9: Osmotic Pressure and Solvation Concepts

Lecture Announcements

• Today's Topics: Properties of Solutions (Brown Ch. 13)
• Specific Topics: The Solution Process, Saturated Solutions and Solubility, Factors Affecting Solubility, Expressing Solution Concentration, Colligative Properties
• Problem Sets: Problem Set 8 due tomorrow, upload on Moodle. Problem Set 9 posted on Moodle; due before Exercise #10 next week.
• Study Center: Wednesdays 6:00 PM - 8:00 PM in ETA F 5
• Office Hours: Prof. Norris and Brisby, Thursdays 5:00 PM - 6:00 PM in LEE P 210
• Resources: Five practice exams available.

Lecture 10

• Next Week's Topics: Chemical Kinetics (Brown Ch. 14)
• Specific Topics: Factors That Affect Reaction Rates, Reaction Rates, Concentration and Rate Laws, The Change of Concentration with Time

Red Thread

• Course Summary: A conceptual overview, connecting topics previously covered with new material.

• Progression: Intro → Gases → Solutions → Thermo → other chemistry topics like Catalysis, Acid-Base, Equilibrium, Batteries, etc.

• Overall Goal: Establish deeper understanding of how molecules interact and affect properties of substances

Review of Lecture 8

• Intermolecular Interactions and Liquids: Key concepts included Van der Waals interactions, molecular dipoles, hydrogen bonding, ion-dipole interactions, and polar solvents.
• Properties of Liquids: Viscosity, surface tension, vapor pressure, phase changes, heat of fusion, heat of vaporization, heat of sublimation, heating curves, phase diagrams, and the Clausius-Clapeyron equation.

Review (Focus on Intermolecular Forces)

• Hierarchy of Intermolecular Forces (strongest to weakest): Ionic bonding, Ion-dipole forces, hydrogen bonding, dipole-dipole forces, dispersion forces (van der Waals)
• Examples of compounds/materials for each type of intermolecular force . (e.g., NaCl, polar solvents, NH3, CH3OH, CH4, Br2)

Lecture 3: Properties of Solutions

• Definition of Solution: Solvent + Solute; homogenous mixture
• Solvation Process: Process of dissolving substances in water (or other solvents). Also known as hydration.
• Key Questions: When do substances dissolve? How much dissolves? What are the properties of solutions?

Thermodynamics of Solutions

• Definition: Change in Gibbs free energy to form a solution.
• Spontaneity: Solutions are spontaneous if the change in Gibbs free energy is less than 0.
• Relationship to Entropy and Enthalpy: ΔG = ΔH - TΔS. ΔSsoln is the change in entropy in mixing solute and solvent, typically positive. The change in enthalpy ΔHsoln, depends on the intermolecular interactions (solute-solute, solute-solvent, solvent-solvent).

Enthalpy Changes During Solvation

• Breaking Intermolecular Forces: Solvation involves breaking solute, solvent, and solute/solvent interactions.
• Steps of the process: Separate solvent, separate solute, mix separated solvent and solute.
• Exothermic vs. Endothermic: Enthalpy changes of solution formation (ΔH_soln) can be positive (endothermic).

Solubility

• Definition: Maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature.
• Factors: Solubility depends on a multitude of factors including: temperature, pressure, and the nature of the solvent and solute.
• Solid vs Liquid Solubility: Solutes can be solids or liquids, and solubility behavior can be different for each type.
• "Like Dissolves Like" (general rule): Polar substances tend to dissolve in polar solvents, and nonpolar substances tend to dissolve in nonpolar solvents

Other Factors Affecting Solubility

• Pressure: Solubility of gases increases with pressure (Henry's Law).
• Temperature: Solubility of many solids increases with temperature, whereas solubility of gases decreases with increasing temperature.
• Example: How dissolving carbon dioxide or other gases in water or other solvents is affected by temperature or pressure .

Expressions for Concentration

• Qualitative: Dilute, Concentrated
• Quantitative Units: Mole fraction (X), Mass %, Molarity (M), Molality (m), ppm, ppb.

How Does Solute Affect Properties of the Solvent (Colligative Properties)

• Boiling Point Elevation: Solute increases the boiling point of the solvent (measured using a change in temperature ΔT_b, dependent on molality 'm' of solute)
• Vapor Pressure Lowering: Solute lowers the vapor pressure of the solvent by Raoult's Law
• Freezing Point Depression: Solute decreases the freezing point of the solvent (measured using a change in temperature from the pure solvent ΔT_f).

Osmosis

• Semi-permeable membrane: Allows the passage of the solvent/solute (water) but not the solvent.
• Osmotic flow occurs when the solvent diffuses to equalize the concentration across a semi-permeable membrane.
• Osmotic pressure : Counteracting osmotic flow, measures the pressure required to prevent more solvent movement.

Ideal Gas vs Ideal Solutions

• Ideal Gas: No intermolecular forces.
• Ideal Solution: Intermolecular forces are identical in each component of the solution (solute, solvent).

Determining Molecular Weight

• Application of osmotic pressure (Π) to find the molar mass (MW) of a solute.

Antifreeze Proteins (AFPs)

• Mechanism of action: AFPs function differently than other antifreeze agents by blocking ice nucleation, not by lowering the freezing point.
• Non-colligative effect
• Kinetics-based mechanism is important

Summary of What We Learned

• Thermodynamics of Solutions (ΔG, ΔH, ΔS): Crucial for assessing the spontaneity and energy changes in mixing processes.
• Solubility Concepts: Various factors impacting solubility (polarity, T, P).
• Expressions of Concentration: Essential ways to quantify solute levels in solutions.
• Colligative Properties: Properties altering with concentration (BP elevation, VP lowering, FP depression, Osmosis).