1ACh13 - Ch 13 Properties of Solutions Solution =...

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Ch. 13 Properties of Solutions Solution = Homogeneous mixture A solution is an Homogeneous mixture (a mixture that has uniform properties through out the material). Water, the Common Solvent Water is the most common solvent that we use. Aqueous Solutions Are solutions with water as the solvent Solvation A solution is formed when one substances mixes uniformly throughout another. The process of solvation occurs when the attractive forces between the solute and solvent are approximately the same or greater then the solute-solute and solvent-solvent attractive forces. When this happens, the solvent particles form a cage around the solute particles. Hydration Hydration is the process of attaching water onto a substance. In the process of solvation, the hydrogen (positive) end of water associates with negative ions and the oxygen end (negative) associates with positive cations. In polar solvents, the solvent tends to form a solvent cage around the solute. In this process, the negative and positive ends of the polar molecules line up with each other. Water and ethanol: hydrogen of water aligns with oxygen of ethanol, hydrogen of ethanol aligns with oxygen of water. For ionic compounds, the positive dipole of the solvent aligns with the anions, and the negative dipole of the solvent aligns with the cation. The cations and anions totally separate in solution. The solvation of an ionic compound in water into cations and anions can be written as: Ca(NO 3 ) 2 (aq) Ca 2+ (aq) + 2NO 3 - (aq) NaCl(aq) Na + (aq) + Cl - (aq) KF(aq) K + (aq) + F - (aq)
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The Energetics of Solvation The enthalpy of solvation essentially consists of three components. We have to break the intermolecular attraction between solvent molecules, we have to break the intermolecular attraction between solute particles (both of these are endothermic), then we make the intermolecular attraction between solute and solvent particles (exothermic). The sum of these components may be endothermic or exothermic. However, the spontaneous direction of flow in this universe is towards lower energy (i.e. exothermic processes. This would mean that only exothermic processes could happen; however, there is another component that influences spontaneous processes: Entropy. Entropy is described as randomness or disorder. It is a measure of the number of possible arrangements of the system. Example: Solid salt in water; the salt ions are packed into a crystal structure that does not allow rearrangement. The water molecules can move around and have a certain amount of entropy (possible arrangements), and there is a limited amount of arrangements the solid salt can take in the bottom of the water. After the salt dissolves, the salt ions can be arranged throughout the water, and the water has more arrangement possibilities with cations, anion, and water molecules to interact with, so the arrangement possibilities has increased significantly, the entropy has increased.
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This note was uploaded on 04/17/2008 for the course CHEM 1 taught by Professor Evans during the Fall '06 term at UC Irvine.

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1ACh13 - Ch 13 Properties of Solutions Solution =...

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