Chemistry: The Central Science - Chapter 13: Properties of Solutions
Properties of Solutions
Definition of Solutions
Solutions are homogeneous mixtures containing two or more pure substances.
In a solution, the solute is uniformly dispersed throughout the solvent.
The ability of different substances to form solutions relies on:
Natural tendency toward mixing.
Intermolecular forces.
Natural Tendency Toward Mixing
The mixing of gases occurs spontaneously.
Each gas behaves independently as it fills the container.
The randomness of molecules increases during mixing; this phenomenon is linked to a thermodynamic quantity known as entropy.
The formation of solutions is favored by the increase in entropy that accompanies mixing.
Intermolecular Forces of Attraction
Intermolecular forces can be the attraction between solute and solvent molecules.
Attractions Involved When Forming a Solution
Solute-Solute Interactions: Must be overcome to disperse solute particles when creating a solution.
Solvent-Solvent Interactions: Must also be overcome to create space for the solute.
Solvent-Solute Interactions: Occur as the particles mix, facilitating solution formation.
Solvation (Hydration)
Involves solvent-solute interactions allowing a solid to dissolve, illustrated with
Crystals of NaCl in water:
Hydrated Cl⁻ ion
Hydrated Na⁺ ion
Energetics of Solution Formation
For an endothermic reaction to occur, the process must be close to the sum of the individual disorders of the system (entropy will matter).
Exothermic solutions tend to be spontaneous, emphasizing energy release.
Aqueous Solution vs. Chemical Reaction
The disappearance of a substance in a solvent does not always indicate dissolution; the substance may react (e.g., nickel with hydrochloric acid).
Opposing Processes
The processes of solution formation and crystallization are oppositional.
A saturated solution exists when the rates of these opposing processes are equal; additional solute will not dissolve unless some crystallizes from the solution.
An unsaturated solution exists when the quantity of solute dissolved is less than the max solubility, preventing crystallization.
Solubility
Definition: Solubility is the maximum amount of solute that can dissolve in a certain quantity of solvent at a specified temperature.
Saturated solutions contain the maximum solute dissolved.
Unsaturated solutions contain less solute than the saturation point.
Supersaturated Solutions:
A rare type where the solvent holds more solute than normally possible at a specific temperature.
They are unstable and can crystallize by adding a “seed crystal” or through mechanical agitation.
Factors That Affect Solubility
Factors impacting solubility include:
Solute-solvent interactions.
Pressure (particularly for gaseous solutes).
Temperature variations.
Solute–Solvent Interactions
Fundamental principle: “Like dissolves like.”
Although this adage informs solubility behavior, it does not encompass all interactions.
Stronger solute-solvent interactions correlate with greater solubility of a solute in that solvent.
In Table 13.1, solubility for gases in water at 20°C at 1 atm shows the relationship between molecular mass of the gas and solubility:
Gas
Molar Mass (g/mol)
Solubility (M)
N₂
28.0
0.69 x 10⁻³
O₂
32.0
1.38 x 10⁻³
Ar
39.9
1.50 x 10⁻³
Kr
83.8
2.79 x 10⁻³
Organic Molecules in Water
Polar organic molecules tend to dissolve in water more effectively than their nonpolar counterparts.
Hydrogen bonding significantly boosts solubility because C–C and C–H bonds are poorly polar.
Liquid/Liquid Solubility
Miscible Liquids: Liquids that mix in all proportions.
Immiscible Liquids: Liquids that do not mix with each other.
Example: Hexane is nonpolar while water is polar, making them immiscible.
Solubility and Biological Importance
Fat-soluble vitamins (e.g., vitamin A) can be stored in body fats due to nonpolar characteristics.
Water-soluble vitamins (e.g., vitamin C) must be consumed regularly to meet dietary needs.
Pressure Effects
Solubility of solids and liquids is largely unaffected by changes in pressure.
Conversely, the solubility of gases is significantly influenced by pressure conditions.
Henry’s Law
States that the solubility of a gas is proportional to the partial pressure of the gas above the solution.
Temperature Effects
General trends include:
For many solids, increased temperature corresponds to increased solubility. However, this may vary; some solids exhibit little change or reduced solubility with temperature changes.
For gases, increased temperatures typically decrease solubility; thus, colder rivers have a higher dissolved oxygen content than warmer ones.
Solution Concentration
Discussion focuses on quantitative measures of solutions, moving beyond qualitative terms like saturated, unsaturated, and supersaturated.
Units of Concentration
The specific measures of concentration include:
Mass percentage.
Parts per million (ppm).
Parts per billion (ppb).
Mole fraction (χ).
Molarity (M).
Molality (m).
Mass Percentage
Defined as the ratio of the mass of solute to the total mass of the solution, multiplied by 100 to express as a percent.
Parts per Million (ppm)
Relates to mass of solute to the total solution mass, conventionally defined as:
PPM: total mass/1,000,000
Parts per Billion (ppb)
Similar to ppm, but on a much smaller scale, defined as:
PPB: total mass/1,000,000,000
Mole Fraction (χ)
Ratios of the moles of a substance to the total moles within a solution.
Applications can include both solutes and solvents.
Molarity (M) and Molality (m)
Molarity: Moles of solute per liter of solution (discussed within Chapter 4).
Molality: Moles of solute per kilogram of solvent.
Molarity vs. Molality
In solutions where water is the solvent, molarity and molality yield similar values for dilute solutions.
Molality remains consistent regardless of temperature, while molarity fluctuates with temperature changes due to volume alterations.
Converting Units
Conversion between molarity and molality requires application of dimensional analysis techniques, incorporating solution density as a crucial factor.
Colligative Properties
Colligative properties rely solely on the quantity of solute particles rather than their identity. Key properties include:
Vapor-pressure lowering.
Boiling-point elevation.
Freezing-point depression.
Osmotic pressure.
Vapor Pressure
Increased concentrations of nonvolatile solutes impede solvent escape into vapor, resulting in lower vapor pressures for solutions compared to pure solvents.
Raoult’s Law
Asserts that the vapor pressure of a volatile solvent over a solution equals the product of the mole fraction of the solvent and the vapor pressure of the pure solvent.
Ideal solutions assume adherence to Raoult’s Law for component mixtures.
Boiling-Point Elevation
Lower vapor pressures necessitate a temperature increase to achieve atmospheric pressure, subsequently raising the boiling point of the solution.
Freezing-Point Depression
Phase diagrams illustrate that the freezing point is depressed while the boiling point is elevated for solutions.
Boiling-Point Elevation and Freezing-Point Depression
The change in temperature correlates directly with molality, influenced by the van’t Hoff factor (number of particles a substance produces when it dissolves).
Table 13.3: Molal Boiling-Point-Elevation and Freezing-Point Depression Constants
Solvent
Normal Boiling Point (°C)
Kb (°C/m)
Normal Freezing Point (°C)
Kf (°C/m)
Water, H₂O
100.0
0.51
0.0
1.86
Benzene, C₆H₆
80.1
2.53
5.5
5.12
Ethanol, C₂H₅OH
78.4
1.22
-114.6
1.99
Carbon tetrachloride
76.8
5.02
-22.3
29.8
Chloroform, CHCl₃
61.2
3.63
-63.5
4.68
Osmosis
Refers to the net movement of solvent molecules from a solution with a lower solute concentration to one with a higher concentration across a semipermeable membrane, with the opposing pressure termed osmotic pressure.
Osmotic Pressure
A colligative property, osmotic pressure indicates that if two solutions separated by a semipermeable membrane share equal osmotic pressure, no osmosis occurs.
Types of Solutions and Osmosis
Isotonic solutions: Same osmotic pressure; solvent moves through the membrane at equal rates.
Hypotonic solution: Lower osmotic pressure; solvent exits at a higher rate than it enters.
Hypertonic solution: Higher osmotic pressure; solvent enters at a higher rate than it exits.
Osmosis and Blood Cells
Red blood cells possess semipermeable membranes:
In hypertonic solutions, water exits the cell, causing crenation (shriveling).
In hypotonic solutions, water enters the cell, leading to hemolysis (bursting).
Intravenous (IV) solutions must maintain an isotonic balance with blood to prevent cellular distress.
Colloids
Defined as suspensions of particles larger than separate ions or molecules, yet too small to settle by gravity.
Colloids serve as the boundary between solutions and suspensions.
Table 13.5: Types of Colloids
Phase of Colloid
Dispersing Substance
Dispersed Substance
Colloid Type
Example
Gas
Gas
Gas
—
None (all are solutions)
Gas
Liquid
Aerosol
Fog
Smoke
Liquid
Gas
Foam
Whipped cream
Liquid-Liquid
Liquid
Liquid
Emulsion
Milk
Liquid-Liquid
Solid
Liquid
Sol
Paint
Liquid-Liquid
Solid
Solid
Solid foam
Marshmallow
Solid-Solid
Solid
Solid
Solid emulsion
Butter
Solid-Solid
Solid
Solid
Solid sol
Ruby glass
Solid-Solid
Tyndall Effect
A phenomenon where colloidal suspensions can scatter light; solutions lack this ability. Examples illustrate the difference between solutions and colloids via visible scattering.
Colloids and Biomolecules
Many biomolecules possess both hydrophilic and hydrophobic characteristics; the hydrophilic part faces outward in water, promoting solubility.
Stabilizing Colloids by Adsorption
Ions can adhere to the surfaces of hydrophobic colloids, facilitating interaction with aqueous solutions, aiding in stabilization.
Colloids in Biological Systems
Colloids play a role in emulsifying fats and oils in aqueous solutions. Emulsifiers enable substances that typically resist dissolution in a solvent to disperse.
Brownian Motion
The agitation of colloids caused by multiple collisions with smaller solvent molecules contributes to their motion within the solution.
Table 13.6: Calculated Mean Free Path for Uncharged Colloidal Spheres in Water at 20°C
Radius of sphere (nm)
Mean Free Path (mm)
1
1.23
10
0.390
100
0.123
1000
0.039
Chapter 13 Homework
Homework problems to focus on:
#5, #7, #9, #15, #22, #25, #27, #29, #35, #39, #41, #45, #47, #51, #53 (a&c), #55, #73, #85, #90
Copyright Information
Copyright protection notices reiterate the proprietary nature of the materials provided. Reproduction or distribution is strictly prohibited without proper authorization.