Qualitative understanding of partition preferences

Introduction
Cavity model
Rules for partitioning
The cavity model in quantitative terms

Selftest
     1) What does the cavity model say?
           Answer
     2) Main interactions ... ?
           Answer
     3) Size of a solute  molecule ... ?
           Answer
     4) Size of the solvent  molecule ... ?
           Answer
     5) Interpretation of data
           Answer
     6) "Like dissolves like"
           Answer
     7) Concept maps
     8) Functional groups
           Answer
     9) Illustration by given data?
           Answer
     10) Evaluation of the software PcKocWIN
     11) H-bonds between given substances?
     12) Tendency to distribute
           Answer
     13) Gas chromatography
           Answer
     14) Henry's Law constant
           Answer
     15) Quiz
Problems
Intermolecular interactions in every day life
FAQ

14) Henry's Law constant

The Henry´s Law constant, KH, describes the partition equilibrium of a chemical between water and air1. For compounds with a low water solubility the Henry´s Law constant is often derived from the saturation vapour pressure and the water solubility of the compound.

1Note that in physical chemistry the term “Henry´s law constant” is in fact used differently than in environmental chemistry: in physical chemistry it stands for any partition constant between the gas phase and a condensed phase in the linear range of the partition isotherm.


Questions:

1) Can this procedure also be used for compounds...

  a)...with relatively high, but still limited, water solubility?

  b)...that are completely miscible with water?

 

2) If the compound is a solid at room temperature which data do you use?

  a) KH = solubility of the subcooled liquid / saturation vapour pressure of the subcooled liquid

  b) KH = solubility of the solid / saturation vapour pressure of the subcooled liquid

  c) KH = solubility of the subcooled liquid / saturation vapour pressure of the solid

  d) KH = solubility of the solid / saturation vapour pressure of the solid

 

 

Answer:

1a) In this case, the resulting KH would be valid for high concentrations and might differ from the one that is valid at low concentrations because the partition isotherm can become non-linear for compounds with a rather high water solubility.

1b) It should be obvious that this cannot work because you would have to divide by infinity. But of course these compounds still possess a Henry´s Law constant. It is just that you have to determine it in a different way.


2) a and d are both correct. It is important that the same reference state is used .

 

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