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Solubility and Its Dependence on Pressure and Temperature

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Solubility and Solution Formation

Exothermicity and Solution Formation

Solution formation does not require exothermicity. Both exothermic and endothermic processes can result in spontaneous solution formation, with entropy playing a crucial role in driving endothermic dissolutions.

  • Exothermic process: Releases energy; often but not always leads to solution formation.

  • Endothermic process: Absorbs energy; can still form solutions if the increase in entropy compensates for the energy input.

  • Entropy: The missing component in understanding spontaneous solution formation; quantifies the disorder or randomness in a system.

Saturation and Types of Solutions

Saturated, Unsaturated, and Supersaturated Solutions

A saturated solution is in dynamic equilibrium with undissolved solute, meaning the rate of dissolution equals the rate of crystallization. The solubility of a solute is the maximum amount that can dissolve in a specific quantity of solvent at a given temperature.

  • Unsaturated solution: Contains less solute than the solubility limit.

  • Supersaturated solution: Contains more solute than the solubility limit; unstable and can precipitate excess solute.

  • Example: The solubility of NaCl in H2O at 25°C is approximately 36 g per 100 mL water.

Solubility as an Equilibrium Process

Dissolution and Crystallization

Solubility is governed by the equilibrium between dissolution and crystallization:

  • Dissolution: Solute particles enter the solution.

  • Crystallization: Solute particles leave the solution and form solid.

  • At equilibrium: Rate of dissolution = Rate of crystallization

  • Equation:

Factors Affecting Solubility

"Like Dissolves Like" Principle

The solubility of substances depends on the nature of solute and solvent:

  • Polar molecules: Soluble in polar solvents due to dipole-dipole forces (e.g., ethanol in water).

  • Ionic solids: Soluble in polar solvents due to ion-dipole interactions (e.g., NaCl in water).

  • Nonpolar molecules: Soluble in nonpolar solvents (e.g., I2 in CCl4).

Miscibility

Miscibility refers to the ability of two liquids to mix in all proportions, forming a homogeneous solution.

  • Miscible liquids: Mix completely (e.g., ethanol and water).

  • Immiscible liquids: Do not mix (e.g., hexane and water).

Solubility of Alcohols and Vitamins

Alcohols in Water and Hexane

The solubility of alcohols in water decreases as the hydrocarbon chain length increases. Alcohols with short chains are miscible in water, while longer chains are more soluble in nonpolar solvents.

Alcohol

Solubility in H2O

Solubility in C6H14

CH3OH (methanol)

∞

0.12

CH3CH2OH (ethanol)

∞

∞

CH3CH2CH2OH (propanol)

∞

∞

CH3CH2CH2CH2OH (butanol)

0.11

∞

CH3CH2CH2CH2CH2OH (pentanol)

0.030

∞

CH3CH2CH2CH2CH2CH2OH (hexanol)

0.0058

∞

Vitamin Solubility

  • Vitamins A, D, E, K: Nonpolar, soluble in nonpolar solvents and fatty tissue.

  • Vitamins B, C: Polar, soluble in water.

Gas Solubility and Pressure

Henry's Law

Henry's Law describes the relationship between the solubility of a gas and its partial pressure above the solution:

  • Equation:

  • = solubility of the gas

  • = Henry's Law constant (depends on gas and solvent)

  • = partial pressure of the gas

  • Higher pressure increases gas solubility due to more frequent collisions with the liquid surface.

Examples of Henry's Law Constants

Gas

k (mol/L·atm)

CO2

3.4 × 10-2

O2

1.3 × 10-3

N2

6.1 × 10-4

He

3.7 × 10-4

Practice Problem: Gas Solubility

At 20°C and 1 atm, the concentration of dissolved O2 in water is lower than that of N2 due to their respective Henry's Law constants.

Temperature Dependence of Solubility

Solids and Gases

  • Solids: Solubility generally increases with temperature due to increased entropy.

  • Gases: Solubility decreases with temperature because higher kinetic energy allows gas molecules to escape from solution.

  • Entropy: Both trends are related to the entropy change during solution formation.

Graphical Trends

  • Solubility of salts (e.g., KNO3, NaCl) increases with temperature.

  • Solubility of gases (e.g., O2, CO2) decreases with temperature.

Crystallization and Supersaturation

Slow Changes in Solubility

  • Supersaturated solutions can crystallize upon disturbance, returning to the saturation point.

  • Crystallization is a key process in purification and precipitation reactions.

Summary of Key Concepts

  • Saturation: Solutions are maximally concentrated at given temperature and pressure.

  • Gas solubility and pressure: Higher pressure increases gas solubility (Henry's Law).

  • Solubility and temperature: Most solids become more soluble at higher temperatures; most gases become less soluble.

  • Both pressure and temperature effects are related to the entropy of solution formation.

Additional info:

  • Dynamic equilibrium in saturated solutions is a foundational concept for understanding chemical equilibria in aqueous systems.

  • Henry's Law is essential for environmental chemistry, physiology, and industrial applications involving gases in liquids.

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