---
title: "AP Chem Unit 5 Review: Kinetics | Fiveable"
description: "AP Chemistry Unit 5 covers Reaction Rates, Introduction to Rate Law, and Concentration Changes Over Time. Study guides, practice questions, and key terms."
canonical: "https://fiveable.me/ap-chem/unit-5"
type: "unit"
subject: "AP Chemistry"
unit: "Unit 5 – Kinetics"
---

# AP Chem Unit 5 Review: Kinetics | Fiveable

## Overview

Unit 5 covers how fast chemical reactions occur and what controls that speed. You will write and use rate laws, apply integrated rate laws to concentration-time data, interpret reaction energy profiles, analyze multistep mechanisms, and explain how catalysts work at the molecular level.

## AP CED Alignment

This unit hub is organized around AP Course and Exam Description topics, skills, and exam task types when they are available in the source data.
- 5.1: Reaction Rates
- 5.2: Introduction to Rate Law
- 5.3: Concentration Changes Over Time
- 5.4: Elementary Reactions
- 5.5: Collision Model
- 5.6: Reaction Energy Profile
- 5.7: Introduction to Reaction Mechanisms
- 5.8: Reaction Mechanism and Rate Law
- 5.9: Pre-Equilibrium Approximation
- 5.10: Multistep Reaction Energy Profile
- 5.11: Catalysis
- 5.2: Rate Laws and Reaction Order
- 5.3: Integrated Rate Laws and Half-Life
- 5.4-5.5: Elementary Reactions and the Collision Model
- 5.6: Reaction Energy Profiles
- 5.7-5.8: Reaction Mechanisms and the Rate-Determining Step
- 5.10-5.11: Multistep Energy Profiles and Catalysis
- Practice 5 - Mathematical Routines
- Practice 4 - Model Analysis
- FRQ 1 – Long Answer
- FRQ 4 – Short Answer
- FRQ 2 – Long Answer

## Topics

- [5.1: Reaction Rates](/ap-chem/unit-5/reaction-rates/study-guide/4V94d3BwjoPaOOyQtDKQ): Define reaction rate as concentration change per unit time, use stoichiometric coefficients to relate rates of different species, and identify factors that increase or decrease rate.
- [5.2: Introduction to Rate Law](/ap-chem/unit-5/intro-rate-law/study-guide/oq5mJS35IadrTLHLjdqZ): Write rate = k[A]^m[B]^n, determine reaction orders using the method of initial rates, calculate k, and assign correct units based on overall reaction order.
- [5.3: Concentration Changes Over Time](/ap-chem/unit-5/concentration-changes-over-time/study-guide/Owtcz3zbxFZXsPI7YfwJ): Apply integrated rate laws to find concentration at any time, identify reaction order from linear concentration-time plots, and calculate first-order half-life using t1/2 = 0.693/k.
- [5.4: Elementary Reactions](/ap-chem/unit-5/elementary-reactions/study-guide/SPsFzzECb4aCre0wFrGg): Write rate laws directly from the stoichiometry of elementary steps, identify molecularity as unimolecular or bimolecular, and explain why termolecular steps are rare.
- [5.5: Collision Model](/ap-chem/unit-5/collision-model/study-guide/rvpVG6AlhGU9AfTvkCdv): Explain that effective collisions require both sufficient energy and correct orientation, use the Maxwell-Boltzmann distribution to show how temperature affects the fraction of reactive collisions.
- [5.6: Reaction Energy Profile](/ap-chem/unit-5/reaction-energy-profile/study-guide/RF1B5UBHtCp0qgt8ayp3): Draw and interpret single-step energy profiles, label activation energy and transition state, connect Ea to the Arrhenius equation, and distinguish exothermic from endothermic profiles.
- [5.7: Introduction to Reaction Mechanisms](/ap-chem/unit-5/intro-reaction-mechanisms/study-guide/ATIcHMtAfJw4fOwFzDGK): Identify reactants, products, intermediates, and catalysts in a mechanism, confirm that elementary steps sum to the overall balanced equation, and explain how detecting an intermediate supports a proposed mechanism.
- [5.8: Reaction Mechanism and Rate Law](/ap-chem/unit-5/reaction-mechanism-rate-law/study-guide/4v22KrcnuKsP5QYwBUu6): Derive the overall rate law from the molecularity of the rate-determining first step, and verify that the derived rate law is consistent with experimental data.
- [5.9: Pre-Equilibrium Approximation](/ap-chem/unit-5/steady-state-approximation/study-guide/uIOD7v3PLyx0Uf6RDqXJ): Apply the pre-equilibrium approximation when the first step is fast and reversible, substitute the intermediate's concentration into the slow step's rate law, and express the final rate law in terms of reactants only.
- [5.10: Multistep Reaction Energy Profile](/ap-chem/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ): Construct energy profiles for multistep mechanisms, label each transition state and intermediate, identify the rate-determining step as the highest barrier, and read the overall energy change from the diagram.
- [5.11: Catalysis](/ap-chem/unit-5/catalysts/study-guide/bkTgdolcJRgD7fG434Ru): Explain how catalysts lower activation energy and change the reaction mechanism, distinguish homogeneous, heterogeneous, and enzyme catalysis, and show catalyst regeneration in a mechanism.

## Hardest Topics And Analytics

Snapshot: practice snapshot
This snapshot uses Fiveable practice activity to show where students tend to miss questions and which review moves are worth prioritizing first.
- **63% average MCQ accuracy** (Across 14k multiple-choice practice attempts for this unit.)
- **14k MCQ attempts** (Practice activity included in this snapshot.)
- **65% average FRQ score** (Across 19 scored free-response attempts for this unit.)
- **5.9: Pre-Equilibrium Approximation**: 51% MCQ miss rate across 797 attempts. Review Pre-Equilibrium Approximation with attention to how the concept appears in AP-style source and evidence questions.
- **5.10: Multistep Reaction Energy Profile**: 41% MCQ miss rate across 3233 attempts. Review Multistep Reaction Energy Profile with attention to how the concept appears in AP-style source and evidence questions.
- **5.8: Reaction Mechanism and Rate Law**: 34% MCQ miss rate across 1575 attempts. Review Reaction Mechanism and Rate Law with attention to how the concept appears in AP-style source and evidence questions.
- **5.3: Concentration Changes Over Time**: 32% MCQ miss rate across 1708 attempts. Review Concentration Changes Over Time with attention to how the concept appears in AP-style source and evidence questions.

## Review Notes

### 5.1: Reaction Rates

Reaction rate measures how quickly reactant concentrations decrease or product concentrations increase over time, expressed in units of mol/L/s. Because stoichiometric coefficients relate the rates of different species, you must divide each species' rate of change by its coefficient to get a single reaction rate. For example, if 2 mol of NO2 are consumed for every 1 mol of O2 produced, the rate of disappearance of NO2 is twice the rate of appearance of O2. Factors that increase rate include higher reactant concentration, higher temperature, greater surface area, and the presence of a catalyst.

- **Reaction rate**: Change in concentration of a reactant or product per unit time; always reported as a positive value.
- **Stoichiometric rate relationship**: Rate = -(1/coefficient) x d[reactant]/dt = (1/coefficient) x d[product]/dt; coefficients scale the individual species rates to a common reaction rate.
- **Factors affecting rate**: Concentration, temperature, surface area, and catalysts all influence how frequently and how energetically particles collide.

**Checkpoint:** Given a balanced equation, can you write the relationship between the rate of disappearance of a reactant and the rate of appearance of a product, and explain why increasing temperature increases rate?

Factor | Effect on rate | Reason
--- | --- | ---
Increase concentration | Increases | More frequent collisions
Increase temperature | Increases | More collisions exceed activation energy
Increase surface area | Increases | More reactant particles exposed
Add catalyst | Increases | Lower activation energy pathway

### 5.2: Rate Laws and Reaction Order

A rate law has the form rate = k[A]^m[B]^n, where m and n are the orders with respect to each reactant and must be determined experimentally using the method of initial rates. To find an order, compare two experiments where only one reactant concentration changes and calculate how the rate changes. The rate constant k is temperature-dependent and has units that vary with overall order: M/s for zeroth order, 1/s for first order, and 1/(M·s) for second order.

- **Rate constant k**: Proportionality factor in the rate law; its value depends on temperature and its units depend on overall reaction order.
- **Reaction order**: The exponent of a reactant's concentration in the rate law; determined experimentally, not from stoichiometric coefficients.
- **Method of initial rates**: Compare initial rate data from experiments where one concentration is held constant to isolate and calculate each reactant's order.
- **Overall order**: Sum of all individual reactant orders in the rate law expression.

**Checkpoint:** Given a table of initial rate data with varying concentrations, can you determine the order with respect to each reactant, write the full rate law, and calculate k with correct units?

Order | Rate law form | Units of k
--- | --- | ---
Zeroth | rate = k | M/s
First | rate = k[A] | 1/s
Second | rate = k[A]^2 or k[A][B] | 1/(M·s)

### 5.3: Integrated Rate Laws and Half-Life

Integrated rate laws relate concentration directly to time, allowing you to predict concentration at any point or determine how long a reaction takes. The key skill is identifying which plot is linear: [A] vs. t for zeroth order, ln[A] vs. t for first order, and 1/[A] vs. t for second order. The slope of the linear plot equals -k for zeroth and first order, and +k for second order. First-order reactions have a constant half-life t1/2 = 0.693/k, meaning each successive half-life reduces concentration by half regardless of starting concentration. Radioactive decay is a classic first-order process.

- **Integrated rate law (first order)**: ln[A]t = ln[A]0 - kt; a plot of ln[A] vs. t is linear with slope -k.
- **Integrated rate law (second order)**: 1/[A]t = 1/[A]0 + kt; a plot of 1/[A] vs. t is linear with slope +k.
- **Half-life (first order)**: t1/2 = 0.693/k; constant and independent of initial concentration, which distinguishes first-order from other orders.
- **Graphical identification**: Test which concentration-time transformation gives a straight line to identify reaction order from experimental data.

**Checkpoint:** Given concentration-time data, can you identify the reaction order from the correct linear plot, extract k from the slope, and calculate the half-life for a first-order reaction?

Order | Linear plot | Slope | Half-life
--- | --- | --- | ---
Zeroth | [A] vs. t | -k | t1/2 = [A]0 / 2k
First | ln[A] vs. t | -k | t1/2 = 0.693/k (constant)
Second | 1/[A] vs. t | +k | t1/2 = 1/(k[A]0)

### 5.4-5.5: Elementary Reactions and the Collision Model

An elementary reaction is a single-step process, and its rate law can be written directly from its stoichiometry: a bimolecular step A + B produces rate = k[A][B]. This is the only situation where you can read orders from coefficients. For a collision to produce a reaction, it must meet two requirements: sufficient energy to overcome the activation energy barrier and correct geometric orientation of the reacting molecules. The Maxwell-Boltzmann distribution shows that only a fraction of molecules at a given temperature have enough energy to react, and that fraction increases as temperature rises.

- **Molecularity**: Number of particles colliding in an elementary step: unimolecular (1), bimolecular (2), or termolecular (3, rare).
- **Effective collision**: A collision that results in a reaction because it has both sufficient energy and correct orientation.
- **Maxwell-Boltzmann distribution**: Graph of the fraction of molecules vs. energy; the area to the right of Ea represents the fraction of molecules that can react.
- **Orientation factor**: Reactant molecules must approach each other in the correct geometry for bonds to break and form properly.

**Checkpoint:** Can you write the rate law for a given elementary step, explain why only some collisions are effective, and describe how a Maxwell-Boltzmann curve changes with increasing temperature?

### 5.6: Reaction Energy Profiles

A reaction energy profile plots potential energy along the reaction coordinate. For a single-step reaction, the diagram shows reactants on the left, a single peak at the transition state, and products on the right. The activation energy Ea is the energy difference between the reactants and the transition state peak. The overall energy change is the difference between reactant and product energy levels. A catalyst lowers the peak but does not change the energy of reactants or products. The Arrhenius equation k = Ae^(-Ea/RT) quantifies how Ea and temperature together determine the rate constant.

- **Transition state**: The highest-energy point on the reaction coordinate; represents the activated complex that must form for the reaction to proceed.
- **Activation energy (Ea)**: Energy difference between reactants and the transition state; the minimum energy required for a successful collision.
- **Arrhenius equation**: k = Ae^(-Ea/RT); shows that k increases exponentially as temperature increases or as Ea decreases.
- **Overall energy change**: Difference between product and reactant energy levels; negative for exothermic, positive for endothermic reactions.

**Checkpoint:** Given a reaction energy profile, can you identify and label Ea for the forward and reverse reactions, the transition state, and the overall energy change, and predict how a catalyst would alter the diagram?

Feature | Exothermic profile | Endothermic profile
--- | --- | ---
Products vs. reactants | Products lower in energy | Products higher in energy
Overall energy change | Negative (energy released) | Positive (energy absorbed)
Ea forward vs. reverse | Ea forward < Ea reverse | Ea forward > Ea reverse

### 5.7-5.8: Reaction Mechanisms and the Rate-Determining Step

A reaction mechanism is a sequence of elementary steps that together produce the overall balanced equation. Intermediates are species produced in one step and consumed in a later step; they do not appear in the overall equation. When the first step of a mechanism is the slowest (rate-determining step), the rate law for the overall reaction equals the rate law for that slow step, written from its molecularity. You validate a proposed mechanism by checking that the steps sum to the overall equation and that the derived rate law matches the experimental rate law.

- **Reaction intermediate**: A species formed in one elementary step and consumed in a subsequent step; present only during the reaction, not in the overall equation.
- **Rate-determining step**: The slowest elementary step in a mechanism; its molecularity sets the rate law for the overall reaction when it is the first step.
- **Mechanism validation**: Elementary steps must add up to the overall balanced equation, and the derived rate law must match experimental data.
- **Catalyst in a mechanism**: Appears as a reactant in an early step and is regenerated in a later step; net concentration is unchanged.

**Checkpoint:** Given a two-step mechanism, can you identify the intermediate, write the rate law from the slow step, and verify that the steps sum to the overall equation?

Species | Appears in overall equation? | Consumed and regenerated?
--- | --- | ---
Reactant | Yes, as starting material | No
Product | Yes, as final product | No
Intermediate | No | Produced then consumed
Catalyst | No (net) | Consumed then regenerated

### 5.9: Pre-Equilibrium Approximation

When the first step of a mechanism is fast and reversible rather than rate-limiting, you cannot directly use its molecularity to write the overall rate law. Instead, apply the pre-equilibrium approximation: set the forward and reverse rates of the fast first step equal to each other, solve for the concentration of the intermediate, and substitute that expression into the rate law for the slow second step. The result is a rate law expressed only in terms of actual reactants, which can then be compared to experimental data.

- **Pre-equilibrium approximation**: Assumes the fast reversible first step reaches equilibrium before the slow step proceeds, allowing the intermediate's concentration to be expressed in terms of reactants.
- **Intermediate substitution**: Replace [intermediate] in the slow step's rate law with the expression derived from the fast equilibrium, eliminating the intermediate from the final rate law.
- **Composite rate constant**: The observed k is a combination of the elementary rate constants from both steps: kobs = k2 x (k1/k-1).

**Checkpoint:** Given a mechanism with a fast reversible first step and a slow second step, can you apply the pre-equilibrium approximation to derive a rate law that contains only reactant concentrations?

Scenario | Rate law source | Approximation needed?
--- | --- | ---
First step is slow (rate-limiting) | Molecularity of slow step directly | No
First step is fast and reversible | Slow step after substituting intermediate | Yes, pre-equilibrium

### 5.10-5.11: Multistep Energy Profiles and Catalysis

A multistep reaction energy profile has one peak per elementary step. Each peak is a transition state and each valley between peaks is an intermediate. The tallest peak corresponds to the rate-determining step and has the largest activation energy. The overall energy change is still the difference between the final products and the initial reactants. A catalyst provides an alternative mechanism with a lower activation energy, which appears on the diagram as a new, shorter peak. The catalyst is consumed in one step and regenerated in a later step, so its net concentration does not change. Homogeneous catalysts are in the same phase as reactants; heterogeneous catalysts are in a different phase and often work by adsorbing reactants onto a surface.

- **Multistep energy profile**: A diagram with multiple peaks (transition states) and valleys (intermediates); the highest peak marks the rate-determining step.
- **Catalyst effect on profile**: Lowers activation energy by providing a new pathway; does not change the energies of reactants or products or the overall energy change.
- **Homogeneous catalysis**: Catalyst is in the same phase as the reactants, such as an acid catalyst in aqueous solution.
- **Heterogeneous catalysis**: Catalyst is in a different phase, such as a solid metal surface catalyzing a gas-phase reaction by adsorbing reactants and orienting them favorably.
- **Catalyst regeneration**: The catalyst is consumed in one elementary step and produced again in a later step, keeping its net concentration constant throughout the reaction.

**Checkpoint:** Can you draw and label a two-step energy profile, identify the intermediate and rate-determining step, and show how adding a catalyst would change the diagram without changing the overall energy difference?

Catalyst type | Phase relationship | How it lowers Ea
--- | --- | ---
Homogeneous | Same phase as reactants | Provides alternative reaction pathway in solution
Heterogeneous | Different phase from reactants | Adsorbs reactants onto surface, improving orientation
Enzyme | Same phase (aqueous) | Binds substrate at active site, stabilizing transition state

## Study Guides

- [5.8 Reaction Mechanism and Rate Law](/ap-chem/unit-5/reaction-mechanism-rate-law/study-guide/4v22KrcnuKsP5QYwBUu6)
- [5.7 Introduction to Reaction Mechanisms](/ap-chem/unit-5/intro-reaction-mechanisms/study-guide/ATIcHMtAfJw4fOwFzDGK)
- [5.6 Reaction Energy Profile](/ap-chem/unit-5/reaction-energy-profile/study-guide/RF1B5UBHtCp0qgt8ayp3)
- [5.11 Catalysis](/ap-chem/unit-5/catalysts/study-guide/bkTgdolcJRgD7fG434Ru)
- [5.1 Reaction Rates](/ap-chem/unit-5/reaction-rates/study-guide/4V94d3BwjoPaOOyQtDKQ)
- [5.3 Concentration Changes Over Time](/ap-chem/unit-5/concentration-changes-over-time/study-guide/Owtcz3zbxFZXsPI7YfwJ)
- [5.4 Elementary Reactions](/ap-chem/unit-5/elementary-reactions/study-guide/SPsFzzECb4aCre0wFrGg)
- [5.2 Introduction to Rate Law](/ap-chem/unit-5/intro-rate-law/study-guide/oq5mJS35IadrTLHLjdqZ)
- [5.5 Collision Model](/ap-chem/unit-5/collision-model/study-guide/rvpVG6AlhGU9AfTvkCdv)
- [5.10 Multistep Reaction Energy Profile](/ap-chem/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ)
- [5.9 Pre-Equilibrium Approximation](/ap-chem/unit-5/steady-state-approximation/study-guide/uIOD7v3PLyx0Uf6RDqXJ)

## Practice Preview

### Multiple-choice practice

- **Stimulus-based practice question**: Practice 5 - Mathematical Routines | Using the proposed mechanism and the data in the figure, what was the initial concentration of $N_2O_5$ in Experiment 2?
- **Stimulus-based practice question**: Practice 4 - Model Analysis | Which statement best identifies species Y?
- **Stimulus-based practice question**: Practice 4 - Model Analysis | Which statement best explains why \ce{ClO} is absent from the overall equation?
- **Stimulus-based practice question**: Practice 4 - Model Analysis | Which letter best represents the intermediate and its concentration?
- **Stimulus-based practice question**: Practice 5 - Mathematical Routines | Which approach gives the overall balanced equation?
- **Stimulus-based practice question**: Practice 5 - Mathematical Routines | Based on the concentration profiles in the graph, which mathematical relationship or definition correctly identifies species $Z$ as a catalyst?

### FRQ practice

- **Nitrogen monoxide and chlorine gas reaction kinetics**: FRQ 1 – Long Answer | Nitrogen monoxide and chlorine gas reaction kinetics
- **Hypochlorite decomposition activation energy and catalysis**: FRQ 4 – Short Answer | Hypochlorite decomposition activation energy and catalysis
- **Nitrogen monoxide and hydrogen gas reaction kinetics**: FRQ 2 – Long Answer | Nitrogen monoxide and hydrogen gas reaction kinetics

## Key Terms

- **Activation Energy**: The minimum energy required for a collision to result in a reaction; the energy difference between the reactants and the transition state on an energy profile.
- **Rate Laws**: Mathematical expressions of the form rate = k[A]^m[B]^n that relate reaction rate to reactant concentrations; orders must be determined experimentally.
- **Rate Constant**: The proportionality factor k in the rate law; its value depends on temperature and its units depend on the overall reaction order.
- **Integrated Rate Law**: An equation relating reactant concentration to time; the form differs by order: [A] vs. t (zeroth), ln[A] vs. t (first), 1/[A] vs. t (second).
- **Half-Life**: The time required for reactant concentration to fall to half its initial value; for first-order reactions, t1/2 = 0.693/k and is constant regardless of initial concentration.
- **Collision Theory**: The model stating that reactions occur only when molecules collide with both sufficient energy to exceed the activation energy and the correct geometric orientation.
- **Effective Collisions**: Collisions that result in a reaction because they meet both the energy requirement (Ea) and the orientation requirement for bond rearrangement.
- **Transition State**: The highest-energy arrangement of atoms along the reaction coordinate; represents the activated complex that must form for reactants to become products.
- **Elementary Step**: A single-step reaction in a mechanism whose rate law can be written directly from the stoichiometry of the colliding particles.
- **Molecularity**: The number of particles that collide in an elementary step: unimolecular (1), bimolecular (2), or termolecular (3, rare).
- **Reaction Intermediate**: A species produced in one elementary step and consumed in a later step; it does not appear in the overall balanced equation.
- **Rate-Determining Step**: The slowest elementary step in a mechanism; when it is the first step, its molecularity directly sets the rate law for the overall reaction.
- **Catalyst**: A substance that increases reaction rate by providing a pathway with lower activation energy; it is consumed in one step and regenerated in a later step, so its net concentration is unchanged.
- **Reaction Order**: The exponent of a reactant's concentration in the rate law; the sum of all exponents gives the overall order of the reaction.

## Common Mistakes

- **Reading reaction orders from the balanced equation**: Reaction orders must come from experimental data. The coefficients in the overall balanced equation do not equal the exponents in the rate law unless the reaction is a single elementary step and that is explicitly stated.
- **Confusing intermediates with catalysts**: An intermediate is produced in one step and consumed in a later step; it is not present at the start. A catalyst is present at the start, consumed in one step, and regenerated later. Both are absent from the overall equation, but for different reasons.
- **Assuming half-life is constant for all reaction orders**: Only first-order reactions have a constant half-life (t1/2 = 0.693/k). For zeroth-order reactions, half-life decreases as concentration decreases. For second-order reactions, half-life increases as concentration decreases.
- **Writing the rate law from the slow step when the first step is fast**: If the first step is fast and reversible, you cannot directly use its molecularity. You must apply the pre-equilibrium approximation to express the intermediate's concentration in terms of reactants before writing the final rate law.
- **Thinking a catalyst changes the overall energy of the reaction**: A catalyst lowers the activation energy and changes the mechanism, but it does not change the energies of the reactants or products. The overall enthalpy change for the reaction is the same with or without a catalyst.

## Exam Connections

- **Data analysis and rate law derivation**: AP Chemistry frequently presents a table of initial rate data and asks you to determine reaction orders, write the rate law, and calculate k. A related task is identifying reaction order from a set of concentration-time graphs by recognizing which plot is linear. Be prepared to justify each order with a ratio calculation rather than just stating the answer.
- **Mechanism analysis and consistency checking**: A common task presents a proposed two- or three-step mechanism and asks you to identify intermediates, derive the rate law from the rate-determining step, and evaluate whether the mechanism is consistent with an experimentally determined rate law. You may also need to apply the pre-equilibrium approximation when the first step is fast and reversible.
- **Energy profile interpretation and catalysis explanation**: Questions often ask you to draw or annotate a reaction energy profile, labeling activation energies, transition states, intermediates, and overall energy change. A follow-up task typically asks you to explain how a catalyst changes the diagram and the mechanism, or to compare homogeneous and heterogeneous catalysis in terms of how each lowers the activation energy.

## Final Review Checklist

- **Write and use rate laws**: Given initial rate data, determine each reactant's order, write the complete rate law, calculate k with correct units, and predict how rate changes when concentration changes.
- **Apply integrated rate laws**: Identify reaction order from linear concentration-time plots ([A] vs. t, ln[A] vs. t, 1/[A] vs. t), extract k from the slope, and use the first-order half-life formula t1/2 = 0.693/k.
- **Interpret reaction energy profiles**: Label activation energy, transition state, intermediates, and overall energy change on both single-step and multistep diagrams, and show how a catalyst alters the profile.
- **Analyze reaction mechanisms**: Identify intermediates and catalysts, verify that elementary steps sum to the overall equation, and derive the rate law from the rate-determining step or via the pre-equilibrium approximation.
- **Connect collision theory to rate**: Use the Maxwell-Boltzmann distribution to explain why temperature increases rate, and describe how activation energy and molecular orientation determine whether a collision is effective.
- **Explain catalysis mechanistically**: Describe how a catalyst lowers Ea, changes the mechanism, is consumed and regenerated, and distinguish homogeneous, heterogeneous, and enzyme catalysis by phase and binding mechanism.
- **Use the Arrhenius equation**: Explain qualitatively how k depends on Ea and temperature using k = Ae^(-Ea/RT), and predict the direction of change in k when temperature or activation energy changes.

## Study Plan

- **Step 1: Understand rate laws and concentration-time data (5.1-5.3)**: Start with the foundational math of kinetics. Practice using the method of initial rates to find reaction orders and k. Then work through integrated rate law problems: given a data table, test which plot is linear, read k from the slope, and calculate half-life for first-order cases. Use the topic guides for 5.1, 5.2, and 5.3 to check your understanding of stoichiometric rate relationships and units of k.
- **Step 2: Build the molecular picture with collision theory and energy profiles (5.4-5.6)**: Review elementary reactions and molecularity, then connect them to the collision model. Practice sketching Maxwell-Boltzmann distributions and explaining how temperature shifts the curve. Draw single-step energy profiles from scratch, labeling Ea, the transition state, and the overall energy change. Make sure you can connect Ea to the Arrhenius equation qualitatively.
- **Step 3: Work through reaction mechanisms (5.7-5.9)**: Practice identifying intermediates and catalysts in written mechanisms, verifying that steps sum to the overall equation, and deriving rate laws. Do problems where the first step is rate-limiting (5.8) and problems where you must apply the pre-equilibrium approximation (5.9). Compare your derived rate law to a given experimental rate law to check mechanism validity.
- **Step 4: Interpret multistep energy profiles and catalysis (5.10-5.11)**: Draw two- and three-step energy profiles, label every transition state and intermediate, and identify the rate-determining step as the highest barrier. Then add a catalyst to the diagram and explain what changes and what stays the same. Review the differences among homogeneous, heterogeneous, and enzyme catalysis using the topic guides for 5.10 and 5.11.
- **Step 5: Practice with mixed problems and use available tools**: Work through the 25+ practice questions and FRQ practice available for this unit, focusing on problems that ask you to move between experimental data, rate laws, mechanisms, and energy diagrams in a single question. Use the AP score calculator to estimate your estimated score range and identify which topic areas need more attention before your exam.

## More Ways To Review

- [Topic study guides](/ap-chem/unit-5#topics)
- [FRQ practice](/ap-chem/frq-practice)
- [Cram archive videos](/cram-archives?subject=ap-chemistry&unit=unit-5)
- [Cheatsheets](/ap-chem/cheatsheets/unit-5)
- [Key terms](/ap-chem/key-terms)

## FAQs

### What topics are covered in AP Chem Unit 5?

AP Chem Unit 5 covers 11 topics in kinetics: Reaction Rates, Introduction to Rate Law, Concentration Changes Over Time, Elementary Reactions, the Collision Model, Reaction Energy Profile, Introduction to Reaction Mechanisms, Reaction Mechanism and Rate Law, Pre-Equilibrium Approximation, Multistep Reaction Energy Profile, and Catalysis. Together these topics explain why reactions happen at different speeds, how rate laws are written and used, and how catalysis speeds up reactions without being consumed. See [AP Chem Unit 5](/ap-chem/unit-5) for matched practice on each topic.

### How much of the AP Chem exam is Unit 5?

AP Chem Unit 5 makes up 7-9% of the AP exam. That weight covers all of kinetics, including rate law, reaction mechanisms, the collision model, and catalysis. It's a focused unit, but the concepts show up in FRQs regularly, so understanding how reaction rates are determined and manipulated pays off on exam day.

### What's on the AP Chem Unit 5 progress check (MCQ and FRQ)?

The AP Chem Unit 5 progress check includes both MCQ and FRQ parts drawn from all 11 kinetics topics. MCQ questions test rate law interpretation, concentration changes over time, the collision model, and reaction energy profiles. FRQ questions typically ask you to determine rate laws from experimental data, analyze reaction mechanisms, or explain how catalysis affects activation energy. The progress check is College Board's built-in check on whether you can apply these concepts, not just recall them. Practicing with questions matched to each topic on [AP Chem Unit 5](/ap-chem/unit-5) is a solid way to prepare for both parts.

### How do I practice AP Chem Unit 5 FRQs?

AP Chem Unit 5 FRQs most often ask you to determine a rate law from experimental data, identify the rate-determining step in a reaction mechanism, or explain how catalysis lowers activation energy. These questions reward clear, step-by-step reasoning, so writing out your logic matters as much as the final answer. Good practice steps:
- Work through rate law problems where you solve for reaction orders from initial rate data.
- Practice drawing and interpreting reaction energy profiles for both catalyzed and uncatalyzed reactions.
- Write out mechanism steps and connect them to the overall rate law. You can find FRQ-style practice questions organized by topic at [AP Chem Unit 5](/ap-chem/unit-5).

### Where can I find AP Chem Unit 5 practice questions?

The best place to find AP Chem Unit 5 practice questions, including MCQ and full practice test sets, is [AP Chem Unit 5](/ap-chem/unit-5). Questions there are organized by topic, so you can target rate law, the collision model, reaction mechanisms, or catalysis specifically rather than hunting through a mixed set. For the most useful practice, start with MCQ on the topics you find hardest, then move to FRQ-style questions that ask you to interpret data or explain mechanisms in writing. That combination covers both question formats you'll see on the real exam.

### How should I study AP Chem Unit 5?

Start AP Chem Unit 5 by building a solid understanding of rate law before moving to mechanisms. If you can't write a rate law from experimental data, the later topics on reaction mechanisms and the pre-equilibrium approximation won't click. A practical study sequence:
1. Learn how to determine reaction orders from initial rate tables (5.2).
2. Practice integrated rate laws and half-life calculations for concentration changes over time (5.3).
3. Study the collision model and reaction energy profiles together, since activation energy connects both (5.5, 5.6).
4. Work through multistep mechanisms and practice identifying the rate-determining step (5.7, 5.8).
5. Finish with catalysis, focusing on how it changes the energy profile without shifting equilibrium (5.11). Kinetics is one of those units where doing problems beats re-reading notes. Use [AP Chem Unit 5](/ap-chem/unit-5) to get topic-specific practice at each step.

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See <a href=\"/ap-chem/unit-5\">AP Chem Unit 5</a> for matched practice on each topic."}},{"@type":"Question","@id":"https://fiveable.me/ap-chem/unit-5#how-much-of-the-ap-chem-exam-is-unit-5","name":"How much of the AP Chem exam is Unit 5?","acceptedAnswer":{"@type":"Answer","text":"AP Chem Unit 5 makes up 7-9% of the AP exam. That weight covers all of kinetics, including rate law, reaction mechanisms, the collision model, and catalysis. It's a focused unit, but the concepts show up in FRQs regularly, so understanding how reaction rates are determined and manipulated pays off on exam day."}},{"@type":"Question","@id":"https://fiveable.me/ap-chem/unit-5#whats-on-the-ap-chem-unit-5-progress-check-mcq-and-frq","name":"What's on the AP Chem Unit 5 progress check (MCQ and FRQ)?","acceptedAnswer":{"@type":"Answer","text":"The AP Chem Unit 5 progress check includes both MCQ and FRQ parts drawn from all 11 kinetics topics. MCQ questions test rate law interpretation, concentration changes over time, the collision model, and reaction energy profiles. FRQ questions typically ask you to determine rate laws from experimental data, analyze reaction mechanisms, or explain how catalysis affects activation energy. The progress check is College Board's built-in check on whether you can apply these concepts, not just recall them. Practicing with questions matched to each topic on <a href=\"/ap-chem/unit-5\">AP Chem Unit 5</a> is a solid way to prepare for both parts."}},{"@type":"Question","@id":"https://fiveable.me/ap-chem/unit-5#how-do-i-practice-ap-chem-unit-5-frqs","name":"How do I practice AP Chem Unit 5 FRQs?","acceptedAnswer":{"@type":"Answer","text":"AP Chem Unit 5 FRQs most often ask you to determine a rate law from experimental data, identify the rate-determining step in a reaction mechanism, or explain how catalysis lowers activation energy. These questions reward clear, step-by-step reasoning, so writing out your logic matters as much as the final answer. Good practice steps:\n- Work through rate law problems where you solve for reaction orders from initial rate data.\n- Practice drawing and interpreting reaction energy profiles for both catalyzed and uncatalyzed reactions.\n- Write out mechanism steps and connect them to the overall rate law. You can find FRQ-style practice questions organized by topic at <a href=\"/ap-chem/unit-5\">AP Chem Unit 5</a>."}},{"@type":"Question","@id":"https://fiveable.me/ap-chem/unit-5#where-can-i-find-ap-chem-unit-5-practice-questions","name":"Where can I find AP Chem Unit 5 practice questions?","acceptedAnswer":{"@type":"Answer","text":"The best place to find AP Chem Unit 5 practice questions, including MCQ and full practice test sets, is <a href=\"/ap-chem/unit-5\">AP Chem Unit 5</a>. Questions there are organized by topic, so you can target rate law, the collision model, reaction mechanisms, or catalysis specifically rather than hunting through a mixed set. For the most useful practice, start with MCQ on the topics you find hardest, then move to FRQ-style questions that ask you to interpret data or explain mechanisms in writing. That combination covers both question formats you'll see on the real exam."}},{"@type":"Question","@id":"https://fiveable.me/ap-chem/unit-5#how-should-i-study-ap-chem-unit-5","name":"How should I study AP Chem Unit 5?","acceptedAnswer":{"@type":"Answer","text":"Start AP Chem Unit 5 by building a solid understanding of rate law before moving to mechanisms. If you can't write a rate law from experimental data, the later topics on reaction mechanisms and the pre-equilibrium approximation won't click. A practical study sequence:\n1. Learn how to determine reaction orders from initial rate tables (5.2).\n2. Practice integrated rate laws and half-life calculations for concentration changes over time (5.3).\n3. Study the collision model and reaction energy profiles together, since activation energy connects both (5.5, 5.6).\n4. Work through multistep mechanisms and practice identifying the rate-determining step (5.7, 5.8).\n5. Finish with catalysis, focusing on how it changes the energy profile without shifting equilibrium (5.11). Kinetics is one of those units where doing problems beats re-reading notes. Use <a href=\"/ap-chem/unit-5\">AP Chem Unit 5</a> to get topic-specific practice at each step."}}]}
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