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Vapor pressure is an important property for the instrumental analysis of many organic compounds. A variety of intermolecular forces can be involved with varying vapor pressure values, but this post focusses on just one; London Dispersion Forces. A second semester student may recall what those Dispersion Forces are about- They are statistically-improbable short-lived dipoles occurring across single atoms. Even though the effect is statistically rare, properties of compounds are greatly influenced because of the induction effect. This means that dipoles are produced in atoms near to other dipole atoms. One can then imagine a cascading effect of forming and disappearing dipoles continuously occurring. It stands to reason, the cascading effect can occur across carbon atoms of a hydrocarbon, as well. Following this concept, the following diagram presents a model for intermolecular bonding by way of London Dispersion Forces.
The diagram model is a simplified view of London Forces with three main representations: 1) a focus on increasing separation of charge across the molecule, indicated with larger (and darker) ovals looking left to right, 2) An increase in intermolecular force with increasing molecule size, indicated with dashed lines of different weights, and 3) the notion that heavier molecular weight alkanes will result in larger "dispersion clusters", also indicated by the model.
Now, if the effects of increasing molecule size on overall increase of London-Dispersion-Force numbers, were as simple as the above model suggests, then the resulting effect on vapor pressure vs MW could be linear. The reality of the situation, however, is much more complex:1) Intermolecular attractions occur in 3-dimensional space (the model above is a Pseudo-2-D space representation), 2) Intermolecular attractions are occurring across all atoms of the molecules, and 3) the molecules are in constant motion. Because of these greater complexities, a very small change in alkane MW causes a dramatic change in vapor pressure, and we find that the relationship is actually exponential! This is why the plot in the above diagram features log of vapor pressures. Mathematically, a logarithm plot of an exponential relationship gives a linear graph.
That's all for this post - Sorry to have been away for so long!
As always, Thank you for reading!
A Publication of http://ExcellenceInLearning.biz
Vapor pressure is an important property for the instrumental analysis of many organic compounds. A variety of intermolecular forces can be involved with varying vapor pressure values, but this post focusses on just one; London Dispersion Forces. A second semester student may recall what those Dispersion Forces are about- They are statistically-improbable short-lived dipoles occurring across single atoms. Even though the effect is statistically rare, properties of compounds are greatly influenced because of the induction effect. This means that dipoles are produced in atoms near to other dipole atoms. One can then imagine a cascading effect of forming and disappearing dipoles continuously occurring. It stands to reason, the cascading effect can occur across carbon atoms of a hydrocarbon, as well. Following this concept, the following diagram presents a model for intermolecular bonding by way of London Dispersion Forces.
The diagram model is a simplified view of London Forces with three main representations: 1) a focus on increasing separation of charge across the molecule, indicated with larger (and darker) ovals looking left to right, 2) An increase in intermolecular force with increasing molecule size, indicated with dashed lines of different weights, and 3) the notion that heavier molecular weight alkanes will result in larger "dispersion clusters", also indicated by the model.
Now, if the effects of increasing molecule size on overall increase of London-Dispersion-Force numbers, were as simple as the above model suggests, then the resulting effect on vapor pressure vs MW could be linear. The reality of the situation, however, is much more complex:1) Intermolecular attractions occur in 3-dimensional space (the model above is a Pseudo-2-D space representation), 2) Intermolecular attractions are occurring across all atoms of the molecules, and 3) the molecules are in constant motion. Because of these greater complexities, a very small change in alkane MW causes a dramatic change in vapor pressure, and we find that the relationship is actually exponential! This is why the plot in the above diagram features log of vapor pressures. Mathematically, a logarithm plot of an exponential relationship gives a linear graph.
That's all for this post - Sorry to have been away for so long!
As always, Thank you for reading!
A Publication of http://ExcellenceInLearning.biz
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