Solutions, Chemical Reactions, and Aqueous Chemistry: Study Guide for Exam 3

Solutions and Their Properties

Definition and Properties of Solutions

A solution is a homogeneous mixture composed of two or more substances. The substance present in the largest amount is called the solvent, while the other substances are solutes.

• Homogeneous means the composition is uniform throughout.

• Common examples: salt water, sugar dissolved in tea.

Types of Solutions

• Solid solutions (e.g., alloys like brass)

• Liquid solutions (e.g., vinegar in water)

• Gaseous solutions (e.g., air)

Concentration Units and Calculations

Common Units of Concentration

• Molarity (M): Moles of solute per liter of solution.

• Molality (m): Moles of solute per kilogram of solvent.

• Percent by mass (% w/w): Mass of solute divided by total mass of solution, multiplied by 100.

• Parts per million (ppm): $\text{ppm} = \frac{\text{g solute}}{\text{g solution}} \times 10^6$

• Parts per billion (ppb): $\text{ppb} = \frac{\text{g solute}}{\text{g solution}} \times 10^9$

• Osmolarity: $\text{Osmolarity} = \frac{\text{mol particles}}{\text{L solution}}$

Dilution Calculations

• To dilute a solution, use the formula: $M_1V_1 = M_2V_2$ or $C_1V_1 = C_2V_2$

• Example: To prepare 100 mL of 0.5 M NaCl from a 2.0 M stock solution, use $V_1 = \frac{M_2V_2}{M_1} = \frac{0.5 \times 100}{2.0} = 25$ mL of stock solution, diluted to 100 mL.

Colligative Properties and Solution Behavior

Colligative Properties

Properties that depend on the number of solute particles, not their identity.

• Vapor pressure lowering

• Boiling point elevation

• Freezing point depression

• Osmotic pressure

These properties are used to predict changes in physical properties when solutes are added to solvents.

Predicting the Effect of Solutes

• Non-volatile solutes lower the vapor pressure of a solvent.

• Boiling point increases and freezing point decreases with added solute.

• Magnitude depends on the number of particles (ions or molecules) in solution.

Solubility and Factors Affecting It

Solubility and Saturation

• Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature.

• Saturated solution: Contains the maximum amount of dissolved solute.

• Unsaturated solution: Can dissolve more solute.

• Supersaturated solution: Contains more solute than is stable; excess may crystallize out.

Factors Affecting Solubility

• Nature of solute and solvent: "Like dissolves like" (polar with polar, nonpolar with nonpolar).

• Temperature: Solubility of solids generally increases with temperature; for gases, solubility decreases as temperature increases.

• Pressure: Affects solubility of gases (Henry's Law: $C = kP$ where $C$ is concentration, $k$ is a constant, $P$ is pressure).

Electrolytes and Nonelectrolytes

Definitions

• Electrolytes: Substances that dissociate into ions in solution and conduct electricity (e.g., NaCl, HCl).

• Nonelectrolytes: Substances that do not form ions in solution (e.g., sugar, ethanol).

• Strong electrolytes: Completely dissociate in water (e.g., NaCl, HNO3).

• Weak electrolytes: Partially dissociate (e.g., acetic acid, NH3).

Precipitation, Acid-Base, and Redox Reactions

Precipitation Reactions

• Occur when two solutions are mixed and an insoluble product (precipitate) forms.

• Use solubility rules to predict if a precipitate will form.

Acid-Base Reactions

• Acids: Proton donors (produce H+ in solution).

• Bases: Proton acceptors (produce OH- in solution).

• Neutralization: Acid + Base → Salt + Water

Redox (Oxidation-Reduction) Reactions

• Involve transfer of electrons between species.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Assign oxidation numbers to identify what is oxidized and reduced.

Stoichiometry in Solution

Solution Stoichiometry

• Use molarity and volume to calculate moles of reactants and products.

• For titrations: $M_1V_1 = M_2V_2$ (for 1:1 reactions).

• Balance chemical equations before performing calculations.

Energy Changes in Chemical Reactions

Endothermic and Exothermic Reactions

• Endothermic: Absorb energy (ΔH > 0).

• Exothermic: Release energy (ΔH < 0).

Activation Energy

• The minimum energy required for a reaction to occur.

• Catalysts lower the activation energy, increasing reaction rate.

Summary Table: Key Solution Calculations

Calculation	Formula	Units
Percent by mass	$\%\ (w/w) = \frac{\text{mass solute}}{\text{mass solution}} \times 100$	%
Percent by volume	$\%\ (v/v) = \frac{\text{volume solute}}{\text{volume solution}} \times 100$	%
ppm	$\text{ppm} = \frac{\text{g solute}}{\text{g solution}} \times 10^6$	ppm
ppb	$\text{ppb} = \frac{\text{g solute}}{\text{g solution}} \times 10^9$	ppb
Molarity	$M = \frac{\text{mol solute}}{\text{L solution}}$	mol/L
Osmolarity	$\text{Osmolarity} = \frac{\text{mol particles}}{\text{L solution}}$	osmol/L
Dilution	$M_1V_1 = M_2V_2$	mol/L, L

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Examples and formulas have been added to ensure the notes are self-contained and exam-ready.