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Work, Energy, and Power


Intermolecular Forces


The Physical Properties of Organic Compounds

Chemical Thermodynamics and the Equilibrium State


Why does 'like dissolves like?' To get at the thermodynamic reason behind the rule, Hess's Law of Heat Summation lets us imagine dissolving a solute in a solvent as a step by step process. Let us use our imagination to take the solution process down a pathway that is easy to analyze in terms of internal energy change.

Imagine pulling the solute apart from itself, pulling the solvent apart from itself, then letting the solute and solvent come together. In the first step, enthalpy must increase to separate the solute molecules from each other (we have to increase the electrostatic potential energy of mutually attracting particles, an internal energy increase, requiring heat flow from the surroundings) and likewise, enthalpy must increase to separate the solvent molecules from each other.

The stronger the respective intermolecular attractions within either the solvent or solute, the more internal energy increase will have been necessary to reach this stage in our imaginary pathway. Now, we can picture the system at a stage where all the particles are separated from each other.

Then, as the next step, imagine the system falling back together into the new arrangement, the solution, the final state of the system. So instead of the solute and solvent particles seeking out their old attractions, the solute associates with solvent and vice-versa.

If the quality of intermolecular attraction in both solute and solvent is similar, the net process will not involve a large increase of internal energy. The energy that went in to the system will now come out. With 'like dissolving like', the overall process will be only slightly endothermic (usually). If, on the other hand, one of the two, the solute is polar and the solvent is nonpolar (or vice versa), the imaginary process would call for the input of a large amount of energy and it would not be recoverable. The polar molecules of the solute would have nothing to grip in a nonpolar solvent, no charge densities in the solvent, to form a solution, and the result would be a very endothermic solution process. In an endothermic process, heat must flow in to the system (positive enthalpy change). Inward heat flow is not likely to happen, which is why the equilibrium points the other way toward insolubility.

The Physical Properties of Organic Compounds


The solubility properties of organic compounds, which the various organic functional groups impart on organic molecules containing them, is an extremely theme in biochemistry.

For example, a major influence upon the conformation of a globular protein in aqueous solution is the solubility of the amino acid side-chains. In aqueous solution, a protein will assume a low internal energy conformation where the regions with many nonpolar side chains are on the interior of the protein, sequestered from water, and the regions with polar side chains are on the exterior, hydrogen bonding with water.

The Physical Properties of Organic Compounds


Another example of the importance of the basic solubility properties of organic functional groups is in the solubility of the simple sugars. Ask yourself, if triglycerides are so much more energy dense than carbohydrates, why are sugars the basic nutrient molecule for metabolism. The answer is solubility. Because of its numerous hydroxyl groups, glucose (and other simple sugars) can readily hydrogen bond with water. Simple sugars are water soluble in any proportion.

The Physical Properties of Organic Compounds


The lipid category of biomolecules is operationally rather than structurally defined, and the operation involved is a solution process.

Lipids are the biomolecules that will be found in the organic phase of an extraction of biological material. Having large regions of non-polar functional groups (lots of carbon and hydrogen), lipids tend to be insoluble or sparingly soluble in water.

The Physical Properties of Organic Compounds

Biological Membranes

The Eukaryotic Cell

The Endocrine System

Continuing on our theme of applying the solubility theme in the biological context, let us introduce a favorite MCAT example: Peptide hormones are water soluble while steroid hormones are not.

This is an interesting distinction because the difference determines the signaling mode. While peptide hormones cannot pass through the nonpolar interior of the phospholipid bilayer, steroid hormones can pass through cell membranes without a channel. Because they cannot pass through membranes, unlike steroid hormones, peptide hormones typically rely on second messenger systems for signal transduction into the cell.

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