Chapter 12: Solutions and Their Properties

Chapter 12: Solutions and Their Properties

Chapter Outline

• Types of Solutions and Solubility

• Energetics of Solution Formation

• Solution Equilibrium and Factors Affecting Solubility

• Expressing Solution Concentration

• Colligative Properties: Vapour Pressure Lowering, Freezing Point Depression, Boiling Point Elevation, and Osmotic Pressure

• Properties of Strong Electrolyte Solutions

Types of Solutions and Solubility

Definition and Components of Solutions

A solution is a homogenous mixture that contains two components:

• Solvent: The component present in the larger amount.

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

Phases of Solutes and Solvents

Solutions can exist in various combinations of solute and solvent phases:

Solute Phase	Solvent Phase	Example
Gas	Gas	Air (mainly oxygen and nitrogen)
Gas	Liquid	Club soda (CO2 and water)
Liquid	Liquid	Vodka (ethanol and water)
Solid	Liquid	Seawater (salt and water)
Solid	Solid	Brass (copper and zinc) and other alloys

Factors Affecting Solubility

• Solubility depends on intermolecular forces, temperature, and entropy.

• Attractive forces between solute and solvent cause the formation of a homogenous solution.

• Solubility of most solids increases with increasing temperature due to increased energy dispersal (entropy).

Types of Intermolecular Forces in Solution Formation

• Dispersion forces (e.g., hexane)

• Dipole-dipole forces (e.g., acetone)

• Hydrogen bonds (e.g., ethanol, water)

• Ion-dipole forces (e.g., aqueous Na+)

Relative Strengths of Interactions

Strength of Solute-Solvent Interactions	Outcome
Solute-solvent > Solvent-solvent and solute-solute	Solution forms
Solute-solvent = Solvent-solvent and solute-solute	Solution forms
Solute-solvent < Solvent-solvent and solute-solute	Solution may or may not form, depending on relative strengths

Energetics of Solution Formation

Steps in Solution Formation

Three main steps are involved in forming a solution:

1. Breaking solvent-solvent intermolecular forces (endothermic)

2. Breaking solute-solute intermolecular forces (endothermic)

3. Forming solute-solvent intermolecular forces (exothermic)

Enthalpy of Solution ($\Delta H_{\text{sol}}$)

The overall enthalpy change for solution formation is the sum of the three steps:

• $\Delta H_{\text{sol}} = \Delta H_{\text{solvent}} + \Delta H_{\text{solute}} + \Delta H_{\text{mix}}$

• If $\Delta H_{\text{sol}} > 0$, the process is endothermic (absorbs energy).

• If $\Delta H_{\text{sol}} < 0$, the process is exothermic (releases energy).

Example: Ion-Dipole Forces

When ionic compounds dissolve in water, strong ion-dipole forces form, which are generally stronger than the forces in pure water.

Example: Dissolving NaCl in water forms Na+(aq) and Cl-(aq) ions surrounded by water molecules.

Additional info:

• Entropy increases as solutions form, favoring mixing due to greater energy dispersal.

• Whether a solution forms depends on both enthalpy and entropy changes.