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Physical Properties of Solutions

Definition of a Solution

• Solution: A homogenous mixture of 2 or more substances.

  • Solute: The substance(s) present in the smaller amount(s).

  • Solvent: The substance present in the larger amount.

Types of Solutions

Saturated, Unsaturated, and Supersaturated Solutions

• Saturated Solution: Contains the maximum amount of solute that will dissolve in a given solvent at a specific temperature.

• Unsaturated Solution: Contains less solute than the solvent can dissolve at a specific temperature.

• Supersaturated Solution: Contains more solute than is present in a saturated solution at a specific temperature.

  • Example: Sodium acetate crystals form when a seed crystal is added to a supersaturated solution of sodium acetate.

Interactions in the Solution Process

Types of Interactions

• Solvent-Solvent Interaction

• Solute-Solute Interaction

• Solvent-Solute Interaction

Enthalpy of Solution

[ \Delta H_{soln} = \Delta H_1 + \Delta H_2 + \Delta H_3 ]

• If solvent-solute attractions are stronger than solute-solute and solvent-solvent:

• [ \Delta H_{soln} < 0 ] (favorable)

• If not, [ \Delta H_{soln} > 0 ] (not favorable)

Solvation and Miscibility

• Solvation: The process of surrounding solute particles with solvent molecules.

• Miscible: When two liquids are soluble in all proportions.

"Like Dissolves Like"

• Substances with similar intermolecular forces are likely to be soluble in each other.

  • Non-polar Solubility: Non-polar molecules are soluble in non-polar solvents (e.g., CCl4 in C6H6).

  • Polar Solubility: Polar molecules are soluble in polar solvents (e.g., C2H5OH in H2O).

  • Ionic Compounds: More soluble in polar solvents (e.g., NaCl in H2O).

Concentration Units

• Concentration: The amount of solute present in a designated quantity of solvent or solution.

Percent by Mass

• [ % \text{ by mass} = \frac{\text{mass of solute}}{\text{mass of solute} + \text{mass of solvent}} \times 100%

Mole Fraction

• [ X_A = \frac{\text{moles of A}}{\text{sum of moles of all components}} ]

Molarity and Molality

• Molarity (M): [ M = \frac{\text{moles of solute}}{\text{liters of solution}} ]

• Molality (m): [ m = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}} ]

Example Calculation

• For a 5.86 M ethanol (C2H5OH) solution with density 0.927 g/mL:

  • Calculate molality:

    • 927 g of solution corresponds to 1000 mL.

    • Mass of solvent = 927 g - 270 g = 657 g = 0.657 kg.

    • [ m = \frac{5.86 \text{ moles}}{0.657 \text{ kg}} = 8.92 \text{ m} ]

Temperature and Solubility

Effect of Temperature on Solubility of Solids

• Solubility generally increases with temperature for substances like KNO3 and NaNO3.

• Solubility generally decreases with temperature for substances like KBr and NaCl.

Fractional Crystallization

• Process: Separation of substances based on differing solubilities.

• Example: 90 g KNO3 mixed with 10 g NaCl dissolved in water at 60°C, results in:

  • NaCl remains in solution, while 78 g of pure KNO3 precipitates.

Temperature and Gas Solubility

• For gases, solubility usually decreases with increasing temperature.

Pressure and Gas Solubility

• Henry’s Law: The solubility of a gas in a liquid is proportional to the pressure of the gas above the solution.

  • [ c = kP ]

  • Where:

    • c = concentration (M) of the dissolved gas

    • P = pressure of the gas over the solution

    • k = constant for each gas (mol/L•atm).

Colligative Properties of Nonelectrolyte Solutions

• Definition: Properties depending on the number of solute particles rather than their identity.

Key Colligative Properties

• Vapor-Pressure Lowering: Using Raoult’s law:

  • [ P_1 = X_1 P^0_A ]

• Boiling-Point Elevation: [ \Delta T_b = K_b m ]

• Freezing-Point Depression: [ \Delta T_f = K_f m ]

• Osmotic Pressure: [ , = MRT ]

Colligative Properties of Electrolyte Solutions

• Electrolytes dissociate into ions, affecting colligative properties.

van't Hoff Factor (i)

• The ratio of the number of particles in a solution to the number of formula units initially dissolved.

Influence on Boiling-Point Elevation and Freezing-Point Depression

• Boiling-Point Elevation: [ \Delta T_b = i K_b m ]

• Freezing-Point Depression: [ \Delta T_f = i K_f m ]

• Osmotic Pressure: [ p = iMRT ]

Applications of Colligative Properties

Example: Freezing-Point Depression Calculation

• For a solution with 478 g of ethylene glycol in 3202 g of water:

  • Calculate molality, freezing-point depression.

Osmosis

• Concept: Selective passage of solvent molecules through a semipermeable membrane.

• Osmotic Pressure Calculation: [ p = MRT ]

Colloid vs. Solution

• Colloid: Dispersion of larger particles throughout another substance.

• Colloidal suspensions are less homogeneous than solutions.

Cleansing Action of Soap

• Soap molecules feature hydrophilic (water-attracting) and hydrophobic (water-repelling) properties.

Desalination Process

• Methods: Solar Still & Reverse Osmosis: Utilizes solar radiation or pressure to separate salt from seawater to obtain fresh water.