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| Michigan State University - Virtual Textbook - Intermolecular Forces and Physical Properties
Excellent tutorial on the relationship between intermolecular force and physical properties. Highly recommended.
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| Special points of emphasis
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Work, Energy, and Power Electricity Intermolecular Forces Functional Groups in Organic Chemistry Thermochemistry The States of Matter | Intermolecular forces are fundamentally electrostatic interactions (although London dispersion forces have a magnetic component also). Applying the classical electrostatic description, we understand that the greater the forces of attraction between molecules, the more the energy of the system must be increased as molecules are separated from each other, as in a phase change such as vaporization.
Phase change is an intermolecular event. Let us apply our understanding of electrostatics to the phenomenon of vaporization to help us begin to develop a sense of the thermochemistry of the process.
We are at a stage in our MCAT preparation where we want to create a bridge between basic electrostatics that runs through our understanding of the types of interactions that occur at the particle level in chemistry to the next stage of chemistry, which is thermodynamics.
The internal energy change associated with vaporization is the electrostatic potential energy increase of separating mutually attracting molecules. The greater the strength of intermolecular force, the greater the internal energy increase required for vaporization, so substances for which the mode of intermolecular force is hydrogen bonding are higher boiling than similarly sized molecules taking part in dipole-dipole interactions. A higher temperature must be reached for the kinetic energy of the molecules to be sufficient to allow them to escape from each other.
The lowest boiling substances only have weak Van der Waals attractions between molecules, so a simplified picture shows them moving only slowly and having sufficient energy to become released from their mutual potential energy well, to become vapor.
Molecular weight and the strength of intermolecular force are the primary determinants of the melting point, vapor pressure, and boiling point. A comparison of the physical properties of structural isomers such as ethanol and dimethyl ether is the kind of direct questions these concepts give rise to on the MCAT.
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Work, Energy, and Power Electricity Intermolecular Forces Thermochemistry Solutions | 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 (against intermolecular force) and pulling the solvent molecules apart (overcoming the self-attraction of the substances involves input of energy).
Next, imagine allowing the solute and solvent to fall together to form the final solution.
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.
On the contrary, though, when solute and solvent have similar modes of intermolecular force, the solution process is generally much less endothermic because the energy increase involved in separating the molecules will be matched by an energy decrease as the solvent and solute molecules come together.
One proviso. Despite the discussion above, keep in mind that most solution processes actually are endothermic. The difference between soluble and insoluble is the difference between being a little endothermic (soluble) and extremely endothermic (insoluble).
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Intermolecular Forces Proteins Carbohydrates Nucleic Acids Lipids | Intermolecular forces play a major roll in determining the physical and chemical properties of biomolecules. Intermolecular forces are central to the most critical processes of biochemistry such as protein conformation and enzyme activity or DNA base-pairing. Note that when you enter the world of biomolecules, the definition of intermolecular force is often expanded to include not only forces between molecules but also forces between widely separated regions of the same molecule.
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Work, Energy, and Power Electricity Intermolecular Forces Thermochemistry The States of Matter Lipids | One favorite topic for MCAT questions from basic biochemistry is that the longer and more saturated the fatty acid chains of a triglyceride, the higher will be the melting point of the fat. These characteristics enable the maximum Van der Waals interaction between the hydrocarbon chains, meaning that each chain experiences a higher net electrostatic force of attraction to its neighbor.
Therefore, the transition from solid to liquid, in which the chains draw apart and allow much more free rotation around bond axes, requires a greater influx of heat with the longer, saturated hydrocarbon chains. With saturated fats, the phase transition represents a larger increase in electrostatic potential energy as the straighter chains pull away from each other.
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