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Chapter_14_Notes - Chapter 14 Notes[14-1 The Dissolution...

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Chapter 14 Notes [14-1 The Dissolution Process Solutions are homogeneous mixtures of two or more substances. Homogenous means at the mixture has uniform composition and properties throughout. o The dissolving medium is called the solvent (water). o The dissolved species are called the solute (Kool-Aid TM mix). There are many different types of solutions! o The three states of matter may be mixed two at a time to give NINE different types of solutions. o Seven of the solutions are homogeneous and two are heterogeneous. Homogeneous solutions Solute Solvent Example *Solid Liquid salt water Liquid Liquid mixed drinks Gas Liquid carbonated drinks *Liquid Solid dental amalgams Solid Solid alloys Gas Solid metal pipes Gas Gas air *Note that these are not the same thing!!! Heterogeneous solutions Solute Solvent Example Solid Gas dust in air Liquid Gas clouds, fog Some dissolution processes are spontaneous and others are not. o Some solutes dissolve easily with no outside influence. o Some solutes have to be “forced” to dissolve. o How do we know when the dissolution process will b e spontaneous and when it won’t ? For solutions in which the solvent is a liquid, there are two factors that affect the dissolution of solutes. o Enthalpy: Represented by the symbol H o Entropy: Represented by the symbol S Enthalpy is the amount of heat transferred into or out of a system as it undergoes a chemical or physical change at constant pressure. o In this case, we are discussing the amount of heat transferred as a solute dissolves in a solvent, so we call it the H solution or H dissolution . o The value of H solution tells us whether or not the dissolution process is spontaneous. o When H solution < 0 the process is exothermic . Dissolution is favored . o When H solution > 0 the process is endothermic .
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Dissolution is NOT favored . Entropy is the change in disorder that accompanies a process. o We are interested in the change in order, or randomness that occurs when we mix a solvent and a solute. We call this the S mixing. o Dissolution is favored when S mixing > 0 . Increasing S mixing indicates a greater degree of disorder which is exactly what occurs as a solute dissolves in a solvent. o Dissolution is not favored when S mixing < 0. Decreasing S mixing indicates less disorder, so the solute does not “want” to dissolve in the solvent. The best conditions for dissolution are when H solution is negative and S mixing is positive. Increased disorder almost always results from the mixing process. o So, S mixing is almost always positive! Solute-solute attractions o Remember that before the solute is placed in the solvent, it is a pure compound with many molecules held together by intermolecular forces . o
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Chapter_14_Notes - Chapter 14 Notes[14-1 The Dissolution...

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