Chapter 5: Reactions in Aqueous Solution

Introduction

This chapter focuses on chemical reactions that occur in aqueous solutions—where water is the solvent. Key reaction types include precipitation reactions, acid-base reactions, and oxidation-reduction (redox) reactions. The chapter also covers solution concentration (molarity) and the classification of substances as strong, weak, or nonelectrolytes based on their ability to conduct electricity in solution.

Learning Goals

• Distinguish between strong, weak, and nonelectrolytes in solution.

• Use solubility rules to predict precipitation reactions.

• Write and balance complete and net ionic equations.

• Classify acids and bases as strong or weak, and recognize their formulas and names.

• Identify acid-base reactions using Arrhenius and Brønsted-Lowry models.

• Assign oxidation numbers and balance redox reactions.

• Distinguish between precipitation, acid-base, gas-evolving, and redox reactions.

• Calculate solution concentrations and perform solution stoichiometry.

Solutions and Aqueous Solutions

Definitions

• Solution: A homogeneous mixture of two or more substances.

• Solute: The component present in the smallest amount.

• Solvent: The component present in the largest amount (in aqueous solutions, water is the solvent).

• Aqueous solution: A solution in which water is the solvent.

When a solute is dissolved in water, it is indicated by writing (aq) after the chemical formula.

Example: Dissolving Glucose

• When glucose (C6H12O6) dissolves in water:

• Glucose molecules are surrounded by water molecules.

• This solution does not conduct electricity; glucose is a nonelectrolyte.

Ions in Solution and Electrolytes

Electrolyte Classification

• Electrolyte: A substance that produces ions when dissolved in water, allowing the solution to conduct electricity.

• Strong electrolyte: Completely dissociates into ions (e.g., NaCl, HCl).

• Weak electrolyte: Partially dissociates into ions (e.g., acetic acid, HC2H3O2).

• Nonelectrolyte: Does not produce ions in solution (e.g., glucose, ethanol).

Conductivity and Electrolyte Strength

• Strong electrolyte solutions are good conductors of electricity.

• Weak electrolyte solutions are poor conductors.

• Nonelectrolyte solutions do not conduct electricity.

Table: Electrolyte Classification and Conductivity

Examples of Electrolyte Behavior

• Glucose solution: Nonelectrolyte, does not conduct electricity.

• Sodium chloride solution: Strong electrolyte, high conductivity.

• Acetic acid solution: Weak electrolyte, low conductivity.

Acids and Bases in Aqueous Solution

Acids

• Acids are molecular compounds that produce H+ ions in water.

• Acidic solutions have pH < 7.

• The chemical formula for an acid usually has one or more hydrogen atoms written first (e.g., HCl, H2SO4).

• Strong acids (memorize): HCl, HBr, HI, H2SO4, HNO3, HClO4

• Weak acids: Most other acids, such as acetic acid (CH3COOH).

Bases

• Bases produce OH- ions in water.

• Basic (alkaline) solutions have pH > 7.

• Strong bases: Soluble hydroxide salts (e.g., NaOH, KOH, Ba(OH)2).

• Weak bases: Ammonia (NH3) and amines (e.g., CH3NH2).

Classification of Electrolytes

Strong Electrolytes

• Soluble ionic compounds (e.g., KCl, Na2CO3).

• Strong acids (see above).

• Strong bases (soluble hydroxides of Group 1A and 2A metals).

Weak Electrolytes

• Weak acids (e.g., HF, HC2H3O2).

• Weak bases (e.g., NH3, CH3NH2).

Nonelectrolytes

• Alcohols and sugars (e.g., C2H5OH, C6H12O6).

• Do not dissociate into ions in water.

Practice: Classification and Dissolution Equations

• Classify compounds as strong acid, weak acid, strong base, weak base, salt, or other.

• Classify as strong electrolyte, weak electrolyte, or nonelectrolyte.

• Write equations showing what happens when the solute dissolves in water.

Example: Na2SO4(s) → 2 Na+(aq) + SO42-(aq) (Salt, Strong Electrolyte)

Review: Ionic Compounds

• Monatomic cations and anions differ from atoms by their charge.

• Use the periodic table to find charges of monatomic ions.

• Polyatomic ions: ions composed of more than one atom (e.g., SO42-, NO3-).

• Memorize names and formulas of common polyatomic ions.

• Empirical formula: the simplest whole-number ratio of ions in an ionic compound.

• Ionic compounds in the solid state exist as a crystal lattice (not individual molecules).

Example: The formula unit of CaCl2 is one Ca2+ ion and two Cl- ions.

Summary Table: Electrolyte Types and Properties

Key Equations

• Molarity (M):

• Dissociation of strong electrolytes (example):

• Dissociation of weak electrolytes (example):

• Nonelectrolyte (example):

Summary

• Understanding the nature of solutes in aqueous solution is essential for predicting chemical behavior.

• Electrolyte strength determines conductivity and is key for classifying reactions in solution.

• Acids and bases are classified by their ability to produce H+ or OH- ions and their strength (strong vs. weak).

• Practice writing dissociation equations and classifying compounds for mastery.