General Chemistry Study Notes: Solutions, Solubility, and Concentration (Chem 166, Lecture 1)

Chapter 13: Solutions

Introduction to Solutions

Solutions are homogeneous mixtures composed of two or more substances, where the solute is uniformly distributed within the solvent. Understanding the nature of solutions is essential for studying chemical reactions, properties of matter, and laboratory techniques in chemistry.

• Solution: A homogeneous mixture of solute and solvent.

• Solute: The substance dissolved in the solvent (minor component).

• Solvent: The substance in which the solute is dissolved (major component).

• Solubility: The amount of solute that dissolves in a given amount of solvent at a specific temperature.

• Soluble: When a solute dissolves in a solvent (e.g., NaCl in water).

• Insoluble: When a solute does not dissolve in a solvent (e.g., oil in water).

Types of Solutions

Solutions can be classified based on the physical states of their components. The following table summarizes common types:

Type	Solute	Solvent	Example
Aqueous Solution	Gas, Liquid, Solid	Liquid (water)	Air, Soda, Sea Water
Gaseous Solution	Gas, Liquid, Solid	Gas	Air, Catalytic Converter
Solid Solution	Solid, Liquid, Gas	Solid	Alloys (Brass, Steel), Dental Amalgams

Intermolecular Forces and Solubility

The ability of a solute to dissolve in a solvent depends on the intermolecular forces (IMFs) between particles. Stronger IMFs between solute and solvent favor dissolution.

• Dispersion Forces: Present in all molecules, stronger in larger molecules.

• Dipole-Dipole Interactions: Occur between polar molecules.

• Hydrogen Bonding: Strong IMF involving H bonded to N, O, or F.

• Ion-Dipole Interactions: Occur when ionic compounds dissolve in polar solvents (e.g., NaCl in water).

Periodic Trends and Dipole Interactions

Dipole interactions depend on the electronegativity of atoms in a molecule. The periodic table can be used to predict polarity and the strength of IMFs.

Energetics of Solution Formation

Forming a solution involves breaking and forming IMFs, which affects the enthalpy change ($riangle H_{soln}$) of the process.

• Step 1: Break apart the solute ($riangle H_{solute} > 0$)

• Step 2: Break apart the solvent ($riangle H_{solvent} > 0$)

• Step 3: Form the solution ($riangle H_{mix} < 0$)

The overall enthalpy of solution formation:

$riangle H_{soln} = riangle H_{solute} + riangle H_{solvent} + riangle H_{mix}$

Exothermic process: $riangle H_{mix} > riangle H_{solute} + riangle H_{solvent}$ Endothermic process: $riangle H_{mix} < riangle H_{solute} + riangle H_{solvent}$

Solvation and Solutions of Ionic Compounds

When ionic compounds dissolve in water, ions become hydrated and are surrounded by water molecules. The formation of ion-dipole attractions is typically exothermic.

Solution Equilibrium & Solubility

The solubility of a solute is the maximum amount that dissolves in a fixed quantity of solvent at a given temperature. The process is governed by changes in enthalpy ($riangle H$) and entropy ($riangle S$):

For a reaction to take place: $riangle G = riangle H - T riangle S$

• Effect of temperature:

  • Solubility increases with temperature if the process is endothermic.

  • Solubility decreases with temperature if the process is exothermic.

Concentration Terms and Units

Definition of Concentration

Concentration is a measure of the amount of solute in a given quantity of solvent or solution.

$ext{Concentration} = rac{ ext{quantity of solute}}{ ext{quantity of solvent or solution (solute + solvent)}}$

Common Concentration Units

• Molarity (M): $M = rac{ ext{moles of solute}}{ ext{liters of solution}}$

• Molality (m): $m = rac{ ext{moles of solute}}{ ext{kg of solvent}}$

• Mole Fraction ($X_A$): $X_A = rac{ ext{moles of A}}{ ext{total moles}}$

• Mole Percent: $ext{mol ext{%}} = X_{solute} imes 100 ext{%}$

• Percent by Mass: $ext{ ext{%} by mass} = rac{ ext{mass of solute}}{ ext{mass of solution}} imes 100$

• Parts per Million (ppm): $ext{ppm} = rac{ ext{mass of solute}}{ ext{mass of solution}} imes 10^6$

• Parts per Billion (ppb): $ext{ppb} = rac{ ext{mass of solute}}{ ext{mass of solution}} imes 10^9$

Table: Solution Concentration Terms

Unit	Definition	Units
Molarity (M)	amount solute (in mol) / volume solution (in L)	mol/L
Molality (m)	amount solute (in mol) / mass solvent (in kg)	mol/kg
Mole fraction ($X_A$)	moles of A / total moles	None
Mole percent	amount solute (in mol) / total amount of solute and solvent (in mol) × 100%	%
Percent by mass	mass solute / mass solution × 100	%
Parts per million (ppm)	mass solute / mass solution × $10^6$	ppm
Parts per billion (ppb)	mass solute / mass solution × $10^9$	ppb

Concentration Term Conversion Examples

• Example 1: What is the molality of a 0.200 M solution of NaCl if its density is 1.15 g/mL? Solution: Convert volume to mass using density, then calculate molality using moles of solute and mass of solvent.

• Example 2: Environmental health application: Calculate if a soil sample exceeds EPA limits for lead using ppm by mass.

• Example 3: Given a mole fraction, calculate the mass percent of a component in a solution.

Intermolecular Forces and Solubility in Life

Solubility and Chemistry of Life

Water-soluble vitamins (e.g., Vitamin C, Folic Acid) contain polar groups and dissolve in water, while fat-soluble vitamins (e.g., Vitamin A, E) are stored in fatty tissues due to their nonpolar nature.

Understanding Dipole and H-Bond Interactions

• Dipole interactions: Depend on molecular polarity and electronegativity differences.

• Hydrogen bonds: Occur when H is bonded to highly electronegative atoms (N, O, F).

• Example: Fluoromethane (CH3F) can form H-bonds due to the presence of F.

Summary Table: Key Solution Concepts

Concept	Definition	Example/Application
Solution	Homogeneous mixture of solute and solvent	Salt water, air, alloys
Solubility	Maximum amount of solute that dissolves in solvent	NaCl in water
Concentration	Amount of solute per unit solvent/solution	Molarity, molality, ppm
IMFs	Forces between molecules affecting solubility	Hydrogen bonding in water

Additional info: These notes expand on the provided lecture slides and handwritten notes, adding definitions, formulas, and examples for clarity and completeness. All equations are presented in LaTeX format for academic rigor.