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Embed code for: AP missed 10-17 end of Ch5
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Effusion and Diffusion Diffusion Describes the mixing of gases Effusion Describes the passage of a gas through a tiny orifice into an evacuated chamber Graham’s Law of Effusion Rate of effusion is inversely proportional to the square root of the mass of its particles M1 and M2 = molar masses of the gases (can be EITHER g/mol or kg/mol, as long as they match) Effusion & Diffusion Example The effusion rate of an unknown gas is measured and found to be 31.50 mL/min. Under identical experimental conditions, the effusion rate of O2 is found to be 30.50 mL/min. If the choices are CH4, CO, NO, CO2, and NO2, what is the identity of the unknown gas? NO Diffusion Can be hard to describe theoretically because of the large number of collisions of gas molecules (with each other and with themselves) In-Class Problems #112, 113, 114 Real Gases No gas exactly follows the ideal gas law…although many come close at low pressures, high temp. and low numbers of moles of gas. Effect of Pressure on Gases Plots of PV/nRT vs. P For an ideal gas, PV/nRT should equal 1 For real gases, PV/nRT approaches 1 only at low pressures Effect of Temp. on Gases Consider N2 at different temps: Plots of PV/nRT vs. P Closer to ideal as temp. increases How do we modify the KMT & the IGL for real gases? KMT assumed particles had no volume But real gases DO have a volume , so the volume available to the particle is actually less than the volume of the container b/c the particle itself occupies some of the space. V in the IGL must be corrected to V-nb, where nb is the volume of the particles. n=# moles of gas, b=constant determined by experimental results Correcting for Real Gases, cont’d KMT assumed there are no attractions between particles. But when real gases come together, attractive forces occur, so they hit the walls of the container slightly less (affecting?____) To correct for attractive forces, we must factor in the concentration of the gas molecules (in mol/L or n/V) The higher the conc., the more likely they will come close enough to attract each other. # of interacting pairs of particles depends on the square of the concentration (n/V)2 (see next diagram) Correcting for Real Gases, cont’d The pressure correction is Pobs = P’–a(n/V)2 a is a proportionality constant & is determined from experimental behavior of the particular gas Correcting for Real Gases, cont’d van der Waals Equation Factoring the volume and pressure corrections into the IGL gives the “Van der Waals” equation: Ideal: P x V = nRT vdW’s: [Pobs + a(n/V)2] x (V-nb) = nRT rearranged: Values for a & b are experimental Notice the units Example Calculate the pressure exerted by 0.5000 mol N2 in a 1.0000 L container at 25.0oC a) using the ideal gas law 12.24 atm b) using the van der Waals equation 12.13 atm c) compare the results IGL is high by 0.11 atm or 0.91% Van der Waals Eqn Cont’d Remember, gases behave more like ideal at high temp. & low pressure. The van der Waals equation accounts for this. In-Class #116