Chapter 4: Reactions in Aqueous Solution

Solutions and Their Components

Solutions are homogeneous mixtures composed of two or more pure substances. The substance present in the greatest amount is called the solvent, while all other substances are solutes. When water is the solvent, the solution is termed an aqueous solution.

• Homogeneous mixture: Uniform composition throughout.

• Solvent: The component in the largest quantity.

• Solute: The component(s) present in lesser quantity.

• Aqueous solution: A solution where water is the solvent.

Aqueous Solutions and Solvation

All substances dissolve by solvation, the process where solvent molecules surround solute particles. The manner in which substances dissolve in water depends on their nature:

• Ionic compounds: Dissolve by dissociation, where water molecules surround and separate the ions.

• Molecular compounds: Disperse in water, but most remain as intact molecules.

• Some molecular substances: Form ions in water upon dissolving (e.g., acids).

Electrolytes and Nonelectrolytes

Substances that dissolve in water can be classified based on their ability to conduct electricity:

• Strong electrolytes: Completely dissociate into ions; solutions conduct electricity well.

• Weak electrolytes: Partially dissociate; solutions conduct electricity weakly.

• Nonelectrolytes: Do not dissociate into ions; solutions do not conduct electricity.

Strong vs. Weak Electrolytes – Chemical Equilibrium

A strong electrolyte dissociates completely, represented by a single arrow in equations. A weak electrolyte only partially dissociates, and its equation uses a double arrow (⇌) to indicate equilibrium.

• Example (strong):

• Example (weak):

Electrolytes and Nonelectrolytes: Examples

• Electrolyte: NaCl(s) forms and

• Electrolyte: CH3CO2H(aq) forms and

• Nonelectrolyte: Glucose (C6H12O6) and sucrose (C12H22O11) dissolve but do not form ions.

Precipitation Reactions

Formation of Precipitates

Precipitation reactions occur when two solutions containing soluble salts are mixed and an insoluble salt (precipitate) forms.

• Precipitate: The solid product formed in the reaction.

Solubility Rules for Ionic Compounds

Not all ionic compounds dissolve in water. Solubility rules help predict which combinations of ions will form soluble or insoluble compounds.

Predicting Precipitate Formation

To predict if a precipitate forms when strong electrolytes are mixed:

1. Identify the ions present in the reactants.

2. Consider all possible cation-anion combinations.

3. Use solubility rules to determine if any combination is insoluble.

Metathesis (Exchange) Reactions

Metathesis reactions involve the exchange of ions between two compounds:

• General form:

• Example:

Completing and Balancing Metathesis Equations

Steps:

1. Determine ions present in reactants.

2. Write formulas for products using cation from one reactant and anion from the other.

3. Check solubility rules for precipitate formation.

4. Balance the equation.

Net Ionic Equations

To derive the net ionic equation:

1. Write the molecular equation (balanced overall reaction).

2. Write the complete ionic equation (all strong electrolytes as ions).

3. Cancel out spectator ions (ions unchanged on both sides) to get the net ionic equation.

• Example:

Acids, Bases, and Neutralization

Acids

Acids are substances that ionize in aqueous solution to form H+ ions (proton donors).

Bases

Bases are substances that react with or accept H+ ions, increasing the concentration of OH- ions in water. Not all bases contain OH- (e.g., NH3).

Strong and Weak Acids and Bases

Strong acids and bases dissociate completely in water, while weak acids and bases only partially dissociate.

Electrolytic Behavior of Compounds

Neutralization Reactions

Neutralization reactions occur when an acid reacts with a base, typically producing water and a salt. These can be represented as molecular, complete ionic, or net ionic equations.

• Example (molecular):

• Example (net ionic):

Neutralization with Gas Formation

Some neutralization reactions produce a gas, such as CO2 or H2S, instead of water.

• Example:

Applications: Antacids

Antacids neutralize excess stomach acid. Common antacid ingredients include NaHCO3, Al(OH)3, Mg(OH)2, and CaCO3.

Oxidation–Reduction (Redox) Reactions

Oxidation and Reduction

Oxidation is the loss of electrons, while reduction is the gain of electrons. These processes always occur together in redox reactions.

Oxidation Numbers

Assigning oxidation numbers helps determine if redox reactions have occurred. The rules are as follows:

1. Atoms in elemental form: 0

2. Monatomic ions: equal to their charge

3. Nonmetals: usually negative, but can be positive in some compounds

4. Sum in a neutral compound: 0; in a polyatomic ion: equals the ion's charge

Displacement Reactions and Activity Series

In displacement reactions, ions oxidize an element, displacing another ion. The activity series ranks metals by their reactivity; elements above hydrogen react with acids to produce H2 gas.

Concentration and Molarity

Definition of Molarity

Molarity (M) is a measure of solution concentration, defined as moles of solute per liter of solution:

Preparing Solutions

To prepare a solution of known molarity, weigh the solute, add it to a volumetric flask, and add solvent up to the calibration mark.

Dilution of Solutions

Solutions can be diluted by adding solvent. The number of moles of solute remains constant:

• and are the molarity and volume of the concentrated solution.

• and are the molarity and volume after dilution.

Solution Stoichiometry and Chemical Analysis

Titration

Titration is an analytical technique to determine the concentration of a solute in solution. A standard solution of known concentration is added to react with the analyte until the reaction reaches the equivalence point, often indicated by a color change (end point).