2. Formation of a Liquid Solution
Separating the solute into its individual components (expanding the solute).
Overcoming intermolecular forces in the solvent to make room for the solute (expanding the solvent).
Allowing the solute and solvent to interact to form the solution.
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3. Steps in the Dissolving Process
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4. Steps in the Dissolving Process
Steps 1 and 2 require energy, since forces must be overcome to expand the solute and solvent.
Step 3 usually releases energy.
Steps 1 and 2 are endothermic, and step 3 is often exothermic.
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5. Enthalpy (Heat) of Solution
Enthalpy change associated with the formation of the solution is the sum of the ΔH values for the steps:
ΔHsoln = ΔH1 + ΔH2 + ΔH3
ΔHsoln may have a positive sign (energy absorbed) or a negative sign (energy released).
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6. Enthalpy (Heat) of Solution
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7. CONCEPT CHECK!
Explain why water and oil (a long chain hydrocarbon) do not mix. In your explanation, be sure to address how ΔH plays a role.
Oil is a mixture of nonpolar molecules that interact through London dispersion forces, which depend on molecule size. ΔH1 will be relatively large for the large oil molecules. The term ΔH3 will be small, since interactions between the nonpolar solute molecules and the polar water molecules will be negligible. However, ΔH2 will be large and positive because it takes considerable energy to overcome the hydrogen bonding forces among the water molecules to expand the solvent. Thus ΔHsoln will be large and positive because of the ΔH1 and ΔH2 terms. Since a large amount of energy would have to be expended to form an oil-water solution, this process does not occur to any appreciable extent.
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8. The Energy Terms for Various Types of Solutes and Solvents
ΔHsoln
Outcome
Polar solute, polar solvent
Large
Large, negative
Small
Solution forms
Nonpolar solute, polar solvent
Large, positive
No solution forms
Nonpolar solute, nonpolar solvent
Polar solute, nonpolar solvent
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9. In General
One factor that favors a process is an increase in probability of the state when the solute and solvent are mixed.
Processes that require large amounts of energy tend not to occur.
Overall, remember that “like dissolves like”.
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10. Affecting aqueous solutions
Structure Effects:
Polarity
Pressure Effects:
Henry’s law
Temperature Effects:
Affecting aqueous solutions
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11. Structure Effects Hydrophobic (water fearing) Non-polar substances
Hydrophilic (water loving)
Polar substances
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12. Pressure Effects C = concentration of dissolved gas k = constant
Little effect on solubility of solids or liquids
Henry’s law: C = kP
C = concentration of dissolved gas
k = constant
P = partial pressure of gas solute above the solution
Amount of gas dissolved in a solution is directly proportional to the pressure of the gas above the solution.
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13. A Gaseous Solute
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14. Temperature Effects (for Aqueous Solutions)
Although the solubility of most solids in water increases with temperature, the solubilities of some substances decrease with increasing temperature.
Predicting temperature dependence of solubility is very difficult.
Solubility of a gas in solvent typically decreases with increasing temperature.
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15. The Solubilities of Several Solids as a Function of Temperature
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16. The Solubilities of Several Gases in Water
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