Reactants are the substances present at the start of a chemical reaction that get consumed as new substances (products) form; in AP Chem they appear on the left side of a balanced equation, in the denominator of K and Q expressions, and as the concentration terms in a rate law.
Reactants are the starting materials of a chemical reaction. They sit on the left side of the arrow in a balanced chemical equation, and as the reaction proceeds, their amounts decrease while products form. A chemical change means the reactants are transformed into new substances with different compositions (EK 4.1.A.2), and you can often spot it through evidence like gas formation, a precipitate, a color change, or heat and light.
On the AP exam, "reactant" is less a vocab word and more a position in every major equation you write. In a rate law, the rate is proportional to each reactant's concentration raised to a power (EK 5.2.A.2). In an equilibrium expression, reactant concentrations go in the denominator of K and Q (Topic 7.3). In a Hess's law style calculation, you subtract the formation enthalpies of the reactants (EK 6.8.A.1). If you can correctly identify what counts as a reactant, and what doesn't, half of these setups take care of themselves.
Reactants are the thread running through Units 4 through 7. Unit 4 (Topics 4.1 and 4.3) asks you to recognize when reactants have actually changed into new substances and to represent that change with particulate models (AP Chem 4.1.A, 4.3.A). Unit 5 builds the whole field of kinetics on how fast reactants disappear: rate is defined as the conversion of reactants to products per unit time (EK 5.1.A.1), the stoichiometry of the balanced equation sets the relative rates of change (EK 5.1.A.2), and rate laws are written in terms of reactant concentrations (AP Chem 5.2.A). Unit 6 puts reactants at the starting point of every energy diagram (AP Chem 6.2.A, 5.6.A) and in the subtracted term of ΔH°rxn = ΣΔH°f products − ΣΔH°f reactants (AP Chem 6.8.A). Unit 7 makes reactants the denominator of Q and K (AP Chem 7.3.A), so the size of K tells you whether reactants or products dominate at equilibrium (AP Chem 7.5.A), and comparing Q to K tells you whether the system will consume reactants or regenerate them (EK 7.7.A.2).
Keep studying AP Chemistry Unit 5
Products (Unit 4)
Products are the flip side of reactants. Every relationship in the course pairs them, since rates compare reactant consumption to product formation, K compares product concentrations to reactant concentrations, and ΔH° is products minus reactants. If you mix up which side is which, the sign or the fraction flips.
Rate Law and Reaction Order (Unit 5)
A rate law only contains reactant concentrations, each raised to a power called the order with respect to that reactant (EK 5.2.A.2-A.3). The catch is that the exponents come from experimental data, not from the coefficients in the balanced equation, unless you're looking at a single elementary step.
Reaction Quotient and Equilibrium Constant (Unit 7)
Reactants live in the denominator of Q and K, raised to their stoichiometric coefficients. That placement does real work, because a very small K means the mixture is mostly reactants at equilibrium (EK 7.5.A.1), and when Q > K the reaction runs in reverse and regenerates reactants (EK 7.7.A.2).
Energy Diagrams and Activation Energy (Units 5-6)
Reactants are the left edge of every energy profile. The climb from the reactants' energy up to the transition state is the activation energy (EK 5.6.A.3), and comparing the reactants' energy to the products' energy tells you whether the process is exothermic or endothermic (AP Chem 6.2.A).
You won't see "define reactant" on the exam. Instead, the exam tests whether you can use reactants correctly inside bigger setups. Multiple-choice questions ask you to write rate laws from mechanisms, like the iodide-catalyzed decomposition of H₂O₂, where you have to recognize that the rate law uses the reactants of the slow step (rate = k[H₂O₂][I⁻]) even though I⁻ isn't a reactant in the overall equation. Equilibrium MCQs test whether you know that K = 1 means reactant and product concentrations are comparable, not equal to zero. On FRQs, reactants show up constantly. The 2017 long FRQ built on N₂ and O₂ reacting to form NO, the 2018 FRQs centered on identifying reactants in redox and NO/NO₂ systems, and the 2019 FRQ used excess Ca(NO₃)₂ as a reactant to drive a precipitation to completion. The recurring tasks are the same every year. Write the equilibrium expression with reactants in the denominator, subtract reactant formation enthalpies in ΔH° calculations, predict which direction a stress shifts the reactant-product balance using Le Châtelier's principle, and explain why a limiting or excess reactant controls the outcome.
A catalyst looks like a reactant in a mechanism because it's consumed in an early step, often the rate-determining one. The difference is that a catalyst gets regenerated in a later step, so its net concentration stays constant (EK 5.11.A.2) and it never appears in the overall balanced equation. A true reactant is permanently consumed. The trap on the exam is a mechanism question where the catalyst appears in the rate law (since it's a reactant of the slow step) even though it isn't a reactant overall. An intermediate is the opposite case. It's produced first and consumed later, and it shouldn't appear in the final rate law at all.
Reactants are the substances on the left side of a balanced equation that get consumed as the reaction converts them into products.
Rate laws are written only in terms of reactant concentrations, and the exponents (orders) come from experimental data, not from the balanced equation's coefficients.
In equilibrium expressions, reactant concentrations or partial pressures go in the denominator of K and Q, raised to their stoichiometric coefficients.
When Q < K, the reaction consumes reactants to make products; when Q > K, it consumes products to regenerate reactants; when Q = K, both happen at equal rates.
In ΔH° calculations, you subtract the sum of the reactants' standard enthalpies of formation: ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants).
A species consumed in an early mechanism step but regenerated later is a catalyst, not a reactant, even though it can appear in the rate law of the slow step.
Reactants are the starting substances in a chemical reaction, written on the left side of the balanced equation. They're consumed over time as the reaction forms products, and their concentrations drive both the rate law (Unit 5) and the denominator of K and Q (Unit 7).
No, not in general. The order with respect to each reactant must be determined from experimental data (EK 5.2.A.2-A.3). The only time coefficients become exponents directly is for a single elementary step in a mechanism, like the slow step that sets the overall rate law.
A reactant is permanently consumed, while a catalyst is consumed in one step and regenerated in a later step, so its net concentration never changes (EK 5.11.A.2). For example, in the iodide-catalyzed decomposition of H₂O₂, I⁻ reacts in step 1 but comes back in step 2, so it's a catalyst even though it appears in the rate law.
No. At equilibrium, forward and reverse reactions run at the same rate, so reactants and products coexist at constant concentrations. A very large K means the reaction goes essentially to completion with few reactants left, while a very small K means the mixture is still mostly reactants (EK 7.5.A.1).
In the denominator. For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b, so each reactant's concentration is raised to its coefficient and divided into the product terms. Pure solids and liquids are left out entirely.