Van der waals parameters a and b

This means that in order to determine which molecule has a higher b value, you would need to consider the size of the molecules and their interactions. In summary, to determine which molecule has a higher value for a or b, you would need to consider the attractive forces between molecules for a and the size of the molecules for b.
  • #1
physgirl
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How, if you're given a pair of molecules, would you decide which one might have a higher value for a or b?

As I understand for instance, a's value will be high if two molecules are attracted to each other and b's value will be high if two molecules strongly repel each other... but how would you determine that?

so for something like H2CO2 vs. CH4, in order to determine which has a higher "a", I guessed it was H2CO2 because it can hydrogen bond with each other and so are attracted to each other whereas CH4 aren't... but with something like He and Ne?

and then for deciding which has a higher b value, if you had something like F2 vs. Br2, where would you begin? how would you decide which ones would repel more?

thanks!
 
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  • #2
The volume correction parameter, b is actually indicative of the size of the molecule. A bigger molecule has a larger b value. The Van der Waals model has no mechanism for repulsion other than "hard sphere interaction" (elastic collisions between spheres).
 
  • #3


Van der Waals parameters a and b are empirical constants used to describe the attractive and repulsive forces between molecules. In general, a higher value for a indicates a stronger attractive force between molecules, while a higher value for b indicates a stronger repulsive force.

In order to determine which molecule has a higher value for a or b, you would need to consider the intermolecular interactions between the molecules. For example, in the case of H2CO2 vs. CH4, you correctly identified that H2CO2 has a higher value for a due to its ability to form hydrogen bonds. Hydrogen bonds are strong intermolecular forces that contribute to a higher value for a. On the other hand, CH4 molecules only have weak London dispersion forces, which contribute to a lower value for a.

In the case of He and Ne, both molecules have similar intermolecular interactions (London dispersion forces), so it may be more difficult to determine which one has a higher value for a or b. In this case, you may need to consider factors such as molecular size and shape, as these can also influence the strength of intermolecular forces.

For determining which molecule has a higher b value, you would need to consider the repulsive forces between the molecules. In the case of F2 vs. Br2, both molecules have similar intermolecular interactions (dipole-dipole forces), so you would need to look at other factors such as molecular size and shape to determine which one has a higher value for b.

Overall, determining the values of a and b for a given pair of molecules can be complex and may require considering multiple factors. However, understanding the types of intermolecular interactions present and their relative strengths can help in making an informed decision.
 

What are Van der Waals parameters a and b?

Van der Waals parameters a and b are constants used in the Van der Waals equation to account for the attractive forces and the volume of molecules in a gas or liquid. These parameters are specific to each molecule and are used to calculate the pressure and volume of the substance.

How are Van der Waals parameters a and b determined?

Van der Waals parameters a and b can be determined experimentally by measuring the critical temperature and pressure of a substance and using them to solve the Van der Waals equation. They can also be estimated using molecular parameters such as molecular size and polarizability.

Why are Van der Waals parameters a and b important?

Van der Waals parameters a and b are important because they allow for a more accurate prediction of the behavior of real gases and liquids. The Van der Waals equation takes into account the attractive forces and molecular volume, which are not considered in the ideal gas law. This makes it a better model for real substances.

What are the units of Van der Waals parameters a and b?

Van der Waals parameter a has units of pressure multiplied by volume squared (Pa·m^6/mol^2), while parameter b has units of volume (m^3/mol). These units are consistent with the units used in the Van der Waals equation, which is expressed in terms of pressure, volume, and temperature.

Can Van der Waals parameters a and b be used for all substances?

No, Van der Waals parameters a and b are specific to each substance and cannot be used for all substances. They are determined experimentally or estimated based on molecular parameters, so they are only valid for the particular molecule that they were calculated for.

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