Diffusion Effusion and Grahams Law What is diffusion
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Diffusion, Effusion, and Graham’s Law
What is diffusion? n The movement of particles from regions of higher concentrations to regions of lower concentration. n Eventually, the particles will disperse evenly throughout the space. Low High
What is effusion? n The passage of a gas under pressure through a tiny opening.
Graham’s law n Thomas Graham studied effusion in detail and determined that the rate of effusion is indirectly proportional to the square root of the molar mass of the gas, if pressure and temperature are kept constant. n In other words: v. A v. B = Molar Mass. B Molar Mass. A
How does that work? n Kinetic Molecular Theory: movement of particles is proportional to the amount of kinetic energy. (KE= ½ m v 2 ) n So, if the KE is constant, ½ m. A v. A 2 = ½ m. B v. B 2 n With a little algebra, we get:
Graham’s Law Formula
Example Problems n At the same temperature, which molecule travels faster, O 2 or N 2? How much faster? n At room temperature, Xe atoms have an average speed of 240 m/s. At the same temperature, what is the speed of H 2 molecules? n What is the molar mass of a gas if it diffuses 0. 907 times the speed of argon gas?
Answers 1) N 2 (lower molar mass) (32/28) = 1. 07 times faster 2) x/240 = (131. 3/2) x/240 = 8. 10 x = 1944 m/s 3) 1/0. 907 = (x/39. 9) (1/0. 907)2 = ( (x/39. 9))2 1. 216 = x/39. 9 x = 48. 5 g/mol
