Chapter 11. Solutions
Recommended Article : 【Chemistry】 Chemistry Table of Contents
1. Classification of Substances
2. Overview
3. Solubility
4. General Properties of Solutions
5. Colloids
## 1. Classification of Substances
⑴ Pure Substances : Classified into elements and compounds
⑵ Mixtures : Classified into homogeneous mixtures and heterogeneous mixtures
⑶ Elements
① Substances composed of only one element
② Example : Copper (Cu), Nitrogen (N2), Iron (Fe), Diamond (C), Aluminum (Al)
⑷ Compounds
① Substances composed of two or more different elements in a fixed ratio
② Example : Carbon Dioxide (CO2), Copper Sulfate (CuSO4), Water (H2O)
⑸ Homogeneous Mixtures (Solutions)
① Mixtures where two or more pure substances are uniformly mixed, and the composition is the same throughout
② Example : Air, Sugar Solution
⑹ Heterogeneous Mixtures
① Mixtures where two or more pure substances are unevenly mixed, and the composition varies depending on the portion taken
② Example : Muddy Water, Milk
2. Overview
⑴ Formation of Solutions
① Solvent : Dissolving substance
○ Example : Water in Saltwater
○ Example : Component with a larger quantity in Ethanol solution
② Solute : Dissolved substance
○ Example : Salt in Saltwater
○ Example : Component with a smaller quantity in Ethanol solution
③ Solvation : The phenomenon where solute dissolves in solvent
○ Solvent surrounds solute particles during solution formation
○ When water is the solvent, it’s called hydration
⑵ Concentration of Solutions
① Mass%, Volume%
② Molarity (M) : Moles of solute per liter of solvent
③ Molality (m) : Moles of solute per kilogram of solvent
④ Mole Fraction (x) : Ratio of the number of particles of solvent to that of solute
⑤ ppm, ppb
⑶ Types of Solutions
① Unsaturated Solution : A solution that can dissolve more solute
② Saturated Solution : A solution with the maximum amount of solute dissolved
③ Supersaturated Solution : A solution containing more solute than a saturated solution, causing precipitation of solute
⑷ Electrolytes and Non-Electrolytes
① Electrolyte : A substance that conducts electricity when dissolved in water
○ Electrolytes are composed of particles with opposite charges that dissociate in solution
○ Example : Salt, Copper(II) Sulfate
② Non-Electrolyte : A substance that does not conduct electricity when dissolved in water
○ Example : Distilled Water, Ethanol, Sugar Solution
③ Strong Electrolyte : A substance that ionizes extensively in solution
○ High ion concentration characterizes strong electrolytes
○ Example : Ionic compounds (NaCl), Strong acids (HCl), Strong bases (NaOH), etc.
④ Weak Electrolyte : A substance that ionizes to a lesser extent in solution
○ Low ion concentration characterizes weak electrolytes
○ Example : Weak acids (CH3COOH), Weak bases (NH4OH), etc.
3. Solubility
⑴ Basic Principles of Solubility
① Enthalpy of Solution
○ ΔHsolution = ΔHlattice + ΔHhydration : M+(g) + N-(g) → M+(aq) + N-(aq)
○ ΔHlattice, lattice (< 0) : M+(g) + N-(g) → MN(s)
○ Smaller metal ion radii lead to greater absolute values of lattice enthalpy
○ ΔHhydration, hydration (0 ± ) : MN(s) → M+(aq) + N-(aq)
② Hydration : Surrounding of solute particles by water molecules in a solution
○ Ionic solutes : Smaller ions and higher charges lead to stronger hydration
○ Hydration leads to ordered water molecules, resulting in a decrease in solvent entropy
○ Hydration disrupts the regular structure of solute particles, leading to an increase in solute entropy
○ Hydration is an entropy-increasing process
○ Entropy change upon evaporation of pure solvent = Gas entropy - Solvent entropy > Gas entropy - Solution entropy = Solution’s evaporation entropy
⑵ Temperature and Solubility
① Solubility of Solids : Increases with temperature, endothermic reaction (ΔHsolution > 0)
② Solubility of Gases (applies to some solids as well) : Decreases with temperature, exothermic reaction (ΔHsolution < 0)
③ Henry’s Law
○ **Formula: **Concentration (C) in solution = Henry’s constant (K) × Partial pressure (P) of gas
○ Gas solubility is directly proportional to the partial pressure of the gas
○ Applicable mostly to gases with low solubility
○ Derivation of Henry’s Law
⑶ Polarity and Solubility : “Like dissolves like”
① Polar solvents dissolve polar solutes effectively
② Nonpolar solvents dissolve nonpolar solutes effectively
4. General Properties of Solutions
⑴ Definition
① Properties related to the number of particles of solute, regardless of the type of solute
② Under the assumption of ideal solutions, entropy is the underlying factor for all general properties
⑵ Van’t Hoff Factor : denoted as i
① Definition
② Example : When NaCl(s) is dissolved in water, it completely dissociates into Na+(aq) and Cl-(aq), so the Van’t Hoff factor is 2
③ Real Solutions : As concentration becomes higher and the ion charges are larger, more ion pairs form in the solution, causing i to decrease
④ Ionization Degree and Van’t Hoff Factor
⑶ Lowering of Vapor Pressure
① Raoult’s Law
○ Content 1. In equilibrium, the partial pressure of each component is proportional to the mole fraction of the component in the liquid mixture
○ Content 2. The volume of the mixture is equal to the sum of the volumes of each component before mixing
○ Content 3. Intermolecular interactions in the mixture are the same as those between pure components
○ Relevant equation
② When both the solvent and solute form vapor pressure
③ Ideal Solutions (↔ Non-ideal Solutions) : Solutions that satisfy Raoult’s Law
Figure 1. Raoult’s Law and Positive Deviations, Negative Deviations [Footnote:1]
○ Solution with P = Super-saturated Solution with P: ΔH Solubilization = 0
○ Solution with P < Super-saturated Solution with P: Solvent dislikes vaporization, vapor pressure decreases ⇔ Strong solute-solvent interaction ⇔ ΔH Solubilization < 0
○ Solution with P > Super-saturated Solution with P: Solvent tends to vaporize, vapor pressure increases ⇔ Weak solute-solvent interaction ⇔ ΔH Solubilization > 0
④ Fractional Distillation of Ideal Solutions: Ideal solutions have different compositions in liquid and gas phases → Repetitive vaporization and condensation allow for the extraction of pure components
⑷ Boiling Point Elevation: 1st approximation, valid for dilute solutions and small temperature changes
⑸ Freezing Point Depression: 1st approximation, valid for dilute solutions and small temperature changes
① Molal Depression Constant Kf is a property of the solvent, not the solute
⑹ Osmotic Phenomenon (osmosis)
① Osmosis
○ Phenomenon where solvent molecules move due to differences in concentration between two solutions separated by a semipermeable membrane (diffusion of free molecules)
○ Osmotic Pressure: Pressure needed to prevent osmosis. Formulated using van ‘t Hoff’s law
π = CRT × i
○ Similarity of osmotic pressure formula to ideal gas equation: Due to the dilute nature of solute molecules in solution, they behave like an ideal gas
② Reverse Osmosis
○ When pressure greater than osmotic pressure is applied, water moves from high concentration to low concentration
○ Theoretical pressure needed in reverse osmosis = Pressure needed to establish equilibrium when some water is removed and osmotic pressure increases
○ Example
5. Colloids
⑴ Overview
① Definition: Particles ranging from 1 μm to 1,000 μm dispersed in a gas or liquid medium
② Particles dispersed in a gas are called aerosols
⑵ Properties: Particle size
① Tyndall Phenomenon
○ Definition: The path of light becomes visible due to particles within the colloid
○ Rayleigh Scattering and Tyndall Phenomenon both scatter specific wavelengths, but conditions and paths of scattering differ
○ No accurate mathematical formula describing Tyndall Phenomenon exists to date
② Dialysis: Diffusion of substances through a semipermeable membrane
○ Dialysis solution concentration > Solution concentration: Substance moves from dialysis solution to solution
○ Dialysis solution concentration = Solution concentration: No substance movement
○ Dialysis solution concentration < Solution concentration: Substance moves from solution to dialysis solution
○ Renal hemodialysis is a representative application
③ Adsorption
④ Brownian Motion: Random motion of colloidal particles in a liquid medium
○ Einstein analyzed it mathematically and received the Nobel Prize
○ Application: Dynamic Light Scattering (DLS) measures scattered light reflecting Brownian motion to determine particle size
○ Draws 1st time-intensity plot
Figure. 2. DLS setup and intensity-time plot
○ Compares two intensity-time plots using cross-correlation to draw delayed time-correlation function plot
Figure. 3. Delayed-correlation function plot
○ Analyzes exponential decay curve to determine translational diffusion coefficient Dt
○ Calculates hydrodynamic diameter Dh according to Stokes-Einstein law
⑶ Properties: Charge
② Zeta Potential
Figure. 4. Concept of Zeta Potential]
Figure. 5. Zeta potential patterns based on particle type
○ Background theory
○ Oppositely charged particles gather around the charged particle to form a primary shell
○ Stern Layer: Primary shell
○ Polar particles gather around the Stern Layer to form a secondary shell
○ Stern Layer moves with the particle
○ Double Layer: Secondary shell, also known as DEL (Double Electrode Layer)
○ Secondary shell can be of the same or different polarity as the initial particle
○ Movement influenced by solvent more than particles
○ Zeta Potential
○ Definition: Potential at the surface of the double layer
○ Measurable unlike surface potential or stern potential
○ Zeta potential can be measured by observing the difference in particle movement speed when applying potential
○ Utility 1: Measures particle polarity
○ Utility 2: Reflects both charge state and particle dispersion
○ If particles have the same charge and large zeta potential, they won’t aggregate
○ Absolute value of zeta potential exceeding 30 mV prevents aggregation between particles
○ Due to the emergence of repulsive forces
③ Aggregation (Flocculation)
○ Colloidal particles aggregate due to electrostatic forces between particles, forming small clumps
○ Differentiated from coagulation by the precipitation of solute and solvent together
○ Schulze-Hardy (S-H) rule: Aggregation strength proportional to electrostatic forces between solutes. Proposed around 125 years ago
○ Factor 1: Ion concentration: Aggregation occurs faster with more positive and negative solutes
○ Factor 2: Higher charge of solute increases aggregation strength
④ Salting-out
Figure. 6. Salting-out phenomenon
○ Differentiated from flocculation by the precipitation of solute only
○ Low concentration salt: Solubility increases with added salt (salting-in) due to salt-induced alteration aiding water penetration
○ High concentration salt: Solubility decreases with added salt (salting-out) as salts reduce interaction between substance and water
○ Peak solubility values increase with higher target substance concentration
○ Initial means of purification, ammonium sulfate used extensively
○ Application: Used in making tofu by adding a coagulant (MgCl2)
Input: 2018.12.30 20:39