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Ch 5_Part3_110111

# Ch 5_Part3_110111 - Molecular Speed(cont Let's determine...

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11/1/2011 1 Molecular Speed (cont.) Let’s determine u rms for N 2 at 298 K. For a sense of scale, this is on the order of the speed of sound (~320 m s -1 )….which isn’t a coincidence.       1 2 -1 -1 -1 -1 3 8.314 J mol K 298 K 3 515 m s 0.0280 kg mol rms RT u M     Smaller mass = greater speed 2 2 kg m 1 J s Comparison of u rms for He and N 2 At 25 o C, which gas will travel faster, He or N 2 ? M(He) = 4.0 g/mol M(N 2 ) = 28 g/mol A car travelling at 60 mph, u car = 26.8 m/s If gases travel so fast, why does it take so long for you to smell perfume from across the room?

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11/1/2011 2 Diffusion Gas molecules travel in a straight line only until they collide with a container wall or another gas molecule. Gas molecules do not have an uninterrupted path in front of them. They are constantly colliding with other gas molecules. Rate of diffusion is proportional to u rms . So lighter particles will have a higher rate of diffusion, and vice versa. Diffusion (cont.) Diffusion is the process of mixing gases. In a closed container, diffusion will eventually lead to a homogeneous mixture. http://www.youtube.com/watch?v =H7QsDs8ZRMI
11/1/2011 3 Effusion Effusion is a special case of diffusion, which exploits the difference in velocities of lighter gas molecules. This process was used during the Manhattan Project to separate 235 U and 238 U isotopes. Effusion (cont.) Effusion is dependent on molecular speed. The molecular speed is in turn inversely dependent on the atomic or molar mass. Recall: Graham’s Law of Effusion: Rates of effusion are inversely dependent on the square root of the mass of each gas: 3 rms RT u M     2 1 1 2 Rate of effusion Rate of effusion M M

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Ch 5_Part3_110111 - Molecular Speed(cont Let's determine...

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