How fast? Reaction rates and rate laws, the collision model and activation energy, reaction energy profiles, multistep reaction mechanisms and the rate-determining step, and catalysis.
Eleven topics from the College Board CED, in order.
Topic 5.1
Reaction Rates
Defining rate as the change in concentration of a reactant or product over time.
Topic 5.2
Introduction to Rate Law
Writing rate laws from experimental data, and determining reaction order with respect to each reactant.
Topic 5.3
Concentration Changes Over Time
Integrated rate laws and half-life for zero-order, first-order, and second-order reactions.
Topic 5.4
Elementary Reactions
Single-step reactions whose rate law can be written directly from their molecularity.
Topic 5.5
Collision Model
Why reaction rate depends on collision frequency, correct orientation, and sufficient energy.
Topic 5.6
Reaction Energy Profile
Potential energy diagrams showing activation energy, the transition state, and overall energy change.
Topic 5.7
Introduction to Reaction Mechanisms
Multistep pathways made of elementary steps that sum to the overall reaction.
Topic 5.8
Reaction Mechanism & Rate Law
Using the rate-determining (slowest) step to predict the overall experimental rate law.
Topic 5.9
Steady-State Approximation
Handling mechanisms where the rate-determining step isn't the first step, using intermediate concentrations.
Topic 5.10
Multistep Reaction Energy Profile
Energy diagrams for multistep mechanisms, identifying intermediates and the rate-determining step's peak.
Topic 5.11
Catalysis
How catalysts speed up reactions by providing a lower-energy pathway without being consumed.
About Unit 5
Unit 5 asks a different question than the units before it: not just whether a reaction happens, but how fast. You'll learn to determine rate laws from experimental data, use the collision model and activation energy to explain why reactions speed up with temperature, and read reaction energy profiles to identify the rate-determining step in a multistep mechanism. The unit closes with catalysis — how catalysts provide a faster pathway without being consumed.
This unit is roughly 7–9% of the AP Chem exam and takes about 13–15 class periods. A critical theme to internalize early: rate laws come from experimental data, not from the coefficients of the overall balanced equation (the one exception being elementary steps, where they do match).
The College Board ties Unit 5 to two core Big Ideas:
Big Idea CE
Chemical Effects — collision frequency, orientation, and energy determine reaction rate
Big Idea TRA
Transformations — reaction mechanisms describe the step-by-step pathway of a chemical transformation