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:
Moles of solute.
Kilograms of solvent.
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:
Vapor-pressure lowering.
Freezing-point depression.
Boiling point elevation.
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.