BackChapter 8: Chemical Reactions – Evidence, Equations, and Types
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Chapter 8: Chemical Reactions
Basic Chemistry Skills
Before studying chemical reactions, it is important to have foundational skills in writing and naming chemical compounds.
Writing Ionic Formulas: Understanding how to write correct formulas for ionic compounds (see Section 6.2).
Naming Ionic Compounds: Applying rules for naming compounds formed from metals and nonmetals (see Section 6.3).
Writing Names and Formulas for Molecular Compounds: Using prefixes and rules for covalent compounds (see Section 6.5).
Chemical Change
A chemical change occurs when a substance is converted into one or more new substances with different properties and compositions.
Example: When silver (Ag) tarnishes, it reacts with sulfur (S) in the air to form black silver sulfide (Ag2S), commonly known as tarnish.
Visual Evidence of a Chemical Reaction
Chemical reactions often produce observable changes. The following are common types of evidence indicating a chemical reaction has occurred:
Type of Evidence | Description | Example |
|---|---|---|
Change in color | Formation of a new substance with a different color | Iron nails turning reddish-brown as they form rust (Fe2O3) |
Formation of a gas (bubbles) | Gas is produced, often seen as bubbles | Bubbles form when CaCO3 reacts with acid |
Formation of a solid (precipitate) | Solid forms from the reaction of two solutions | Yellow solid forms when potassium iodide is added to lead nitrate |
Heat (or flame) produced or absorbed | Temperature change or visible flame | Methane gas burns in air with a hot flame |
Writing a Chemical Reaction
Chemical reactions are represented by chemical equations, which show the reactants (starting substances) and products (new substances formed).
Reactants are written on the left side of the equation; products are on the right.
Multiple formulas on one side are separated by a plus sign (+).
The arrow (→) means "reacts to form" or "yields".
Physical states are indicated: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).
The delta symbol (Δ) above the arrow indicates heat is used.
Example: The combustion of charcoal (carbon) in oxygen:
Word equation: Carbon + Oxygen → Carbon dioxide
Chemical equation:
Summary Table: Common Symbols in Chemical Equations
Symbol | Meaning |
|---|---|
+ | Separates two or more formulas |
→ | Reacts to form products |
(s) | Solid |
(l) | Liquid |
(g) | Gas |
(aq) | Aqueous (dissolved in water) |
Δ | Reactant is heated |
Key Terms
Chemical change: A process in which substances are transformed into new substances with different properties.
Reactant: A starting material in a chemical reaction.
Product: A substance formed as a result of a chemical reaction.
Precipitate: A solid that forms from a solution during a chemical reaction.
Example Applications
Tarnishing of Silver: Silver reacts with sulfur in the air to form black silver sulfide, indicating a chemical change.
Combustion of Methane: Methane gas burns in air, producing carbon dioxide, water, and heat.
Additional info: These notes cover the introductory concepts of chemical reactions, including how to recognize chemical changes and how to write and interpret chemical equations. Later sections would typically include balancing equations and classifying reaction types.
