AP Chemistry · AP Chemistry CED Unit 4: Chemical Reactions · 14 min read
1. Core Definitions and Exam Context★☆☆☆☆⏱ 3 min
This foundational topic makes up 1-3% of the total AP Chemistry exam score, per the official CED. It appears in both multiple-choice (MCQ) and conceptual warm-up sections of free-response questions (FRQ), most often paired with particulate diagram reasoning.
2. Key Difference: Intermolecular vs Intramolecular Changes★★☆☆☆⏱ 4 min
The most reliable method to classify any change is to assess which interactions are altered during the process. Intermolecular forces (IMFs) are weak attractive forces between separate molecules or formula units that hold bulk matter together. Intramolecular bonds (covalent, ionic, metallic) are strong bonds that hold atoms together within a single molecule or formula unit, giving a substance its unique chemical identity.
In a physical change, only intermolecular forces are broken or formed; intramolecular bonds remain completely intact, so chemical identity does not change. In a chemical change, intramolecular bonds are broken, atoms rearrange, and new intramolecular bonds form to create new chemical substances.
3. Conservation of Mass★★☆☆☆⏱ 3 min
The law of conservation of mass states that mass is neither created nor destroyed in any physical or chemical change, as long as the system being measured is closed (no mass can enter or leave the system from the surroundings). This rule holds equally for both types of change: all atoms present before the change remain present after, just rearranged.
AP exam questions frequently test understanding of mass changes in open systems, where mass can be exchanged with the surroundings. A measured mass change in an open system does not violate conservation of mass—it just means matter entered or left the system.
4. Interpreting Particulate Diagrams★★★☆☆⏱ 4 min
A common AP Chemistry question type asks you to classify a change as physical or chemical based on a particulate diagram showing particles before and after the change. The key to solving these problems is to check whether the same chemical species exist before and after the change.
If the particles are just separated, spread out, or rearranged but the same molecules or formula units are present, the change is physical. If new molecules or formula units are present that did not exist before (atoms rearranged into new combinations), the change is chemical.
Common Pitfalls
Why: Students confuse breaking ionic bonds between formula units in a crystal lattice with breaking intramolecular bonds to form new substances; hydration only forms new intermolecular interactions, not new chemical species.
Why: Students memorize an oversimplified rule from introductory chemistry that is not universally true.
Why: Students forget the law of conservation of mass applies to closed systems that do not exchange matter with the surroundings.
Why: Students assume all phase changes are physical without considering if decomposition occurs at boiling temperature.
Why: Students confuse separation of molecules (breaking intermolecular forces) with separation of atoms within a molecule (breaking intramolecular bonds).