8.3 & 8.4 Factors that Affect Solubility and Rate of Dissolving

Learning Goals

  • Identify factors affecting the RATE of dissolving

  • Identify factors affecting the AMOUNT of dissolving (solubility)

  • Use a solubility curve for solving solubility problems

Collision Theory

  • Chemical reactions require collisions with proper orientation and sufficient kinetic energy to break bonds and form new bonds.

Dissolving

  • A solute dissolves when its particles become surrounded by solvent molecules.

  • Dilute Solution: Small amount of solute in solvent.

  • Concentrated Solution: Large amount of solute in solvent.

Dissolving Ionic Compounds

  • Ionic compounds dissolve when the attraction between water and ions (ion-dipole interactions) is stronger than the intermolecular forces holding the solid compound together.

Dissolving Molecular Compounds

  • Polar compounds dissolve when the attraction between water and the polar compound (dipole-dipole attraction) is stronger than the forces between solid polar molecules.

Factors Affecting the Rate of Dissolving

  1. Temperature

    • Higher temperature increases kinetic energy, facilitating more frequent collisions with solute.

    • Example: Salt dissolves faster in hot water compared to room temperature.

  2. Agitation (Stirring/Shaking)

    • Increases solute exposure to solvent by spreading out particles.

  3. Particle Size (Surface Area)

    • Smaller particles expose more surface area, increasing dissolution rate.

Solubility

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

Factors Affecting Solubility

  1. Type of Solute

    • Different solutes exhibit varying solubilities in the same solvent.

    • Example: Salt is more soluble in water than baking soda.

  2. Temperature

    • Increasing temperature generally enhances solubility by allowing more energy to pull solute apart.

Ionic Compound Solubility

  • Ion Size: Larger ions are easier to separate due to weaker attraction; larger ions tend to be more soluble.

  • Ion Charge: Greater charges create stronger attractions, making solubility lower for higher charged ions.

    • Example: KCl is more soluble than MgCl2.

Molecular Compound Solubility

  • Molecule Size: Smaller molecules are generally more soluble than larger ones.

    • Example: Ethanol (C2H5OH) is more soluble than butanol (C4H9OH).

Solubility in Water

  • Water dissolves:

    • Ionic compounds > Polar compounds > Non-polar compounds.

  • Like Dissolves Like: Polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents.

Ranking Solubility Example

  • Rank from least soluble to most soluble:

    • Mg(OH)2 < HCl < C2H6 < CH4 < LiOH < NaOH.

Solubility Curves

  • Graphs showing how much solute dissolves in 100g of water across different temperatures.

  • Points on the curve indicate:

    • Saturated Solutions: On the line.

    • Supersaturated Solutions: Above the line.

    • Unsaturated Solutions: Below the line.

Practical Applications of Solubility Curves

  • Example questions using the solubility curve to determine the mass of solute that will dissolve at given temperatures.

Review Questions

  • Identify factors that affect the RATE of dissolving.

  • Identify factors that affect the AMOUNT of dissolving (solubility).

  • Utilize a solubility curve to answer solubility problems.