A decomposition reaction is a chemical reaction in which a single compound breaks down into two or more simpler substances (A → B + C). On the AP Chem exam, decomposition reactions are the classic setup for Unit 5 mechanism problems where you derive the rate law from the slow step.
A decomposition reaction is the "one becomes many" reaction. A single compound breaks apart into two or more simpler products, written generically as A → B + C. It's the exact reverse of a combination (synthesis) reaction, where multiple reactants merge into one product.
In AP Chem, you'll see decomposition in two ways. First, as a reaction type you should recognize on sight (one reactant, multiple products). Second, and more importantly, as the go-to example in Topic 5.8: Reaction Mechanism and Rate Law. Decomposition reactions are perfect for mechanism questions because they're simple enough to break into elementary steps. A reaction like 2A → B + C might happen in one bimolecular collision, or through a slow unimolecular first step followed by fast steps. The overall equation looks the same either way, but the rate law is completely different. Figuring out which mechanism fits the data is the whole game.
Decomposition reactions live in Unit 5: Kinetics, specifically Topic 5.8, supporting learning objective 5.8.A: identifying the rate law from a mechanism whose first step is rate limiting. The essential knowledge (5.8.A.1) says the rate law comes from the molecularity of the slowest elementary step, not from the overall balanced equation. Decomposition reactions make this idea concrete. If the slow step is A → products (unimolecular), the rate law is first order, rate = k[A]. If the slow step is 2A → products (bimolecular), it's second order, rate = k[A]². Same reactant, totally different kinetics. This is also where the famous first-order clue shows up. A constant half-life that doesn't depend on initial concentration screams first-order decomposition, which connects directly to integrated rate laws elsewhere in Unit 5.
Keep studying AP Chemistry Unit 5
Rate-Determining Step (Unit 5)
Decomposition mechanism problems almost always hinge on the slow step. If the first step of a decomposition is rate limiting, you write the rate law straight from that step's molecularity and ignore everything that happens afterward.
Elementary Step (Unit 5)
A decomposition that looks simple on paper (2A → B + C) may actually be a sequence of elementary steps with intermediates. The big rule is that you can only read a rate law directly off an elementary step, never off the overall equation.
Combination Reaction (Unit 4/5)
Combination is decomposition run in reverse. Two or more substances merge into one compound instead of one compound splitting apart. Recognizing the pair helps you classify reactions instantly on MCQs.
Redox Reaction (Unit 4)
Many decompositions are also redox reactions, since breaking a compound apart often changes oxidation states (think a metal chlorate decomposing into a metal chloride and oxygen gas). One reaction can wear both labels.
Decomposition reactions show up most often as the scaffolding for kinetics questions. A typical multiple-choice stem gives you a decomposition like 2A → B + C with a proposed mechanism, tells you which step is slow, and asks for the consistent rate law or what happens to the rate when [A] is tripled (with a bimolecular slow step, tripling [A] multiplies the rate by nine). Another classic move is the reverse direction. You're given experimental evidence, like a half-life that stays constant no matter the starting concentration, and you have to infer that the slow step must be unimolecular, A → products. Watch for traps where a reactant in the overall equation, like B in 2A + B → C, doesn't appear until a fast step. That reactant is zero order, and the rate law leaves it out entirely. Released free-response questions, including the 2019 halogens FRQ, have also used decomposition reactions as real chemical context, so be ready to combine the classification with thermodynamics or bonding reasoning.
They're mirror images. A decomposition reaction takes one compound and breaks it into two or more simpler substances (A → B + C), while a combination (synthesis) reaction takes two or more substances and joins them into a single compound (B + C → A). Quick check: count the reactants. One reactant splitting apart means decomposition. Multiple reactants forming one product means combination.
A decomposition reaction has one reactant breaking into two or more simpler products, the opposite of a combination reaction.
The rate law for a decomposition comes from the molecularity of the rate-determining step, not from the coefficients of the overall equation (EK 5.8.A.1).
A unimolecular slow step (A → products) gives a first-order rate law, while a bimolecular slow step (2A → products) gives a second-order rate law.
A half-life that stays constant regardless of initial concentration is the fingerprint of first-order decomposition.
If a reactant only appears in a fast step after the rate-determining step, it does not appear in the rate law, so changing its concentration leaves the rate unchanged.
It's a reaction where one compound breaks down into two or more simpler substances, written generically as A → B + C. In AP Chem it's the standard setup for Unit 5 mechanism and rate law problems.
No. The order depends on the molecularity of the slow step, not on the fact that it's a decomposition. A → products as the slow step gives first order, but 2A → products as the slow step gives rate = k[A]², which is second order.
They're exact opposites. Decomposition splits one compound into multiple products, while combination (synthesis) merges multiple reactants into one compound. Count the reactants and products to tell them apart instantly.
No, and this is one of the most common AP Chem traps. The exponents in a rate law come from the rate-determining elementary step, not the overall stoichiometric coefficients. You need the mechanism or experimental data to know the order.
It tells you the reaction is first order in the decomposing compound, which means the rate-determining step is unimolecular (a single A molecule falling apart). Only first-order reactions have a half-life that is independent of initial concentration.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.