Chapter 7: Chemical Reactions – Study Notes for Introductory Chemistry

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Chemical Reactions

Introduction to Chemical Reactions

Chemical reactions are processes in which substances (reactants) are transformed into new substances (products) with different properties. These reactions are fundamental to both everyday life and industrial processes. For example, the reaction between baking soda and vinegar produces carbon dioxide gas, which is visible as bubbling in a classic grade school volcano experiment. Reactions that produce a gas in a liquid are called gas-evolution reactions.

Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen to produce heat and often light. Hydrocarbons, such as octane in gasoline, react with oxygen to form carbon dioxide and water, releasing energy. Combustion reactions are a type of oxidation–reduction (redox) reaction, where electrons are transferred between substances.

Combustion of octane in an auto engine forming CO2 and H2O

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid called a precipitate. For example, when laundry detergent reacts with hard water ions, insoluble solids form and settle out, removing the ions from the water.

Soap scum forming in a sink due to precipitation reaction

Evidence of a Chemical Reaction

Observable Signs

Although we cannot see atoms and molecules directly, chemical reactions often produce observable changes, including:

Color change
Formation of a solid (precipitate) in a previously clear solution
Formation of a gas (bubbling or fizzing)
Emission of light
Emission or absorption of heat

Examples of evidence for chemical reactions: color change, precipitate, gas, light, heat

Physical Changes vs. Chemical Changes

Not all observable changes indicate a chemical reaction. For example, boiling water produces bubbles (steam), but no new substance is formed—this is a physical change, not a chemical reaction.

Chemical Equations

Representing Chemical Reactions

Chemical equations use chemical formulas to represent the reactants and products of a reaction. The substances on the left are reactants, and those on the right are products. The physical state of each substance is indicated in parentheses: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).

Balancing Chemical Equations

To reflect the conservation of mass, chemical equations must be balanced so that the number of each type of atom is the same on both sides. This is done by adjusting coefficients (numbers in front of formulas), not subscripts (numbers within formulas).

For example, the combustion of methane is represented as:

Steps for balancing equations:

Write correct formulas for all reactants and products.
Balance elements that appear in only one reactant and one product first.
Balance free elements last.
Check that all atoms are balanced.
Only change coefficients, never subscripts.

Aqueous Solutions and Solubility

Solubility Terminology

A compound is soluble if it dissolves in a liquid, and insoluble if it does not. An aqueous solution is a homogeneous mixture of a substance with water. Ionic compounds that dissolve in water usually dissociate into their component ions, forming strong electrolytes that conduct electricity.

NaCl(aq) contains Na+ and Cl- ions in solution AgNO3(aq) contains Ag+ and NO3- ions in solution AgCl(s) does not dissolve in water and remains as a solid

Solubility Rules

Solubility rules help predict whether an ionic compound will dissolve in water. Compounds containing alkali metal ions or nitrate (NO3-) are generally soluble, while many silver, lead, and carbonate compounds are insoluble.

Precipitation Reactions

Formation of a Precipitate

When two solutions of soluble compounds are mixed, an insoluble compound may form as a precipitate. For example, mixing potassium iodide and lead(II) nitrate produces a yellow precipitate of lead(II) iodide:

Formation of yellow PbI2 precipitate in a precipitation reaction Ions in KI(aq) and Pb(NO3)2(aq) before mixing All ions present after mixing KI(aq) and Pb(NO3)2(aq) PbI2(s) precipitate and KNO3(aq) after reaction

No Reaction Example

If all possible products are soluble, no precipitate forms and no reaction occurs. For example, mixing potassium iodide and sodium chloride results in no reaction because both possible products are soluble.

Mixing KI(aq) and NaCl(aq) results in no reaction

Molecular, Complete, and Net Ionic Equations

Types of Equations

Molecular equation: Shows complete, neutral formulas for all compounds.
Complete ionic equation: Shows all strong electrolytes as ions.
Net ionic equation: Shows only the species that actually participate in the reaction (omitting spectator ions).

Acid–Base Reactions

Neutralization Reactions

Acid–base reactions (neutralization reactions) occur when an acid reacts with a base to form water and an ionic compound (salt). The net ionic equation for many acid–base reactions is:

Acids produce H+ ions in solution, while bases produce OH- ions.

Gas-Evolution Reactions

Formation of Gases

Some reactions in aqueous solution produce a gas, either directly or through the decomposition of an unstable intermediate. For example, the reaction of an acid with a carbonate produces carbon dioxide gas.

Oxidation–Reduction (Redox) Reactions

Electron Transfer

Redox reactions involve the transfer of electrons between substances. Oxidation is the loss of electrons, and reduction is the gain of electrons. These reactions are responsible for processes such as rusting, combustion, and battery operation.

Example of a redox reaction: decomposition of water by electrolysis

Combustion Reactions

Characteristics

Combustion reactions are a type of redox reaction where a substance reacts with oxygen to form one or more oxygen-containing compounds, usually releasing heat. Hydrocarbons combust to form carbon dioxide and water.

Classifying Chemical Reactions

Types of Reactions by Atomic Changes

Synthesis (Combination) Reaction: Two or more simple substances combine to form a more complex substance.
Decomposition Reaction: A complex substance breaks down into simpler substances.
Single-Displacement Reaction: One element displaces another in a compound.
Double-Displacement Reaction: Elements or groups in two compounds exchange places, often forming a precipitate, gas, or water.

Double-displacement reaction example

Review and Learning Objectives

Identify evidence of a chemical reaction.
Write and balance chemical equations.
Determine solubility and predict precipitation reactions.
Write molecular, complete ionic, and net ionic equations.
Identify and write equations for acid–base, gas-evolution, redox, and combustion reactions.
Classify chemical reactions by type.