Chapter13 - Chapter 13 Properties of Solutions Solutions...

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Chapter 13 Properties of Solutions Solutions Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent. The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles. How Does a Solution Form? As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them. If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal. Energy Changes in Solution Simply put, three processes affect the energetics of the process: h Separation of solute particles h Separation of solvent particles h New interactions between solute and solvent The enthalpy change of the overall process depends on H for each of these steps. Why Do Endothermic Processes Occur? Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered. Yet we know that in some processes, like the dissolution of NH 4 NO 3 in water, heat is absorbed, not released. Enthalpy Is Only Part of the Picture The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system. So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered. Student, Beware! Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved. Dissolution is a physical change—you can get back the original solute by evaporating the solvent.
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If you can’t, the substance didn’t dissolve, it reacted. Types of Solutions Saturated h Solvent holds as much solute as is possible at that temperature. h Dissolved solute is in dynamic equilibrium with solid solute particles. Types of Solutions Unsaturated h Less than the maximum amount of solute for that temperature is dissolved in the solvent. Supersaturated
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This note was uploaded on 12/09/2011 for the course CHEM 1 taught by Professor Staff during the Summer '11 term at Simon Fraser.

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Chapter13 - Chapter 13 Properties of Solutions Solutions...

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