Aqueous Reactions

Introduction to Aqueous Solutions

Aqueous reactions are chemical processes that occur in water as the solvent. Understanding how substances dissolve and react in water is fundamental to general chemistry, especially for predicting reaction outcomes and analyzing solution properties.

Solutions and Solvents

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

Types of Dissolution in Water

Substances dissolve in water in different ways:

• Ionic Compounds: Dissolve by dissociation, where water surrounds and separates the ions.

• Molecular Compounds: Interact with water but generally do not dissociate; they dissolve without forming ions.

Electrolytes and Nonelectrolytes

An electrolyte is a substance that dissociates into ions when dissolved in water, allowing the solution to conduct electricity. A nonelectrolyte dissolves in water but does not dissociate into ions, so it does not conduct electricity.

• Strong Electrolyte: Dissociates completely (e.g., soluble ionic compounds, strong acids, strong bases).

• Weak Electrolyte: Dissociates partially (e.g., weak acids, weak bases).

• Nonelectrolyte: Does not dissociate (e.g., most molecular compounds).

Solubility Guidelines

Solubility rules help predict whether a compound will dissolve in water. Certain ions are always soluble, while others have important exceptions.

Precipitation Reactions

When two solutions containing soluble salts are mixed, an insoluble salt may form and precipitate out of solution. This solid is called a precipitate.

Metathesis (Double Replacement) Reactions

Metathesis reactions involve the exchange of ions between two compounds, often resulting in the formation of a precipitate, gas, or water. The general form is:

• Molecular Equation: Shows reactants and products as compounds.

• Complete Ionic Equation: Shows all strong electrolytes as ions.

• Net Ionic Equation: Shows only the species that change during the reaction.

Example:

• Molecular:

• Complete Ionic:

• Net Ionic:

Acids and Bases

Acids and bases are defined by their behavior in water:

• Arrhenius Acid: Increases H+ concentration in water.

• Brønsted-Lowry Acid: Proton donor.

• Arrhenius Base: Increases OH- concentration in water.

• Brønsted-Lowry Base: Proton acceptor.

Strong and Weak Acids/Bases

Strong acids and bases dissociate completely in water, while weak acids and bases only partially dissociate. The pH of a solution is a measure of its H+ concentration:

• Lower pH indicates higher acidity.

• Example: For 0.2M HNO3, and

Neutralization Reactions

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

Gas-Forming Reactions

Some metathesis reactions produce a gas, such as when carbonates or bicarbonates react with acids:

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons:

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• OIL RIG: Oxidation Is Loss, Reduction Is Gain

Assigning Oxidation Numbers

Oxidation numbers help identify redox reactions:

• Elemental form: Oxidation number is 0

• Monatomic ion: Oxidation number equals its charge

• Sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion, it equals the ion's charge

• Oxygen is usually -2, except in peroxides or with fluorine

Activity Series and Displacement Reactions

The activity series ranks elements by their tendency to be oxidized. Elements higher in the series are more reactive and more likely to exist as ions. In single-displacement reactions, a more reactive element displaces a less reactive one from a compound.

Chemical Quantities and Solution Calculations

Molarity

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

Preparing Solutions

To prepare a solution of known molarity:

1. Weigh out a known mass of solute.

2. Add solute to a volumetric flask.

3. Add solvent to the calibration mark.

Dilution

To dilute a solution, use the equation:

• Where and are the molarity and volume of the concentrated solution, and and are those of the diluted solution.

Stoichiometric Calculations Using Molarity

Molarity is used in stoichiometric calculations to relate volumes and concentrations of reactants and products.

Titration

Titration is an analytical technique used to determine the concentration of a solute in a solution. A solution of known concentration (standard solution) is added to the unknown solution until the reaction reaches the equivalence point.

Titration Calculations

At the equivalence point, the moles of solute in the standard solution equal the moles of solute in the unknown solution, allowing calculation of the unknown concentration.

Additional info: These notes cover the essential concepts of aqueous reactions, solution chemistry, and related calculations, suitable for General Chemistry students preparing for exams or coursework.