AP Chemistry Unit 4 Cheatsheet: Chemical Reactions

Start with the first 3 sections of the Unit 4 one-page cheatsheet below. The full sheet is a PDF you can print.

Physical vs. Chemical Changes

Physical Change	Chemical Change
Substance properties change, but composition stays the same	New substances form with different composition
Phase changes (solid ↔ liquid ↔ gas)	Atoms rearrange into new combinations
Mixture formation/separation	Evidence: heat/light, gas formation, precipitate, color change
In a physical change, only interparticle attractions (IMFs, ion–ion lattice attractions) are broken and/or formed — no covalent bonds within the substance's particles are made or broken.	Chemical bonds break and/or form

Gray area: Salt dissolving in water involves breaking ionic bonds but may be classified as physical or chemical depending on context.

Equation Forms & When to Use

Molecular equation: Shows all compounds as written formulas. Use for general representation.
Complete ionic equation: Shows all ions and molecules separately. Reveals what's actually in solution.
Net ionic equation: Removes spectator ions (ions that don't participate). Shows only the actual reaction occurring.

All three forms must be balanced for atoms AND charge. Mass and charge are conserved in every reaction.

Brønsted-Lowry Acid-Base Pairs

Acid: Proton (H⁺) donor
Base: Proton (H⁺) acceptor
Conjugate acid-base pair: Two species differing by one H⁺

Example: HCl + H₂O → H₃O⁺ + Cl⁻
HCl is acid; Cl⁻ is conjugate base.
H⁺ + H₂O ⇌ H₃O⁺
H₂O is base; H₃O⁺ is conjugate acid.

Water is amphoteric—it can donate or accept protons depending on what it reacts with.

The rest of the Unit 4 sheet

6 more sections are on the one-page PDF:

Balancing Redox with Half-Reactions
Titration: Equivalence Point & Endpoint
Oxidation Numbers & Redox Identification
Identifying Reaction Types
Stoichiometry: Mole Ratios & Calculations
Particulate-Level Representation

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