatures are absolute positive numbers d Relationship between P V n and T i The Ideal Gas Law is a combination of the 3 laws 1 V nT P V R nT P PV nRT a R 0 082057 L atm mol K b We often define standard temperature and pressure STP i Standard T 0 C 273 15 K ii Standard P 1 atm Density of Gases g L a Gases have much lower densities than solids and liquids i D m V 1 mass m number of moles n x molar mass M a m n M n m M 2 PV mRT M m V PM RT D PM RT a Note that this law is only dependent upon pressure molar mass and temperature NOT VOLUME Assumption behind calculations a Dalton s Law of Partial Pressure 1801 i It states that the total pressure of a mixture of gases is the sum of their individual pressures 1 P depends on moles of gas not on chemical nature ii Ptotal PA PB PC iii Ptotal XAPtotal XBPtotal 1 PA XAPtotal XA PA Ptotal a XA mole fraction of a mixture of gases Kinetic Molecular Theory a Large separations between molecules b Random motion with collisions c Average kinetic energy is proportionate to gas temperature d Energy Distribution e X i From physical chemistry PV 1 3 nM 2 where n is the number of moles of gas M is the molar mass of the gas and 2 is the average of the square of speeds 1 Root Mean Square is the weighted average speed a PV nRT 1 3 nM 2 2 3RT M i R 8 314 J mol K ii Answer in m s Gas Diffusion and Effusion a Gas diffusion is the gradual mixing of molecules of one gas with molecules of another i Graham s Law 1832 At a constant pressure and temperature the rate of diffusion of gases is proportional to the inverse of the square root of the molar masses Rate1 M2 Rate2 M1 b Gas effusion is the process by which gases escape out of a small opening in a container i Gas effusion is also governed by Graham s Law 1 Furthermore the rate of the escaping of gases is inversely proportional to time slower rate longer time 1 a XI Rate1 Rate2 M2 t2 M1 t1 Non ideal gases a PV nRT assumes no interactions between molecules and assumes that molecules have no volume i Non ideal gas law P a n V 2 V bn nRT XII 1 a n V 2 corrects for intermolecular force assumption stickiness 2 bn corrects for molecular volume assumption ii This formula becomes an issue at high pressures and low temperatures 1 High pressures less space so V becomes more important 2 Low temperatures slower motion so it increases stickiness Problems involving gases a PV nRT can be used to get moles of gas from P V and T Once you have moles then it is just a normal limiting reagent or stoichiometry or titration b Gas law shortcuts all revolve around the fact that in gases V and P do not depend on the nature of the gas i Implications 1 The ratio of n V or n P is constant at a given T and P or V a So 2 moles of gas C takes up twice the volume of 1 mole of gas A under the same conditions Chapter 13 Intermolecular Forces Liquids and Solids I …