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CHAPTER 18

CHAPTER 18. GASES. KINETIC THEORY OF GASES. A given amt. of gas will occupy the entire volume of its container. Changes in temp. have a greater effect on the vol. of a gas than on a liquid or solid. KINETIC THEORY OF GASES. Gas particles are in constant random motion.

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CHAPTER 18

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  1. CHAPTER 18 GASES

  2. KINETIC THEORY OF GASES • A given amt. of gas will occupy the entire volume of its container. • Changes in temp. have a greater effect on the vol. of a gas than on a liquid or solid

  3. KINETIC THEORY OF GASES • Gas particles are in constant random motion. • not held in fixed position by attractive forces • size of gas molec. is insignificant in comparison w/ the dist. betw. molecs. \ we assume gas particles have no effect on ea. other

  4. KINETIC THEORY OF GASES • Gas particles are treated as Point Masses • considered to have no vol. or diameter • Ideal Gas - imaginary gas composed of molecs. w/ mass but no vol. and no mutual attraction betw. particles

  5. KINETIC THEORY OF GASES • Vol. of gas, # of gas particles, press. of gas, & temp. of gas are variables that depend on ea. other. • The # of particles in a vol. of gas depends on the press. & temp. of the gas \ it’s necessary to give temp. & press. of gas along w/ vol. when discussing quantity of a gas.

  6. KINETIC THEORY OF GASES • Standard Pressure - 101.325 kPa • Standard Temp. - 0 oC • STP - Standard temp. & press.

  7. BOYLE’S LAW • Gas press. depends on 2 factors: 1. # of molecs. per unit volume 2. Avg. kinetic energy of the molecs - temp. • A change in either will change the press. of a gas

  8. BOYLE’S LAW • If the # of molecs. in a constant vol. incr., press. incr. • If # of molecs. & vol. remain constant, but K.E. of molecs. incr., press incr. • If temp. & # of molecs. remain constant, but vol. is decr., press. is incr.

  9. BOYLE’S LAW • What happens when volume is decr. by half? • press. doubles • same # of molecs. in 1/2 the volume • molecs. hit the wall of container twice as often & w/ same force per collision \ @ constant temp., press. varies inversely as vol. • the product of press. & vol. is constant

  10. BOYLE’S LAW • BOYLE’S LAW - If the amt. & temp. of a gas remains constant, the press. exerted by the gas varies inversely as the vol. • PV = k • k - constant - takes into account # of molecs. & temp. • Press. varies directly w/ # of molecs.

  11. APPLYING BOYLE’S LAW • Not all experiments can be carried out @ STP • In order to compare vols., we adjust them to standard conditions • V1P1 = V2P2 • V1, P1 - original conditions • P2, V2 - new conditions

  12. Dalton’s Law of Partial Pressure • Gas is often obtained by bubbling it through water • collecting gas over water or by water displacement • gases collected must be practically insoluble in water • Water vapor will be present in the gas

  13. Dalton’s Law of Partial Pressure • Dalton’s Law of Partial Pressure - The total pressure in a container is the sum of the partial pressures of the gases in the container • ea. gas exerts the same press. it would if it alone were present @ the same temp. • Press. exerted by an indiv. gas in a mixture is its Partial Pressure.

  14. Dalton’s Law of Partial Pressure • Air contains ~ 78% nitrogen \ 78% of press. is due to nitrogen • partial press. of N in air @ std. conditions is 78% x 101.325 = 79 kPa

  15. Dalton’s Law of Partial Pressure • If gas is collected over water, the press. in the container = the sum of the partial press. of the gas & the water vapor \ to find the press. of the gas alone (dry gas), subtract the water vapor press. for that temp.

  16. CHARLES’ LAW • Jacque Charles found a relationship betw. vol. & temp. • For ea. C o incr. in temp., the vol. of a gas is incr. by 1/273 of its vol. @ 0 oC. • Examples?

  17. CHARLES’ LAW • Suggests that @ -273 oC (0 K) a gas will have no volume • Not true - all gases liquefy before this temp. • relationship holds true only for gases

  18. CHARLES’ LAW • CHARLES’ LAW - The vol. of a quantity of gas @ constant press. varies directly w/ the kelvin temp. • experimental info led to formation of the Kelvin Scale K = oC + 273 • Zero pt. of Kelvin scale is absolute zero • triple pt. of water is 273.16 K

  19. APPLYING CHARLES’ LAW • For a direct proportion, the quotient is constant • V/T = k • Temperature must be in Kelvin • If temp. goes up, vol. goes up • V1 = V2 T1 T2

  20. COMBINED GAS LAW • Usually need to correct for both temp. & press. of a gas • Can do this by applying Boyle’s Law, then taking new vol. & putting it into Charles’ Law • Can also be done in one step • Temp. must be in Kelvin • P1 V1 = P2 V2 T1 T2

  21. Diffusion & Graham’s Law • Gas molecs. travel in straight lines betw. collisions • If NH3 is opened in back of room, can soon be detected in front of room. • Molecs. travel from back to front of room in straight lines betw. collisions • collide w/ air molecs.

  22. Diffusion & Graham’s Law • Diffusion - random scattering of gas molecs. • as gas molecs. diffuse, they become more evenly distributed throughout the room or container

  23. Diffusion & Graham’s Law • All gases do not diffuse @ the same rate • rate varies w/ velocity • @ same temp. molecs. w/ lower mass diffuse faster than molecs. w/ larger mass bec. they travel faster. • They also pass thru a sm. hole - effuse - more rapidly than higher mass molecs.

  24. Diffusion & Graham’s Law • @ the same temp: V1 = M2 V2 M1 \ relative rates of diffusion of 2 gases vary inversely w/ the square root of their molecular masses

  25. Diffusion & Graham’s Law • Graham’s Law - the relative rates @ which 2 gases under identical conditions of temp. & press. will diffuse vary inversely as the square roots of the molecular masses of the gases.

  26. Gas Density • Usually expressed in g/dm3 • May calculate density of a gas @ any temp. & press. • A decr. in temp. will decr. vol. & incr. density • D2 = D1 x T1 x P2 T2 P1

  27. Deviations of Real Gases • @ low press., real gases behave like ideal gases • molecs. are far apart - vol. molecs. occupy is small compared to total vol. • vol. is mostly empty space

  28. Deviations of Real Gases • @ higher press., real gas molecs. are forced closer together. • molecs. begin to occupy a significant portion of total vol. • If molecs. have slowed down enough, van der Waals forces will have an effect.

  29. Deviations of Real Gases \ Assumption that there’s no attractive forces betw. gas molecs. is not always true. • If gas molecs. are polar, gas behaves significantly diff. than an ideal gas would • weak forces will cause some diff.

  30. Deviations of Real Gases • For most common gases, ideal gas laws are accurate to 1% @ normal lab temps. & press. \ assume these gases have ideal gas properties • He approaches ideal behavior closer than any other

  31. Deviations of Real Gases • A property of real gases which depends upon the attractive forces betw. molecs. • Joule-Thomson Effect - If a highly compressed gas is allowed to escape through a sm. opening, its temp. decr. • In order to expand, the molecs. must do work to overcome attractive forces betw. molecs. • this energy comes from their kinetic energy \ as K.E. decr., temp. decr.

  32. Deviations of Real Gases • This can be seen when spraying an aerosol can. • As product & propellant are released through nozzel, can & contents become cooler • Adiabatic System - a syst. completely insulated so no heat exchange can take place w/ surroundings.

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