Moore 5e_Ch13 Solutions 2022

Chapter 13: The Chemistry of Solutes and Solutions

  • Focus on the role of intermolecular forces on solubility and colligative behavior.

  • Explore energy considerations and observable properties related to solutions.

Solutions

  • Definition: A homogeneous mixture of two or more substances with uniform physical and chemical properties.

    • Components:

      • Solute: Dissolved species (e.g., salt in water).

      • Solvent: Dissolving medium (e.g., water).

  • Types of Solutions:

    • Gas in a gas (e.g., air)

    • Gas in a liquid (e.g., soda water)

    • Gas in a solid (e.g., H2 in palladium)

    • Liquid in a liquid (e.g., ethanol in water)

    • Solid in a liquid (e.g., NaCl in water)

    • Solid in a solid (e.g., brass, CuZn)

Energy Considerations in Solubility

  • Enthalpy Changes:

    • Exothermic Process: DH is negative.

    • Endothermic Process: DH is positive.

  • Gibbs-Helmholtz Equation:

    • DG = DH - TDS

    • Spontaneous processes occur when DG is negative at constant temperature (T) and pressure (P).

Dissolution and Precipitation

  • Dissolution Equation for Na2SO4:

    • Na2SO4(s) ⇌ 2 Na+(aq) + SO42–(aq)

    • Entropy (DS) increases during dissolution (+DS).

  • Precipitation example:

    • PbSO4(s) formed from Pb2+(aq) + SO42–(aq)

    • Entropy decreases for precipitation (-DS).

Heat of Solution

  • DHsolution: The sum of three types of interactions:

    • Solvent-solvent interactions

    • Solute-solute interactions

    • Solute-solvent interactions

Intermolecular Forces Affecting Solubility

  • Types of Intermolecular Forces:

    • London Forces

    • Dipole-dipole Forces

    • Hydrogen Bonding

  • Classifying Compounds: Understand interactions (e.g., ionic charge and size).

    • Ionic compounds often interact more strongly than molecular compounds.

Crystal Lattice Energy

  • Energy change associated with the formation of a crystal lattice from gaseous particles:

    • M+(g) + X-(g) ➔ MX(s) + Lattice Energy (DH is negative)

    • Factors:

      • Smaller cations have larger lattice energies due to stronger forces.

Solvation and Hydration

  • Hydration: When water is the solvent, solvation is often a very exothermic process for ionic or polar covalent compounds.

  • Hydration Enthalpy Chart:

    • Demonstrates different enthalpy changes for various anions.

Solubility Rules and Trends

  • Like-dissolves-like: Solutions are likely to form when solute and solvent have similar intermolecular forces.

    • Polar molecules dissolve in polar solvents (water).

    • Nonpolar compounds dissolve in nonpolar solvents.

  • Influence of Temperature on Solubility: Generally, solubility increases with temperature for solids in liquids but can vary for gases.

Henry's Law and Gas Solubility

  • Henry's Law: Relationship between gas concentration in solution and partial pressure:

    • Pgas = k * Cgas

    • Effect of Pressure: Increased partial pressure of a gas increases its solubility in liquids.

Colligative Properties

  • Properties dependent on the number of solute particles:

    • Vapor Pressure Lowering

    • Boiling Point Elevation

    • Freezing Point Depression

    • Osmotic Pressure

Calculating Colligative Properties

  • Molarity (M): moles of solute/liter of solution

  • Molality (m): moles of solute/kg of solvent

  • Calculating Apparent Molality:

    • mapparent = i * mcalculated where i accounts for particle dissociation (for electrolytes).

Changes in Physical Properties

  • Vapor Pressure: A nonvolatile solute lowers the vapor pressure of the solution compared to the pure solvent.

  • Raoult's Law:

    • Psolution = Xsolvent * Posolvent

    • Vapor pressure changes due to solute concentration.

Applications and Examples

  • Calculations: Use examples for calculating freezing point, boiling point, and osmotic pressure.

  • Discussion: Differences in molecular weights from freezing point depression experiments in different solvents.

Summary Points

  • Review of key concepts:

    • Intermolecular forces, enthalpy changes, dissociation in solutions, colligative properties, and their calculations.

  • Importance of understanding the molecular-level interactions that govern solubility and solution behavior.