Chapter 12 - Introduction to General and Organic Chemistry...

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1 Chapter 12 Properties of Solutions Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry II
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2 Learning Objectives - I - Be able to define solute and solvent. - See that the tendency of any system to disorder is one major driving force for the formation of solutions. - Understand the rule of thumb, “like dissolves like.” - Be able to tell the difference between an exothermic and endothermic heat of solution, H soln . - Be able to tell that solutions can be formed from
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3 Solutions Definition : Homogeneous mixtures of small particles with average diameter of 0.05 to 0.25 nm. This can be summarized as the rule of thumb “like dissolves like” Factors Promoting Solution Formation : Spontaneity Solute-to-solvent attractions
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4 Spontaneous mixing is a strong driving force in nature A system, left to itself, will tend towards the most probable state Formation of the most probable state is also a driving force for liquid solutions Attractive forces are not important in the gas phase but are very important in liquid solutions
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5 The attractive forces between solute and solvent hold the solution together The strength of the intermolecular attractive forces depends on both the solute and solvent Ethanol and water can be mixed in any proportion These compounds are said to be miscible Benzene is virtually insoluble in water
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6 Benzene and water are said to be immiscible When the strengths of the intermolecular attractions are similar in solute and solvent, solutions form Both ethanol and water are polar molecules that form hydrogen bonds. Solvent and solute are “similar” and a solution will form Benzene is nonpolar and very different from water. Solvent and solute are very “different” and no solution forms.
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12.1. Substances mix spontaneously when there is no energy barrier to 7 Learning Check Which of the following are miscible in water? water acetic acid carbon disulfide ammonia
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04/25/10 12.1. Substances mix spontaneously when there is no energy barrier to 8 Your Turn! Which of the following are likely to be miscible with water? A. CH 3 CH 2 CH 2 CH 3 B. C 6 H 6 C. CH 3 CO 2 H D. All are expected to be miscible
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9 This rule “like dissolves like” also applies. The basic principles remain the same when the solutes are solids Ex. When sodium chloride dissolves in water, the sodium and chloride ions are hydrated , i.e. surrounded by water molecules The general term for surrounding a solute particle by solvent molecules is solvation Solid - in – Liquid Solutions
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10 Dissolution of NaCl: Before the solution forms, water molecules are only attracted to other water molecules and the ions in NaCl are attracted only to other ions in the solid. In the solution, the ions have water molecules to replace their oppositely charged counterparts. In addition, water molecules are more strongly attracted to ions than other water molecules Hydration of a Polar molecule:
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Chapter 12 - Introduction to General and Organic Chemistry...

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