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⏱️ Unit 5 · Kinetics 🏠 Unit Hub 🗂 Flashcards 🗺 Cheat Sheet ⭐ Essentials 🎨 Visual Review 📝 MC Practice ✍️ SAQ Practice

AP Chemistry Unit 5 Essentials

The must-know terms and big ideas for Unit 5: Kinetics. Every vocabulary word and concept you need to master.

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Big Idea 1
Rate laws come from experiments, not coefficients
Unlike stoichiometry, where the balanced equation's coefficients directly give you mole ratios, the overall rate law of a reaction must be determined experimentally — by observing how rate changes as you vary the concentration of each reactant. The one exception is an elementary step, where the rate law genuinely can be written directly from its molecularity, because an elementary step describes an actual single collision event.
Rate Law Reaction Order Elementary Reactions
Big Idea 2
The slowest step controls the whole reaction's speed
A reaction mechanism breaks an overall reaction into a sequence of elementary steps. Just like a multi-lane highway narrowing to one lane, the slowest step — the rate-determining step — acts as a bottleneck that controls the overall observed rate, no matter how fast the other steps are. The collision model and activation energy together explain why some steps are slow (high activation energy, poor collision geometry) and others are fast. Catalysts work by opening up an entirely new, lower-energy pathway, lowering the activation energy of the rate-determining step without being consumed.
Reaction Mechanisms Rate-Determining Step Catalysis
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Reaction rate
The change in concentration of a reactant or product per unit time, typically expressed as mol/(L·s).
Reaction Rates
Rate law
An equation relating reaction rate to the concentrations of reactants, each raised to an experimentally determined power (order).
Rate Law
Rate constant (k)
The proportionality constant in a rate law, specific to a given reaction at a given temperature; larger k means a faster reaction.
Rate Law
Reaction order
The exponent on a reactant's concentration in the rate law, describing how strongly that reactant's concentration affects rate. Determined experimentally, not from the balanced equation (except for elementary steps).
Rate Law
Overall reaction order
The sum of all individual reaction orders in the rate law.
Rate Law
Integrated rate law
A rate law expressed as concentration as a function of time, derived by integrating the differential rate law; takes a different mathematical form for zero-, first-, and second-order reactions.
Concentration vs. Time
Half-life
The time required for a reactant's concentration to decrease to half its initial value. Constant for first-order reactions regardless of starting concentration.
Concentration vs. Time
Elementary reaction
A single-step reaction that occurs in one collision event; its rate law can be written directly from its molecularity (the coefficients of the elementary step).
Elementary Reactions
Molecularity
The number of reactant particles involved in an elementary step — unimolecular (one), bimolecular (two), or termolecular (three, rare).
Elementary Reactions
Collision theory
The model stating that for a reaction to occur, reactant particles must collide with sufficient energy and proper orientation.
Collision Model
Activation energy (Ea)
The minimum energy required for reactant collisions to result in a successful reaction; higher activation energy means a slower reaction at a given temperature.
Collision Model
Arrhenius equation
An equation relating the rate constant to temperature and activation energy, showing that rate constants increase sharply as temperature increases or activation energy decreases.
Collision Model
Reaction energy profile
A graph of potential energy versus reaction progress, showing the activation energy barrier and the overall energy change between reactants and products.
Energy Profile
Transition state (activated complex)
The highest-energy, unstable arrangement of atoms at the peak of the activation energy barrier during a reaction step.
Energy Profile
Reaction mechanism
The sequence of elementary steps that together make up an overall reaction; the steps must sum to give the overall balanced equation.
Reaction Mechanisms
Intermediate
A species produced in one step of a reaction mechanism and consumed in a later step; it does not appear in the overall balanced equation.
Reaction Mechanisms
Rate-determining step
The slowest elementary step in a mechanism, which controls the overall rate of the reaction since it acts as a bottleneck.
Reaction Mechanisms
Steady-state approximation
A method for deriving the overall rate law from a mechanism in cases where the rate-determining step isn't the first step, by treating an intermediate's concentration as approximately constant.
Reaction Mechanisms
Catalyst
A substance that increases reaction rate by providing an alternative pathway with lower activation energy, without being permanently consumed in the overall reaction.
Catalysis
Homogeneous vs. heterogeneous catalyst
A homogeneous catalyst exists in the same phase as the reactants (e.g., dissolved in the same solution); a heterogeneous catalyst exists in a different phase (e.g., a solid catalyzing a gas-phase reaction).
Catalysis