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Chapter 5

Chapter 5. Gases. What are some things you remember about gases and their behavior?. List, Pass, Amass. H. He. Solid. Liquid. Li. Be. B. C. N. O. F. Ne. Gas. Na. Mg. Al. Si. P. S. Cl. Ar. K. Ca. Sc. Ti. V. Cr. Mn. Fe. Co. Ni. Cu. Zn. Ga. Ge. As. Se. Br.

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Chapter 5

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  1. Chapter 5 Gases

  2. What are some things you remember about gases and their behavior? List, Pass, Amass

  3. H He Solid Liquid Li Be B C N O F Ne Gas Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Ls Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 5 - 2 Elemental States at 25oC

  4. Stuff You Already Know State Property Solid Liquid Gas Density Shape Compressibility Thermal expansion High High Low Fixed Takes shape Expands of lower part to fill the of container container Small Small Large Very Small Moderate Small

  5. More You Remember In this state, the particles have sufficient energy to overcome all forces that attract them to each other. Each particle is completely separated from the others. This results in low densities and the fact that gases completely fill the container that holds them.

  6. Gas Pressure Gases exhibit pressure on the containers that hold them. Pressure = Force / Area Units: N/m2 a.k.a. Pascal Standard Pressures 760 mmHg = 1 atm = 760 torr = 1.01x105 Pa

  7. Manometer measures air pressure of a gas in a contained system. vacuum Barometer measures ambient air pressure. One atm 760 mmHg 29.9 inHg

  8. Boyles Law P1V1 = P2V2 Pressure and volume are inversely proportional. Examine the graph of the data: Derivative! Data!

  9. Charles’ Law V1/T1 = V2/T2 Volume and Temperature are directly proportional. Check out data: All gases intersect the x axis at a temperature of 0 Kelvin(-273 oC). At temperatures lower than 0 Kelvin, gases would have a negative volume (impossible, as is negative Kelvin degrees)

  10. Avogadro’s Law P1/n1 = P2/n2 Guy-Lussac’s Law P1/T1 = P2/T2 Also, at constant pressure and temperature, the volumes of gases involved in a chemical reaction are related by small whole numbers. (ie: 2 “volumes” of hydrogen and 1 “volume” of oxygen combine to form 2 “volumes” of water…we can use volume and moles similarly when it comes to gas stoichiometry.)

  11. Ideal Gas Law PV = nRT R = 0.0821 Latm/Kmol When you hold variables constant, this equation can be manipulated into one of the others that we just talked about. Since ideal gases don’t exist, this equation expresses the behavior a real gas approaches as pressure is lowered and temperature is increased.

  12. Gas Stoichiometry At STP, the molar volume of a gas is 22.4 L / mole S.T.P. Temperature = 273.15 K (0 oC) Pressure = 1 atm Gas stoichiometry problems can involve a gas law and/or the molar volume of a gas.

  13. Demo and Thoughts Answer Now What is inside the can? Why does this happen? Which gas law(s) does it illustrate? Rearrange the ideal gas law to get an equation to solve for the molar mass of a gas. CRUSH THE CAN

  14. Section 5.5:Dalton’s Law of Partial Pressure • For a mixture of gases in a container, the total pressure exerted is the sum of the pressures that each gas would exert independently. Ptotal= P1 + P2 + P3, etc… Air is a mixture of gases: Pair = PN2 + PO2 + PAr + PCO2+ PH2O

  15. The law of partial pressures also works for moles. Ptotal= (n1 + n2 +…)(RT/V) In a mixture of ideal gases, it is the TOTAL NUMBER OF MOLES OF PARTICLES, not the identity of the substance, that determines the pressure.

  16. Partial Pressure Try Me Mixtures of helium and oxygen are used in scuba diving tanks to help prevent “the bends.” For a particular dive, 46 liters of O2 and 12 liters of He were pumped in to a 5 liter tank. Both gases were added at 1.0 atm pressure at 25oC. Determine the partial pressure for both gases in the scuba tank at 25oC. Then, determine the total pressure in the tank.

  17. P total = P He + P O2 P total = 9.3 + 2.4 P total = 11.7 atm

  18. Mole Fractions • Another way to determine the partial pressures of a gas. X = n1 = P1 ntotal Ptotal Chi, mole fraction

  19. When Gas Is Collected Over Water • You must include the partial pressure of water in your calculations. • The vapor pressure of water is a constant value at each temperature. (you would need to look this up on a table or it would be given to you.) At 22oC, PH2O is 21 torr

  20. Section 5.6:Effusion and Diffusion Effusion occurs when a gas passes through a small opening in a barrier. Diffusion occurs when two gases mix randomly and spontaneously.

  21. More on Diffusion, Effusion, and speed of molecules • The speed of diffusion is much less than the theoretical velocity of the gas because of COLLISIONS. • The average speed of a gas that is effusing is proportional to the averages speed squared. R = 8.314 J/Kmol

  22. Even more on velocity of molecules that are effusing and/or diffusing • Graham’s Law: The rates of the gases is proportional to the square root of the inverse of the molar masses of the gases.

  23. Kinetic Molecular Theory • Kinetic Molecular Theory is an attempt to describe the behavior of an ideal gas • The particles are so tiny and spread out that their volume is negligible. • Particles in constant motion collisions w/ walls cause pressure. • Particles exert no intermolecular force. • Average KE is directly proportional to Kelvin temperature.

  24. Can we explain each gas law using KMT? • Boyle’s Law • Gay-Lussac • Charles • Avogadro

  25. KMT tutorial www.wwnorton.com/college/ chemistry/gilbert/tutorials/ch8.htm

  26. Maxwell-Boltzman Distribution Curves • Seen in wwnorton tutorial. • Illustrates the number of particles at a given velocity and temperature.

  27. Gases are not Perfect • Particles DO have mass and volume • Small attractive forces between particles ARE sometimes significant • Higher concentration of gas, more forces • Under extreme pressures or at low temperature, gases will condense to liq./sol.

  28. Ideal vs Real behavior Nitrogen at Various temp. Various Gas Deviations

  29. Well, Now What? The ideal gas law has to be adjusted to reflect REAL gas behavior. But who would want to do that?!! I DO!!! Johannes van der Waals …Yes, he’s the intermolecular force guy, and its no coincidence!

  30. What Johannes Did… 1 Ideal gas law 2 Adjusted for volume of gas 3 Adjusted for intermolecular forces 4 Glued Together The Official Equation 5

  31. When should you use vanderWaals equation? • Whenever you want to know the REAL behavior of the gas, not just the ideal, limiting value. • Whenever you are working at low temperatures or high pressures • Whenever you need to employ KMT to explain the behavior of a gas …AND BE CORRECT ALL THE TIME.

  32. But how do you know the values of a and b? • They are constants. You can look them up!

  33. Show Me Problem Calculate the pressure exerted by 0.500 mol of nitrogen in a 1.00L container at 25.0oC using • The Ideal Gas Law • The van der Waals Equation • The Ideal Gas Law Way • PV =nRT • P(1.00) = (.500)(.0821)(298) • P = 12.2 atm b) van der Waals Way (Pobs+a(n/V)2)(V-nb)=nRT (Pobs+1.39(.500/1.00)2)(1.00-.500(.0391))=(0.500)(0.0821)(298) (Pobs +5.56)(.98045)=12.23 Pobs=(12.23/0.98045)-5.56 = 6.92 atm

  34. Try Me Problem Calculate the pressure exerted by 0.250 mol of argon in a 1.00L container at 23.0oC using • The Ideal Gas Law • The van der Waals Equation

  35. Atmospheric Chemistry Xe • Air is a homogeneous mixture! • The heavier gases exist in the lower atmosphere, while lighter gases are present in the upper atmosphere. • Radon gas found in basements because it is very heavy. • Oxygen is less prevalent at the tops of mountains. • Hydrogen is more prevalent as you gain altitude. • Troposphere (closest to us) is most greatly impacted by our every-day routines. H2 Kr He N2 Ar H2O O2 Ne CH4 NO CO2

  36. Causes of Air Pollution and Acid Rain Causes: -Generation of electricity Coal burning plants -Transportation Combustion of petroleum

  37. Acid Rain Effects on Buildings • Erosion of the stone • Blackened streaks on bricks

  38. Why? Limestone (calcium carbonate) reacts with acid rain (sulfuric acid): CaCO3 + H2SO4 CaSO4 + H2CO3 H2CO3 CO2 gas + H2O What's it all mean? Calcium sulfate is soluble in water, which means that the limestone will wash away as the rain pours over it.

  39. Chemistry of Acid Rain as caused by burning coal

  40. Prevention of Acid Rain • At coal plants: Injection of limestone and air into combustion chamber before products are allowed to escape. (product becomes CaSO3, which is unusable, but non-toxic)

  41. Air Pollution Effects on People • Burning eyes • Trouble breathing • Headaches • Fatigue • Photochemical smog

  42. Chemistry of Air Pollution as caused by excessive traffic

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