Solutions: Properties, Formation, and Factors Affecting Solubility

Solutions

Introduction to Solutions

A solution is a homogeneous mixture composed of two or more substances. The substance present in the largest amount is called the solvent, while the other substances are called solutes. In aqueous solutions, water acts as the solvent.

• Solute: The substance dissolved in the solvent (e.g., CO2 in soda pop).

• Solvent: The substance present in the greatest amount (e.g., H2O in soda pop).

• Solubility: The maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature.

• Miscible: Liquids that mix in all proportions (e.g., ethanol and water).

• Entropy: A measure of disorder or randomness; mixing increases entropy.

Solution Formation

Energetics of Solution Formation

The formation of a solution depends on the balance of enthalpy and entropy changes. The process can be divided into three conceptual steps:

1. ΔHsolute: Energy required to separate solute particles (endothermic).

2. ΔHsolvent: Energy required to separate solvent particles (endothermic).

3. ΔHmix: Energy released when solute and solvent particles interact (exothermic).

The overall enthalpy change for solution formation is:

$\Delta H_{solution} = \Delta H_{solute} + \Delta H_{solvent} + \Delta H_{mix}$

• If the overall process is exothermic or only slightly endothermic, and entropy increases, solution formation is favored.

Example: In soda pop, CO2 is the solute and H2O is the solvent. The dissolution involves overcoming intermolecular forces and is influenced by both enthalpy and entropy changes.

Hess's Law in Solution Formation

Hess's Law states that the total enthalpy change for a process is the sum of the enthalpy changes for each step, regardless of the pathway taken.

$\Delta H_{overall} = \sum \Delta H_{steps}$

This principle allows us to analyze the energetics of solution formation by considering the separate steps described above.

Solution Equilibrium and Types of Solutions

Dynamic Equilibrium

When a solute dissolves in a solvent, the process eventually reaches a dynamic equilibrium where the rate of dissolution equals the rate of crystallization.

• Saturated solution: Contains the maximum amount of dissolved solute at equilibrium.

• Unsaturated solution: Contains less solute than the equilibrium amount.

• Supersaturated solution: Contains more solute than can remain dissolved at equilibrium; unstable and can precipitate solute.

Recrystallization is a technique used to purify solids by dissolving them in hot solvent and then cooling the solution to crystallize the pure compound.

Factors Affecting Solubility

Temperature Dependence

• The solubility of most solids in water increases with increasing temperature.

• The solubility of gases in water decreases with increasing temperature.

• This is due to the nature of intermolecular forces and the energetics of dissolution.

• Lowering the temperature can cause a dissolved solid to crystallize out of solution (basis for recrystallization).

Example: Cold soda pop retains more dissolved CO2 than warm soda pop, which loses gas more rapidly.

Pressure Dependence (Henry's Law)

• The solubility of gases in water increases with increasing pressure above the solution.

• When pressure is reduced, the solubility decreases and the dissolved gas escapes (e.g., bubbles in soda when opened).

Henry's Law quantifies this relationship:

$S_{gas} = k_H P_{gas}$

• Sgas: Solubility of the gas

• Pgas: Partial pressure of the gas above the solution

• kH: Henry's law constant (varies by gas and temperature)

Gas	kH (M·atm−1)
O2	1.3 × 10−3
N2	6.1 × 10−4
CO2	3.4 × 10−2
NH3	5.8 × 10−1
He	3.7 × 10−4

Key Terminology

• Solution

• Solute

• Solvent

• Solubility

• Miscible

• Entropy

• Hess's Law

• ΔHsolution or ΔHsoln

• ΔHhydration

• Dynamic equilibrium

• Saturated/Unsaturated/Supersaturated solution

• Recrystallization

• Henry's Law

Summary Table: Factors Affecting Solubility

Factor	Effect on Solids	Effect on Gases
Temperature	Usually increases solubility	Decreases solubility
Pressure	No significant effect	Increases solubility (Henry's Law)

Additional info: These notes synthesize and expand upon the provided slides and handwritten notes, ensuring all key concepts from General Chemistry Chapter 14 (Solutions) are covered for exam preparation.