10 8 Molecular Effusion and Diffusion Advanced Chemistry
- Slides: 14
10. 8 Molecular Effusion and Diffusion Advanced Chemistry
Introduction urms (root-mean-square speed) – the square root of the average of the squared speeds of the gas molecules in a gas sample This is the speed of a molecular possessing a kinetic energy identical to the average kinetic energy of the sample. The rms speed is not quite the same as the average speed, uav, but the difference is small.
Root-Mean-Square Speed
Calculating a Root-Mean-Square Speed Calculate the rms speed of the molecules in a sample of N 2 gas at 25⁰C.
More Practice What is the rms speed of an atom in a sample of He gas at 25⁰C?
Extra Information
Effusion & Diffusion The dependence of molecular speed on mass has two interesting consequences. (1) Effusion: the escape of gas molecules through a tiny hole Light atoms or molecules escape through the hole faster than heavier ones Smaller atoms can fit through the hole better Faster atoms are more likely to hit the hole, and smaller atoms are faster at the same temp (2) Diffusion: the spread of one substance throughout a space or throughout a second substance Faster Slower for light molecules than for heavier ones than effusion due to the random motion of molecular collisions. There is no net direction of motion
Diffusion and Mean Free Path Due to molecular collisions, the direction of motion of a gas molecule is constantly changing. Mean Free Path: average distance traveled by a molecule between collisions High pressure short mean free path Low pressure long mean free path
Graham’s Law of Effusion
Applying Graham’s Law An unknown gas composed of homonuclear diatomic molecules effuses at a rate that is 0. 355 times the rate at which O 2 gas effuses at the same temperature. Calculate the molar mass of the unknown and identify it.
More Practice Calculate the ratio of the effusion rates of N 2 gas and O 2 gas.
More Practice If a molecule of neon gas travels at an average speed of 400 m/s at a given temperature. Find the average speed of a molecule of butane gas, C 4 H 10, at the same temperature.
More Practice Compare the rates of effusion of H 2 and O 2 at the same temperature and pressure.
More Practice A sample of hydrogen effuses through a porous container about 9 times faster than an unknown gas. Calculate the molar mass of the unknown gas.