AP Chemistry · AP Chemistry CED Unit 5 Kinetics · 14 min read
1. Introduction to Reaction Energy Profiles★★☆☆☆⏱ 3 min
A reaction energy profile (also called a reaction coordinate diagram or potential energy profile) is a graphical plot that maps changes in potential energy of a chemical system as reactants convert to products along the reaction pathway. The x-axis represents the reaction coordinate, which tracks the progress of bond breaking and formation from reactants (left) to products (right), **not elapsed time**. The y-axis always represents the potential energy of the system, typically reported in kJ/mol.
This topic appears regularly on both multiple-choice (MCQ) and free-response (FRQ) sections of the AP Chemistry exam, and questions often combine energy profile interpretation with thermodynamics and reaction mechanism concepts. Common exam tasks include labeling profile components, calculating energy values, identifying rate-determining steps, and comparing catalyzed and uncatalyzed pathways.
2. Key Components of Single-Step Reaction Profiles★★☆☆☆⏱ 4 min
For a single-step (elementary) reaction, the energy profile has three core features: reactants at the left starting point, a single peak corresponding to the transition state (activated complex), and products at the right ending point. The transition state is the highest-energy, unstable state along the pathway where old bonds are partially broken and new bonds are partially formed.
Forward activation energy ($E_{a,\text{fwd}}$): Energy required to go from reactants to the transition state
Reverse activation energy ($E_{a,\text{rev}}$): Energy required to go from products back to the transition state
Enthalpy change of reaction ($\Delta H$): Overall change in potential energy for the forward reaction
\Delta H = E_{\text{products}} - E_{\text{reactants}}
A negative $\Delta H$ means the reaction is exothermic (products have lower energy than reactants), while a positive $\Delta H$ means the reaction is endothermic.
Exam tip: AP FRQ questions always require the correct sign for $\Delta H$. Missing the positive/negative sign will almost always cost you a point, so double-check the sign before moving on.
3. Multi-Step Profiles and Rate-Determining Step Identification★★★☆☆⏱ 3 min
A multi-step reaction has one elementary step per activation energy barrier, so the profile will have one peak (transition state) per elementary step. Valleys between adjacent peaks correspond to reaction intermediates: species that are formed in one early elementary step and consumed in a later step, so they do not appear in the overall balanced reaction. Intermediates are stable enough to be detected experimentally, so they occupy energy valleys that are lower than adjacent transition states, but usually higher than the initial reactants or final products.
The rate-determining step (RDS), the slowest step that limits the overall reaction rate, is always the step with the highest activation energy barrier. On an energy profile, this corresponds to the transition state with the highest energy relative to the initial starting reactants.
Exam tip: Never mislabel the x-axis as "time" on FRQ drawn responses. The x-axis is the reaction coordinate (progress of bond changes), not elapsed time, so labeling it "time" will cost you a point.
4. Catalyzed Reaction Energy Profiles★★★☆☆⏱ 3 min
A catalyst speeds up a reaction by providing an entirely alternative reaction mechanism (different reaction pathway) with a lower overall activation energy than the uncatalyzed reaction. A lower activation energy means a larger fraction of reactant molecules have enough kinetic energy to overcome the energy barrier at a given temperature, which increases the reaction rate.
A common misconception is that catalysts change the energy of reactants or products. In reality, catalysts do not affect the potential energy of the starting reactants or final products, so the overall enthalpy change $\Delta H$ of the reaction is identical for catalyzed and uncatalyzed reactions. Catalyzed pathways often have more elementary steps (and thus more peaks) than the original uncatalyzed pathway, but the highest peak (maximum activation energy) of the catalyzed pathway is always lower than the highest peak of the uncatalyzed pathway.
Exam tip: If a question asks how a catalyst affects $\Delta H$, the answer is always no change. Never state that a catalyst lowers $\Delta H$ — this is one of the most common errors tested on AP exams.
5. AP-Style Practice Problems★★★★☆⏱ 5 min
Common Pitfalls
Why: Students mix up the "final minus initial" rule for enthalpy with the "peak minus starting" rule for activation energy, leading to inverted signs
Why: Students assume every step produces an intermediate that persists to the end of the reaction
Why: Students confuse faster reaction rate with higher equilibrium yield
Why: Students rely on $E_{a,\text{rev}} = E_{a,\text{fwd}} - \Delta H$ without checking the sign of $\Delta H$ for exothermic reactions
Why: Students confuse activation energy of the individual step with the overall activation energy for the entire reaction
Why: Students intuitively associate reaction progress with elapsed time