General Properties of Aqueous Solutions

Definition and Components

Solutions are homogeneous mixtures composed of two or more pure substances. The solvent is the component present in the greatest amount, while solutes are the substances dissolved in the solvent. For example, in a mixture of sugar and water, water is the solvent and sugar is the solute.

Electrolytic Properties of Solutions

Electrolytes and Nonelectrolytes

Substances that produce ions in aqueous solution are called electrolytes (e.g., NaCl), while those that do not form ions are nonelectrolytes (e.g., C12H22O11). Electrolytes conduct electricity due to the movement of ions, whereas nonelectrolytes do not.

• Electrolyte: Substance whose aqueous solution conducts electricity (e.g., NaCl).

• Nonelectrolyte: Substance whose aqueous solution does not conduct electricity (e.g., sucrose).

Ionic Compounds in Water

Dissociation and Solvation

When ionic compounds dissolve in water, they undergo dissociation, where each ion separates from the solid structure and disperses throughout the solution. Water molecules surround (solvate) the ions, preventing them from recombining.

• Dissociation Example:

• Solvation: Water molecules stabilize ions in solution by surrounding them.

Water as an Effective Solvent

Polarity of Water

Water is an electrically neutral molecule but is highly effective at dissolving ionic compounds due to its polarity. The oxygen atom carries a partial negative charge (δ−), while the hydrogen atoms carry partial positive charges (δ+).

• Polarity: Uneven distribution of electron density in the water molecule.

• Solvation: Polar water molecules interact with and stabilize ions.

Molecular Compounds in Water

Dissolution Without Ionization

Molecular compounds, such as methanol (CH3OH), typically dissolve in water as intact molecules without forming ions. Thus, their solutions do not conduct electricity.

• Molecular Compound Example: Methanol (CH3OH) dissolves but does not ionize.

Ionic vs. Molecular Compounds

Classification and Exceptions

Ionic compounds are generally formed from metals and non-metals, while molecular compounds are formed from non-metals only. Some molecular substances, such as HCl, ionize in water. Ammonium compounds (NH4+) are considered ionic due to their behavior in solution.

• Ionic: NaCl, KCN, MgO, NH4Br

• Molecular: CH3OH, C12H22O11, HCl (ionizes in water)

Strong and Weak Electrolytes

Degree of Ionization

The strength of an electrolyte depends on its ability to exist as ions in solution:

• Strong electrolytes: Completely ionize in solution (e.g., NaCl).

• Weak electrolytes: Partially ionize in solution (e.g., CH3COOH).

Example Equations:

• Strong: (100% ionization)

• Weak: (1% ionization)

Precipitation Reactions

Formation of Insoluble Products

Precipitation reactions occur when two solutions are mixed and an insoluble product, called a precipitate, forms. This happens when pairs of oppositely charged ions attract each other strongly enough to form an insoluble ionic solid.

• Example:

Solubility Guidelines for Ionic Compounds

Predicting Solubility

The solubility of a substance is the amount that can dissolve in a given quantity of solvent at a specific temperature. Compounds with solubility less than 0.01 mol/L are considered insoluble. The following table summarizes common solubility rules:

Exchange (Metathesis) Reactions

Double Replacement Pattern

Exchange (metathesis) reactions involve the exchange of cations and anions between reactants. Precipitation and many neutralization reactions follow this pattern:

• General form:

• Example:

Ionic Equations

Molecular, Complete Ionic, and Net Ionic Equations

Chemical reactions in solution can be represented in three ways:

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

• Complete ionic equation: Shows all strong electrolytes as ions.

• Net ionic equation: Includes only the ions and molecules directly involved in the reaction; spectator ions are omitted.

Example:

• Molecular:

• Complete ionic:

• Net ionic:

Acids, Bases, and Neutralization Reactions

Definitions and Examples

Acids are substances that ionize in aqueous solution to form hydrogen ions (H+), acting as proton donors. Bases accept H+ ions and often produce hydroxide ions (OH−) in water. Some bases, like ammonia (NH3), generate OH− by reacting with water.

• Acid Example:

• Base Example:

Strong and Weak Acids and Bases

Degree of Ionization

Strong acids and bases are completely ionized in solution and are strong electrolytes. Weak acids and bases are only partially ionized and are weak electrolytes. Only soluble ionic solids are considered strong electrolytes.

• Strong Acids: HCl, HBr, HI, HNO3, H2SO4, HClO3, HClO4

• Strong Bases: Group 1A metal hydroxides (e.g., NaOH), heavy Group 2A metal hydroxides (e.g., Ba(OH)2)

Neutralization Reactions and Salts

Formation of Water and Salts

A neutralization reaction occurs when an acid reacts with a base to produce water and a salt. The net ionic equation for a strong acid and strong base is:

Concentrations of Solutions

Molarity and Parts Per Million (ppm)

Concentration expresses the amount of solute in a given quantity of solvent or solution. Molarity (M) is defined as moles of solute per liter of solution:

Parts per million (ppm) is another unit, defined as:

Dilution of Solutions

Preparation of Dilute Solutions

To prepare a dilute solution from a concentrated stock, water is added until the desired concentration is reached. The relationship is:

Solution Stoichiometry

Calculations Involving Reactions in Solution

Solution stoichiometry involves using the volume and concentration of a reactant or product to determine the amount of another substance in the reaction. Always start with a balanced chemical equation.

• Example: How many grams of Ca(OH)2 are needed to neutralize 25.0 mL of 0.100 M HNO3?

• Steps: Convert volume to liters, use molarity to find moles, apply stoichiometry, and convert to grams using molar mass.

Key Equation: