Download - Section 13.2 and 13.3 Control of Solubility
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Section 13.2 and 13.3 Control of Solubility
Bill ViningSUNY Oneonta
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Control of Solubility
In these sections…
Molecular structure control:a.Entropy and thermodynamic controlb.Gas mixturesc. Mixtures of neutral molecules in liquidsd.Mixtures of ionic compounds in water
External (conditions) control:e. pressure f. temperature
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Enthalpy and Entropy
Enthalpy: Bond strength- stronger bonds = lower enthalpy = more stable
Entropy: Free motion of atoms- freer mobility = higher entropy = more stable
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Example: Enthalpy and Entropy of Evaporation
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Enthalpy and Entropy Control of Dissolution
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Gas Mixtures
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Liquid-Liquid Mixtures
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Predicting Miscibility of Two Liquids
General rules: 1. polar solutes dissolve in polar solvents2. nonpolar solutes dissolve in nonpolar solvents
“Like dissolves Like”
Oil and water don’t mix: is oil polar or nonpolar?
For now, we are examining molecular compounds- not ionic compounds.
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Hydrophilic groups:
Hydrophobic groups:
–OH and –NH groups
Examples: CH3OH, NH3, H-O-O-H
Long hydrocarbon groups:
CH3CH2CH2CH2CH2CH2
Examples: CH3CH2CH2CH2CH3
CH3CH2CH2CH2Cl
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Which of these will not dissolve in water?
1. NH3
2. CH3CH3
3. CH3OH
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Which of these is least soluble in water?
1. CH3OH
2. CH3CH2CH2OH
3. CH3CH2CH2CH2OH
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Why don’t oil and water mix?
What controls Solubility:
Enthalpy (enthalpy of solution)
Negative if new forces are stronger than original forces
Entropy
Depends on the entropy change of both the water and the solute.
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Assume they mix fully
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Assume water retains its hydrogen bonds
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Which is it?
Experiments show mixing water and oil has:
H close to zero
S highly negative
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Control of Solubility of Ionic Compounds
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Control of Solubility of Ionic Compounds
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Enthalpy Control
Entropy Control
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Control of Solubility of Ionic Compounds
Enthalpy: lattice energies and hydration energies follow same trends and mostly offset
Entropy: smaller ions and ions of high charge have larger hydration spheres and are entropy disfavored from dissolving
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Control of Solubility of Ionic Compounds
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External Control of Solubility
Pressure Effects: Solubility of Gases in Liquids
As P of a gas increases, its solubility increases.
Henry’s Law:
What is the solubility of O2 under a pressure of 0.66 atm?
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External Control of Solubility
Temperature Effects
In general,
• if Hdissolution is exothermic, solubility decreases as temperature increases
• if Hdissolution is exothermic, solubility decreases as temperature increases
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In general, but not always:
• if Hdissolution is exothermic, solubility decreases as temperature increases
• if Hdissolution is endothermic, solubility increases as temperature increases
What do we expect for ammonium nitrate?