MadSci Network: Chemistry Re: Van der Waals constant(b)for neon Date: Fri feb1313:30:141998 Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College Chemistr ID:886784809.Ch Message While teaching the Van der Waal's equation for real gases, I came across something i did not completely understand the b constant is correlated to molecular volume, why would the b value for neon be smaller than the value for H le? I checked the Handbook of chemistry and Physics, and the closest I could come to an explanation was the fact that the b value was also correlated to compressibility factor By the way, the van der Waals equation is P+ b=nrT According to P W. Atkins(Physical Chemistry, 3d Edition), a relates to the density of the gas and b to the total volume occupied by the gas molecules. It i important to recognize that these constants are derived from experiment, that is they are empirical. The first thing i did was to check my handy Sargent-Welch periodic table. It gives atomic and covalent radii. A few calculations (and a check of the Handbook of Chemistry and Physics) gave the following information: Atomic Molecular van der Waals van der Waals Ele ement Radius Volume a constant*k b constant* Hydr 0.79 0.24 0.027 Helium 49 0.49 0.03 0.024 Neon 0.51 0.55 0.21 0.017 k van der Waals constants taken from the Handbook of Chemistry and Physics, 61 Edition For hydrogen(a diatomic molecule), we need the covalent radius (0. 32) to convert the information into a molecular volume: i assumed cylinder, with hemispheric ends, with radius 0. 79 and length 2.22( 2*0. 32+2*0. 79). The volume is then given by 丌r+2o×r where r is the covalent radius of hydrogen.(The two hemispheres add to the volume of a sphere of radius r, and the remainder of the volume is a cylinder with radius r and height 2r.)MadSci Network: Chemistry Re: Van der Waals constant (b) for neon Date: Fri Feb 13 13:30:14 1998 Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College Area of science: Chemistry ID: 886784809.Ch Message: While teaching the Van der Waal's equation for real gases, I came across something i did not completely understand: If the "b" constant is correlated to molecular volume, why would the b value for neon be smaller than the value for H or He? I checked the Handbook of Chemistry and Physics, and the closest I could come to an explanation was the fact that the b value was also correlated to a compressibility factor. By the way, the van der Waals equation is According to P.W. Atkins (Physical Chemistry, 3d Edition), a relates to the density of the gas and b to the total volume occupied by the gas molecules. It is important to recognize that these constants are derived from experiment, that is, they are empirical. The first thing I did was to check my handy Sargent-Welch periodic table. It gives atomic and covalent radii. A few calculations (and a check of the Handbook of Chemistry and Physics) gave the following information: For hydrogen (a diatomic molecule), we need the covalent radius (0.32) to convert the information into a molecular volume; I assumed a cylinder, with hemispheric ends, with radius 0.79 and length 2.22 (= 2*0.32+2*0.79). The volume is then given by: where rcov is the covalent radius of hydrogen. (The two hemispheres add to the volume of a sphere of radius r, and the remainder of the volume is a cylinder with radius r and height 2rcov.) Element Atomic Radius Molecular Volume van der Waals a constant* van der Waals b constant* Hydrogen 0.79 3.3 0.24 0.027 Helium 0.49 0.49 0.03 0.024 Neon 0.51 0.55 0.21 0.017 * van der Waals constants taken from the Handbook of Chemistry and Physics, 61st Edition