4.1 Introduction for Reactions

Chemical reactions rearrange atoms

A chemical reaction occurs when substances (reactants) transform into new substances (products) through the breaking and forming of bonds. Atoms are neither created nor destroyed, so a balanced chemical equation must have equal numbers of each type of atom on both sides, satisfying the law of conservation of mass.

Representing reactions

Reactions can be shown using molecular, ionic, or net ionic equations, and represented with particulate diagrams showing atoms/molecules rearranging. Physical states are indicated with symbols: (s), (l), (g), (aq). Coefficients show relative mole ratios, not masses, between reactants and products.

Types of chemical change

Reactions include synthesis, decomposition, single/double replacement, and combustion. Evidence of a reaction includes color change, gas production, precipitate formation, or temperature change. Recognizing patterns in reaction types helps predict products when writing balanced equations.

Key terms

Chemical reaction

A process in which reactants are transformed into products through the breaking and forming of chemical bonds.

Law of conservation of mass

Atoms are neither created nor destroyed in a chemical reaction; total mass of reactants equals total mass of products.

Balanced equation

An equation with equal numbers of each type of atom on both the reactant and product sides.

Coefficients in equations

Numbers placed before formulas that show the relative mole ratios of reactants and products.

Evidence of chemical reaction

Signs such as color change, gas formation, precipitate formation, or temperature change indicating a reaction has occurred.

Try an AP-style question

When C₃H₈ + O₂ → CO₂ + H₂O is balanced with whole numbers, what is the coefficient of O₂?

1. 7
2. 5
3. 4
4. 3