P 1 2 V V V T V V. AP Chemistry A. Allan Chapter 5 - Gases

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1 A Chemistry A. Alla Chapter 5 - Gases 5. ressure A. roperties of gases. Gases uiformly fill ay cotaier. Gases are easily compressed 3. Gases mix completely with ay other gas 4. Gases exert pressure o their surroudigs a. ressure force/area B. Measurig barometric pressure. he barometer a. Ivetor - Evagelista orricelli (643). Uits a. mm Hg (torr) () 760 torr Stadard pressure b. ewtos/meter pascal (a) () 0,35 a Stadard pressure c. atmospheres () atmosphere Stadard pressure 5. he Gas Laws of Boyle, Charles, ad Avogadro A. Boyle's Law (Robert Boyle, 67-69). the product of pressure times volume is a costat, provided the temperature remais the same k a. is iversely related to b. he graph of versus is hyperbolic c. olume icreases liearly as the pressure decreases (/). At costat temperature, Boyle's law ca be used to fid a ew volumes or a ew pressure a. k or 3. Boyle's law works best at low pressures 4. Gases that obey Boyle's law are called Ideal gases B. Charles' Law (Jacques Charles, ). he volume of a gas icrease liearly with temperature provided the pressure remais costat a. b / b () / b / or

2 b. emperature must be measured i degrees Kelvi () K C + 73 () 0 K is "absolute zero" C. Avogadro's Law (Amedeo Avogadro, 8). For a gas at costat temperature ad pressure, the volume is directly proportioal to the umber of moles, a. a / a () / a / 5.3 he Ideal Gas Law A. Derivatio from existig laws. k/ b a ( k)( b)( a). Costats k, b, a are combied ito uiversal gas costat, R R or R R L atm K mol B. Limitatios of the Ideal Gas Law. Works well at low pressures ad high temperatures. Most gases do ot behave ideally above atm pressure 3. Does ot work well ear the codesatio coditios of a gas C. Solvig for ew volumes, temp or pressure ( remaiig costat). Combied law (from geeral chem). R or

3 5.4 Gas Stiochiometry A. Stadard temperature ad pressure (S). 0 C, 73 K. 760 torr, atm B. Molar volume. Oe mole of a ideal gas occupies.4 liters of volume at S C. higs to remember Desity mass volume grams of molar subs ta ce mass 5.5 Dalto's Law of artial ressures (Joh Dalto, 803) A. Statemet of law. "For a mixture of gases i a cotaier, the total pressure exerted is the sum of the pressures each gas would exert if it were aloe". It is the total umber of moles of particles that is importat, ot the idetity or compositio of the gas particles B. Derivatio OAL OAL ( ) OAL 3 5. OAL OAL C. Mole Fractio. he ratio of the umber of moles of a give compoet i a mixture to the total umber of moles i the mixture. For a ideal gas, the mole fractio (x): x OAL OAL

4 5.6 he Kietic Molecular heory of Gases (KM) A. ostulates of the KM Related to Ideal Gases. he particles are so small compared with the distaces betwee them that the volume of the idividual particles ca be assumed to be zero. he particles are i costat motio. Collisios of the particles with the walls of the cotaier cause pressure 3. Assume that the particles exert o forces o each other. 4. he average kietic eergy of a collectio of gas particles is assumed to be directly proportioal to the Kelvi temperature of the gas B. Explaiig Observed Behavior with KM. ad ( costat) a. As is decreased, icreases: decrease causes a decrease i the surface area. Sice is force/area, the decrease i causes the area to decrease, icreasig the. ad ( costat) a. As icrease, icreases he icrease i causes a icrease i average kietic eergy. Molecules movig faster collide with the walls of the cotaier more frequetly, ad with greater force 3. ad ( costat) a. As icreases, also icreases Icreased creates more frequet, more forceful collisios. must icrease proportioally to icrease the surface area, ad maitai 4. ad ( ad costat) a. As icreases, must icrease Icreasig the umber of particles icreases the umber of collisios. his ca be balaced by a icrease i to maitai costat 5. Dalto's law of partial pressures a. is idepedet of the type of gas molecule KM states that particles are idepedet, ad is assumed to be zero. he idetity of the molecule is therefore uimportat C. Root Mea Square elocity. elocity of a gas is depedet o mass ad temperature.. elocity of gases is determied as a average a. M mass of oe mole of gas particles i kg b. R J/K mol () joule kg m /s urms 3 M D. Mea Free ath. Average distace a molecule travels betwee collisios a. x 0-7 m for O at S

5 5.7 Effusio ad Diffusio A. Effusio. Movemet of a gas through a small opeig ito a evacuated cotaier (vacuum). Graham's law of effusio Rate of Rate of effusio effusio for for gas gas M M B. Diffusio. he mixig of gases. Diffusio is complicated to describe theoretically ad mathematically 5.8 Real Gases ad va der Waals Equatio (Johaes va der Waals, 873) A. olume. Real gas molecules do have volume. olume available is ot 00% of the cotaier volume a. umber of moles b. b is a empirical costat, derived from experimetal results Ideal Real ' b B. ressure. Molecules of real gases do experiece attractive forces a. a proportioality costat determied by observatio of the gas obs ' a C. Combiig to derive va der Waal's eq obs ad the rearragig + b a a ( b ) obs

6 5.9 Characteristics of Several Real Gases A. Itermolecular Forces of Attractio. Larger itermolecular forces of attractio icrease deviatio from ideal behavior. For these molecules, the icrease i va der Waals forces predicts this icrease i deviatio from ideal behavior H < N < CH 4 < CO 5.0 Chemistry i the Atmosphere A. Compositio of the roposphere Compositio of dry air (sea level Compoet Mole Fractio Nitroge Oxyge Argo Carbo dioxide Neo Helium Methae B. hotochemical Smog - the problem of itroge oxides (NO x ). Auto exhaust cotais small amouts of NO, which is quickly oxidized NO(g) + O (g) NO (g). Radiat eergy causes NO to decompose NO (g) NO(g) + O(g) 3. Free oxyge atoms combie with oxyge molecules to form ozoe O(g) + O (g) O 3 (g) 4. Ozoe may absorb light eergy ad decompose to excited oxyge atoms ad excited oxyge molecules O 3 (g) O * + O* 5. Excited oxyge atoms react with water to form the hydroxyl radical O* + H O OH 6. Hydroxyl ca react with NO to form itric acid OH + NO HNO 3 C. Coal ad acid rai. Most coal, especially cheap coal, cotais sulfer S (i coal) + O (g) SO (g). Sulfur dioxide is oxidized i air SO (g) + O (g) SO 3 (g) 3. Acid rai forms at the SO3 combies with water i the air SO 3 (g) + H O (l) H SO 4 (aq)

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