Chapter 15 Notes

Chapter 15 Notes - Chapter 15 The Chemistry of Solutes and...

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Chapter 15 The Chemistry of Solutes and Solutions
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Solution = homogeneous mixture of substances. It consists of: solvent - component in the greatest amount. solute - all other components (may be >1). Solvent-solute interactions determine if a substance will dissolve in a particular solvent. Solubility & Intermolecular Forces
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Solutions: Exist in all 3 physical states. Can be mixtures of solids, liquids and gases. Type of Solution Examples Gas in gas Air Gas in liquid Carbonated drinks. Gas in solid H 2 (g) in Pd(s) (safe fuel storage). Liquid in liquid Motor oil, vinegar. Solid in liquid Ocean water, sugar-water. Solid in solid Bronze, pewter, 14K gold. Solubility & Intermolecular Forces
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Similar… the pair will be soluble. polar dissolves polar; non-polar dissolves non-polar. Dissimilar… insoluble. Like dissolves like If solute and solvent intermolecular forces are: Solute-Solvent Interactions
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Substances dissolve when: solvent-solute attraction > solvent-solvent attraction, and > solute-solute attraction. Solute-Solvent Interactions
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Miscible liquids dissolve in all proportions. e.g. ethanol and water (both H-bonded polar liquids). Immiscible liquids form distinct separate phases. e.g. gasoline (non-polar) and water (polar). Solute-Solvent Interactions Carefully layered Stir… …settle
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Solute-Solvent Interactions Water solubility decreases as alcohols grow larger: Solute-solute attraction grows. Solute-solvent attraction stays ≈ constant. Name Formula Solubility (g/100g H 2 O @20 C) methanol CH 3 OH miscible ethanol C 2 H 5 OH miscible 1-propanol C 3 H 7 OH miscible 1-butanol C 4 H 9 OH 7.9 1-pentanol C 5 H 11 OH 2.7 1-hexanol C 6 H 13 OH 0.6 London forces increasing
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Alternate view Alcohols: polar head (–OH) on a non-polar tail. The head is hydrophilic (“water loving”) The tail is hydrophobic (“water hating”) As the tail gets bigger, it is harder and harder to dissolve. Solute-Solvent Interactions
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Δ H soln = Δ H step a + Δ H step b + Δ H step c Enthalpy, Entropy & Dissolving Solutes
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Sugar dissolving in water (concentration gradients are visible) On mixing, solute and solvent molecules spread out over a larger V . • This increases entropy ( S ). Processes with Δ S > 0 tend to be product-favored. For NH 4 NO 3 + H 2 O: Δ H soln > 0 (unfavorable). But Δ S soln > 0 (drives solution formation).
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Unsaturated [Solute] < solubility. All added solid is dissolved. Can dissolve more solute. Solutions may be: Saturated No more solute will dissolve. Undissolved solid is often present. Supersaturated • Have more than the equilibrium amount dissolved.
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solute is in dynamic equilibrium: solute + solvent solution Solute constantly moves in and out of solution. Solubility & Equilibrium
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Chapter 15 Notes - Chapter 15 The Chemistry of Solutes and...

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