Understanding intermolecular force is fundamental to understanding solution chemistry, one of the most important MCAT topics. What is the reason for the rule of 'like dissolves like'? The rule derives from the role of intermolecular force in determining the solubility relationship between solutes and solvents. To illustrate this point, let us use conceptualize the solution process as a step-wise path. Hess's Law of Heat Summation assures us our conclusions regarding overall thermodynamic change will be valid.

First, imagine pulling the solute molecules apart from one another. You are pulling against intermolecular force. Now imagine pulling the solvent molecules apart. You have to make room for the solute. Picture what you have done. You have input energy and the system is in a state of separated solute molecules and separated solvent molecules.

Next, imagine allowing the solute and solvent to fall together to form the final solution. This step represents an internal energy decrease.

Now sum up the energy. Internal energy increases as the molecules of solute and solvent are separated, then decreases as new relationships between solute molecules and solvent molecules are formed. If the solute and solvent are unlike (have different kinds of intermolecular force), the summation will point to an internal energy that is very positive (the solute and solvent don't attract each other nearly as strongly as they attract themselves). This in turn would point to a positive standard enthalpy change for the process. This is why a nonpolar solvent won't dissolve a polar solute.

On the contrary, though, when solute and solvent have similar modes of intermolecular force, the solution process is generally much less endothermic, or maybe even exothermic, because the energy increase involved in separating the molecules will be matched by an energy decrease as the solvent and solute molecules come together.
Solution process may be endothermic or exothermic.

The solution process may be endothermic or exothermic (though usually endothermic, even for soluble mixtures).


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