Chemical Reactions in Aqueous Solutions: General Chemistry Study Notes

Chapter 9: Chemical Reactions in Aqueous Solutions

9.1 General Properties of Aqueous Solutions

An aqueous solution is a homogeneous mixture in which water is the solvent. The substance present in the largest amount is the solvent, while other substances are solutes. A substance that dissolves in a solvent is said to be soluble in that solvent.

• Electrolytes: Substances that dissolve in water to yield a solution that conducts electricity. They dissociate into ions.

• Nonelectrolytes: Substances that dissolve in water but do not conduct electricity. Their molecules remain intact.

Strong vs. Weak Electrolytes

• Strong electrolytes: Dissociate completely in water (e.g., water-soluble ionic compounds, strong acids, strong bases).

• Weak electrolytes: Partially dissociate in water, existing mostly as molecules with a small fraction as ions (e.g., weak acids, weak bases).

The double arrow (⇌) denotes a reaction that occurs in both directions, indicating dynamic chemical equilibrium.

Classification Flowchart

9.2 Precipitation Reactions

A precipitate is an insoluble product that separates from a solution. A precipitation reaction forms a precipitate when two solutions are mixed.

Solubility Guidelines for Ionic Compounds in Water

• Water is a polar molecule, making it an excellent solvent for ionic compounds.

• Hydration: Water molecules surround and separate ions from an ionic solid, allowing it to dissolve.

• Solubility: The maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature.

Solubility Guidelines: Soluble Compounds	Insoluble Exceptions
Alkali metal cations, NH4+	None
Nitrate, acetate, chlorate ions	None
Chloride, bromide, iodide ions	Ag+, Hg22+, Pb2+
Sulfate ion	Ag+, Hg22+, Pb2+, Ca2+, Sr2+, Ba2+

Molecular, Ionic, and Net Ionic Equations

• Molecular equation: Shows all reactants and products as compounds.

• Ionic equation: Shows all strong electrolytes as ions.

• Net ionic equation: Shows only the species that actually participate in the reaction; spectator ions are omitted.

9.3 Acid–Base Reactions

Acids and bases can be strong or weak, depending on their degree of ionization in water.

• Strong acids: Ionize completely in water (e.g., HCl, HNO3, H2SO4).

• Strong bases: Dissociate completely in water (e.g., NaOH, KOH).

• Weak acids/bases: Partially ionize or dissociate.

Brønsted Acids and Bases

• Brønsted acid: Proton (H+) donor.

• Brønsted base: Proton (H+) acceptor.

• In water, acids donate protons to form hydronium ions (H3O+).

Polyprotic Acids and Bases

• Monoprotic acid: Donates one proton (e.g., HCl).

• Diprotic acid: Donates two protons in steps (e.g., H2SO4).

• Monobasic base: Produces one hydroxide per mole (e.g., NaOH).

• Dibasic base: Produces two hydroxides per mole (e.g., Ba(OH)2).

Acid–Base Neutralization

A neutralization reaction occurs when an acid reacts with a base to produce water and a salt.

9.4 Oxidation–Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons between reactants. Oxidation is the loss of electrons, and reduction is the gain of electrons.

Oxidation Numbers

• The oxidation number (oxidation state) is the charge an atom would have if electrons were transferred completely.

• Rules for assigning oxidation numbers:

  • Elemental form: 0

  • Sum of oxidation numbers equals the overall charge

  • Monatomic ion: equals the ion's charge

  • Some elements have reliable oxidation numbers (see table below)

Oxidation of Metals in Aqueous Solutions

• Displacement reaction: An atom or ion in a compound is replaced by an atom of another element.

• The activity series ranks metals by their ease of oxidation.

Other Types of Redox Reactions

• Combination reactions: Two or more substances form one product.

• Decomposition reactions: A single compound breaks down into two or more products.

• Disproportionation reactions: One element undergoes both oxidation and reduction.

• Combustion reactions: A substance reacts with oxygen, releasing energy.

9.5 Concentration of Solutions

Molarity (M) is the number of moles of solute per liter of solution:

$M = \frac{\text{moles of solute}}{\text{liters of solution}}$

Dilution

Dilution is the process of preparing a less concentrated solution from a more concentrated one. The number of moles of solute remains constant:

$M_c V_c = M_d V_d$

9.6 Aqueous Reactions and Chemical Analysis

Gravimetric Analysis

Gravimetric analysis is an analytical technique based on measuring mass to determine the amount of a substance in a sample. It is highly accurate and used for reactions that go to completion.

Acid–Base Titrations

Titration is a volumetric technique used to determine the concentration of an acid or base. The equivalence point is when the acid has been completely neutralized by the base, often indicated by a color change due to an indicator.

The pH Scale

The pH of a solution is a measure of its acidity or basicity, defined as:

$\text{pH} = -\log[\text{H}_3\text{O}^+]$

$[\text{H}_3\text{O}^+] = 10^{-\text{pH}}$

Solution Stoichiometry

• Soluble ionic compounds are strong electrolytes and dissociate completely in solution.

• Stoichiometry can be used to relate concentrations of ions and compounds in solution.

Example: For a 0.35 M Na2SO4 solution, [Na+] = 0.70 M and [SO42–] = 0.35 M.

Additional info: These notes are based on Chapter 9 of "Chemistry: Atoms First" (Burdge & Overby, 3rd Edition), covering aqueous reactions, solution stoichiometry, and related analytical techniques.