Chapter 13 Lecture Notes - CHEM 1113 Broering

Chemistry: An Atoms-Focused Approach

Chapter 13: Solutions

The Solution Process

• Solution: A homogeneous mixture of two or more substances.

  • Can involve any two phases of matter.

• Components of a Solution:

  • Solute: The substance that is dissolved (smaller quantity).

  • Solvent: The substance in which the solute is dissolved (present in the largest amount).

• Solubility: The measure of how much solute can dissolve in a given volume of solvent.

Solubility Rules: "Likes Dissolves Like"

• Substances tend to dissolve in one another if the solute and solvent can form intermolecular attractions.

• Predict solubility by examining:

  • Strength and number of interactions among solute particles.

  • Interactions of solvent molecules.

• Energy dynamics:

  • Energy is released when forming solute-solvent attractions; this balances the energy needed to overcome solute-solute and solvent-solvent attractions.

• Example: Most ionic compounds dissolve in water due to ion-dipole attractions between charged ions and polar water molecules.

Interaction Types

• Water:

  • Forms various interactions: Dispersion forces, dipole-dipole interactions, and hydrogen bonding.

• Example: Glucose is a covalent molecule that dissolves in water via its interactions but does not dissociate into ions; thus, predictions of solubility rely on polar interactions.

Competing Intermolecular Forces

• Polarity and Solubility:

  • Molecules with both polar and nonpolar groups require analysis of interaction strength.

  • If nonpolar groups outnumber polar groups, solubility decreases (like dissolves like).

• Hydrophilic vs. Hydrophobic:

  • Hydrophilic: Water-attracting substances that increase solubility in water.

  • Hydrophobic: Water-repelling substances that decrease solubility in water.

• Miscibility: Two liquids that are infinitely soluble in each other (e.g., water and ammonia are miscible; water and gasoline are immiscible).

Solubility of Gases

• For gases, solubility decreases as temperature increases.

  • Higher temperatures provide gas molecules with sufficient kinetic energy to escape into the gas phase.

• Example: Carbon dioxide is dissolved in soft drinks at high pressure, becoming less soluble when the can is opened.

Concentration Units

• Percent by Mass:

  • Formula: Percent by mass = (mass of solute / total mass of solution) × 100%

  • Example: For a 285 g solution with 85.0 g solute, percent by mass can be calculated.

• Molality (m):

  • Alternative to molarity/

  • Formula: Molality = (moles of solute / kilograms of solvent).

Concentration Calculations

• Example: Calculation of molality when dissolving 29.5 g NaCl in 212.7 g H2O involves:

  1. Moles of solute.

  2. Kilograms of solvent.

  3. Final division to find molality.

Mole Fraction

• Mole Fraction (X): Number of moles of component divided by the total number of moles of all components.

  • Formula: X_A = (moles of A) / (moles of A + moles of B)

• Sum of mole fractions in a solution is always equal to 1.

Colligative Properties

• Definition: Properties that depend on the concentration of dissolved particles, not their identity.

• Examples include:

  1. Vapor-pressure lowering.

  2. Freezing-point depression.

  3. Boiling point elevation.

  4. Osmotic pressure.

Vapor-Pressure Lowering

• Solute particles disrupt the vaporization of the liquid, leading to a lower vapor pressure compared to the pure solvent.

Raoult’s Law

• States that the vapor pressure of a solvent in a solution can be calculated using:

  • P_solution = X_solvent * P_pure_sovent

Freezing-Point Depression & Boiling-Point Elevation

• The addition of solutes changes the freezing and boiling points, interfering with the alignment of molecules needed for freezing and allowing higher temperatures for boiling.

• Equation for Freezing-Point Depression:

  • ΔT_f = K_f * m * i

• Equation for Boiling-Point Elevation:

  • ΔT_b = K_b * m * i

Electrolytes vs. Non-Electrolytes

• Nonelectrolytes dissociate in solution to produce one mole of particles, while electrolytes dissociate into multiple ions, affecting colligative properties more significantly.

  • Example: 1 mole of NaCl produces 2 moles of ions.

Osmosis

• Definition: Movement of solvent through a semipermeable membrane from lower to higher solute concentration.

• Osmotic Pressure: Pressure applied to stop solvent movement.

  • Derived from the ideal gas law, involves calculating pressure based on concentration and temperature.

Chemistry: An Atoms-Focused Approach

Chapter 13: Solutions

• Solution: A homogeneous mixture of two or more substances (solute dissolves in solvent).

• Components:

  • Solute: Dissolved substance (smaller quantity).

  • Solvent: Substance that dissolves the solute (largest amount).

  • Solubility: Measure of how much solute dissolves in a solvent.

• Solubility Rules: "Likes Dissolves Like"; substances dissolve if intermolecular attractions are formed.

• Water Interactions: Involves dispersion forces, dipole-dipole interactions, and hydrogen bonding. Example: glucose dissolves in water without dissociating into ions.

• Polarity and Solubility: Molecules with both polar and nonpolar groups' solubility depends on interaction strength.

  • Hydrophilic: Water-attracting, increases solubility.

  • Hydrophobic: Water-repelling, decreases solubility.

• Gas Solubility: Decreases as temperature increases due to gas molecules escaping.

• Concentration Units:

  • Percent by Mass: (mass of solute / total mass of solution) × 100%.

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

• Colligative Properties: Dependent on concentration of dissolved particles, including vapor-pressure lowering, freezing-point depression, boiling-point elevation, and osmotic pressure.

  • Raoult’s Law: P_solution = X_solvent * P_pure_solvent.

  • Freezing/Boiling Calculations: ΔT_f = K_f * m * i and ΔT_b = K_b * m * i.

  • Electrolytes vs. Non-Electrolytes: Electrolytes dissociate into multiple ions, affecting properties more significantly.

• Osmosis: Movement of solvent through a semipermeable membrane from lower to higher solute concentration, involving osmotic pressure calculations.