Intermolecular Forces

During our most recent lab, we experimented with the mechanisms behind solvation, in particular, intermolecular forces. These forces are responsible for the manner in which a solvent surrounds a solute, which is closely related to adhesion and cohesion, and differences in solubility.

In chemistry, solubility is described as the relative capability of being dissolved, or the amount of a solute that will be dissolved in a solvent. Solvation, while similar, is defined as “an interaction of a solute with the solvent, which leads to stabilization of the solute species in the solution”. During this process, the solvent surrounds the solute and pulls the solid molecules apart, surrounding them according to the charges involved (slight positive to slight negative). Simply put, solubility is the likelihood to dissolve, while solvation is the actual process that occurs.

Chromatography, a method that has been utilized used previously in labs related to differences in solubilty, is a set of labratory techniques used to separate solutions. Chromatography is used specifically to separate substances with different solubilities. The mixture at hand is dissolved in what is termed the “mobile phase”, which carries it through another material called the stationary phase. Due to the difference in the rates of travel of the various components of the mixture, which is based off of intermolecular forces and solubility, separation occurs.

Intermolecular forces are the attractive and repulsive forces that act between molecules of a substance, which can be atoms, molecules, or ions. These are not to be confused with intramolecular forces, which act inside molecules between the individual atoms. These forces determine the solubility level of a substance, and play a part in the difference in rates of travel that chromatgraphy is based upon.

In regards to solvation, these forces play a large role in the surrounding of solute by solvent molecules. The intermolecular forces are strong enough to actually pull the solute apart and separate it into ions, allowing the ions to be surrounded by solvent. This was observed in our lab today with the sugar cube and water. Adhesion and cohesion are also affected by intermolecular forces. Cohesion, defined as the “action or property of two of the same molecules sticking together, being mutually attractive”, relates to IMF’s because the level of intermolecular forces determines whether or not molecules are attracted to each other (and to what extent). Adhesion is based off of the same principles, but occurs between two different molecules. Both of these processes are dependent on the intermolecular forces present.

The phrase “like dissolves like” is frequently employed to explain differences in solubility because if a solvent and solute have similar intermolecular forces, solvation is much more likely to occur. This translates seamlessly to chromatography results because the substance component with the intermolecular forces most like the mobile phase travels at a faster rate than the component with differing forces, explaining why substances can be separated in this manner.


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