Chapter 13 - Physical Properties of Solutions A solution is...

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Physical Properties of Solutions
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A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount
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Three types of interactions in the solution process: solvent-solvent interaction solute-solute interaction solvent - solute interaction H soln = H 1 + H 2 + H 3
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solvent-solvent separation: ΔH solvent solute-solute separation: ΔH solute solvent-solute mixing: ΔH mix ΔH solvent + ΔH mix = ΔH solvation (For aqueous solutions, ΔH solvation = ΔH hydration ) So, for the following process: solute (undissolved) solute (dissolved) ΔH solution = ΔH solute + ΔH solvation/hydration
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For ionic compounds dissolving in water: ΔH solution = – ΔH lattice + ΔH hydration (where ΔH lattice is always negative and ΔH hydration is always negative) This is just like Hess’s Law: ΔH° solution = ΣΔH° f (products) – ΣΔH° f (reactants) For example: ΔH° solution = ΣΔH° f (NaCl(aq)) – ΣΔH° f (NaCl(s)) for NaCl(s) NaCl(aq) (° = 1 molal (1m) aqueous solute)
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If ΔH latt of KBrO 3 = – 745 kJ/mol and the combined ΔH hyd = – 704 kJ/mol, what is the ΔH soln for the dissolution of KBrO 3 in water?
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Answer: ΔH solution = – ΔH lattice + ΔH hydration = – (– 745 kJ/mol) + (– 704 kJ/mol) = + 41 kJ/mol But wait! If KBrO 3 dissolves readily in water (solubility = 6.9 g/100mL), then why is ΔH solution for KBrO 3 a positive value (endothermic process)?. ....................
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If the ΔH solution is +, then endothermic; if ΔH solution is –, then exothermic. This contributes to the ENTHALPIC favorability of solution formation. But sometimes a solution has a + ΔH solution , but a solution still forms. Why? Entropy (S) – the natural tendency of most systems to become more disordered (disperse) – entropy tends to increase Solutions form naturally; pure solutes and solvents do not Solution Process involves TWO factors: 1) ΔH 2) ΔS The relative magnitudes of these factors determine whether a solution forms: the increase in S accompanying dissolution always favors solution formation, but ΔH can go either way to favor or not favor solution formation.
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like dissolves like Two substances with similar intermolecular forces are likely to be soluble in each other. non-polar molecules are soluble in non-polar solvents CCl 4 in C 6 H 6 polar molecules are soluble in polar solvents C 2 H 5 OH in H 2 O ionic compounds are more soluble in polar solvents NaCl in H 2 O or NH 3 ( l )
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Solubility of a solute = the maximum amount that dissolves in a fixed amount of a particular solvent at a specified temperature, given that excess solute is present NaCl: S = 39.12 g/100 mL H 2 O at 100°C AgCl: S = 0.0021 g/100 mL H 2 O at 100°C Why the difference? Depends on relative strength of IMF within and between solute and
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Chapter 13 - Physical Properties of Solutions A solution is...

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