5.10 Multistep Reaction Energy Profile

A multistep reaction energy profile (a potential-energy vs. reaction-coordinate diagram) shows the energy changes for each elementary step in a reaction mechanism. On the graph you plot: reactants at left, products at right, and for each elementary step draw a peak (transition state) and a valley (reaction intermediate). Label each peak’s activation energy (Ea from valley to peak) and the overall ΔH (difference from reactants to final products). Identify the highest Ea peak as the rate-determining step. Mark intermediates in the wells between peaks. Show exothermic steps dropping to lower product energy and endothermic steps rising. You can add a catalyzed pathway with a lower-energy peak to show catalysis. Use terms: activation energy, transition state, intermediate, reaction coordinate, energy barrier, overall enthalpy change, Hammond postulate. This is directly tested in AP free-response (represent Ea and ΔH). For a quick visual guide, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ). For more practice, use Fiveable’s AP Chem problem set (https://library.fiveable.me/practice/ap-chemistry).

Because a multistep reaction is actually several elementary steps, its energy profile shows one peak (a transition state) for each elementary step. Between peaks you get energy wells that represent reaction intermediates. Each peak’s height = that step’s activation energy; the tallest peak is the rate-determining step (slowest step) and controls the observed reaction rate. The overall ΔH is just the energy difference between starting reactants and final products, not the sum of peak heights. You can also use the Hammond postulate to relate the structure of a transition state to the stability of intermediates (early TS for exothermic steps, late TS for endothermic). This is exactly what Topic 5.10 asks you to represent on a potential energy diagram (activation energy, transition state, intermediate, rate-determining step, overall enthalpy). For a clear guide and practice drawing these, check the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and more Unit 5 resources (https://library.fiveable.me/ap-chemistry/unit-5).

Yes—each elementary step in a multistep mechanism has its own activation energy. On an energy profile, every step is shown as its own energy hill (reactants → transition state → products for that step), and Ea for that step is the energy difference between the step’s reactants (or intermediate) and its transition state. The step with the largest Ea is usually the rate-determining step because it’s the highest energy barrier the system must overcome. The overall ΔH of the reaction is just the difference between the very first reactants and the final products, not the sum of the individual E a’s. Remember Hammond’s postulate: the structure of a transition state resembles the nearest stable species (reactant-like for exothermic, product-like for endothermic), which helps you interpret profiles. This is exactly what AP 5.10.A asks you to represent on a multistep energy diagram. For a quick review and examples, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).

Overall energy change (ΔHnet) for a multistep reaction is just the difference between the potential energy of products and reactants—or equivalently the algebraic sum of the enthalpy changes for each elementary step. On an energy profile, draw a horizontal line at reactants and one at products; ΔHoverall = Eproducts − Ereactants (negative = exothermic). If you only have step values, add them (ΔH1 + ΔH2 + ... = ΔHoverall). Activation energies and transition states matter for rate and the rate-determining step, but they don’t change ΔHoverall. On the AP exam you may be asked to label activation energy, intermediates (energy wells), and the overall ΔH on the profile (CED 5.10.A). For extra practice and examples, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ), the Unit 5 overview (https://library.fiveable.me/ap-chemistry/unit-5), and practice problems (https://library.fiveable.me/practice/ap-chemistry).

Elementary reactions are the individual steps in a mechanism—each has its own energy barrier (activation energy, Ea), transition state (peak), and can form short-lived reaction intermediates (energy wells). On an energy profile you draw a separate hump for each elementary step; the tallest hump usually corresponds to the rate-determining step. The overall reaction is the net change from starting reactants to final products: its ΔH (overall enthalpy change) is the vertical difference between the reactants’ and products’ energy levels, not the sum of peak heights. So: elementary steps → multiple activation energies and intermediates (show multiple peaks/wells); overall reaction → single ΔH (showed as start-to-end vertical gap) and the effective barrier often determined by the highest Ea. This is exactly what the CED expects you to represent for Topic 5.10 (5.10.A)—see the multistep energy-profile study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ). For extra practice on related kinetics ideas check Unit 5 (https://library.fiveable.me/ap-chemistry/unit-5) and the AP practice set (https://library.fiveable.me/practice/ap-chemistry).

Intermediates show up on a multistep energy diagram as local minima—little “valleys” or wells between two peaks. They are lower in potential energy than the transition states (peaks) but can be higher or lower than the original reactants or final products. Each valley represents a short-lived, relatively stable species formed after one elementary step and before the next. Important exam points: label each transition state (peak), the activation energy for each step (distance from a valley to the next peak), and the overall ΔH (reactants → products)—this is exactly what CED 5.10.A expects you to represent. The rate-determining step corresponds to the highest activation barrier (tallest peak). Hammond’s postulate helps predict whether a transition state looks more like reactants or products depending on whether the step is exothermic or endothermic. For a quick review, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and try practice problems at (https://library.fiveable.me/practice/ap-chemistry).

Think of a multistep mechanism as a series of hills you must climb (reaction coordinate diagram). Each elementary step has its own energy barrier (activation energy, Ea) and transition state. The overall reaction rate is controlled by the slowest “hill”—the step with the highest Ea—because the rate constant k depends exponentially on Ea (Arrhenius: k = A e^(−Ea/RT)). A much larger Ea makes k very small, so that step produces/reacts intermediates far slower than the others; downstream faster steps can’t speed up what’s waiting. That highest-Ea step is the rate-determining step (RDS). In some cases a pre-equilibrium exists (fast reversible step before a slow RDS), but the bottleneck still sets the observed rate law because concentrations of intermediates adjust around it. For AP exam framing, be ready to identify the RDS on an energy profile and connect it to the rate law and transition state (CED keywords: activation energy, transition state, reaction intermediate, rate-determining step). See the Topic 5.10 study guide for examples (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ). For more practice, check Unit 5 resources and 1000+ practice problems (https://library.fiveable.me/ap-chemistry/unit-5 and https://library.fiveable.me/practice/ap-chemistry).

Look at the energy profile and find the step with the largest activation energy—the tallest energy barrier between a reactant/intermediate valley and a transition-state peak. That elementary step with the highest Ea (highest peak above its preceding energy well) is the rate-determining step (RDS) because it’s the slowest step kinetically. Don’t confuse overall ΔH (product minus reactant energy) with the RDS; ΔH tells you exo/endothermicity, not the rate. Label each transition state and intermediate, measure Ea for each step (vertical difference from valley to peak), and the biggest Ea = RDS. The Hammond postulate can help predict whether the transition state resembles reactants or products, but it doesn’t change which peak is tallest. This matches CED 5.10.A (activation energy, transition state, intermediates, and rate-determining step). For more examples and practice problems, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and Unit 5 resources (https://library.fiveable.me/ap-chemistry/unit-5).

In a multistep reaction the potential energy profile is just the sum of each elementary step’s energies. Each step has its own energy barrier (activation energy, Ea) and a transition state peak; between peaks you get energy wells (reaction intermediates). The overall enthalpy change ΔH for reactants → products equals the algebraic sum of the ΔH values for each step (Hess’s-law style), so ΔH is path-independent even if individual Ea’s differ. The rate is controlled by the step with the largest Ea (rate-determining step). A catalyst provides an alternate multistep pathway with lower Ea’s (lower peaks) but doesn’t change overall ΔH. On AP problems you may be asked to label Ea, transition states, intermediates, and overall ΔH on the diagram—practice that using the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and Unit 5 review (https://library.fiveable.me/ap-chemistry/unit-5). For extra practice, check the AP-style problems on Fiveable (https://library.fiveable.me/practice/ap-chemistry).

Think of each elementary step as its own little hill on the same hiking trail (reaction coordinate). To build the overall energy diagram: - Put reactants at the left energy level and products on the right; overall ΔH is the vertical difference (products − reactants). This is what AP wants you to represent (5.10.A). - For each elementary reaction draw a peak (transition state) and a valley (intermediate) in sequence. The peak height above the preceding valley = that step’s activation energy (Ea). - The highest peak (largest Ea) is the rate-determining step—label it. Intermediate wells are species that aren’t reactants or products. - Add the individual ΔH values stepwise: the sum of step ΔH’s = overall ΔH (so exothermic steps drop, endothermic steps rise). - Use Hammond postulate to relate transition-state structure to reaction energetics when interpreting shapes. If you want step-by-step examples and practice, check the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and Unit 5 resources (https://library.fiveable.me/ap-chemistry/unit-5). For extra practice, try problems at (https://library.fiveable.me/practice/ap-chemistry).

Look at the overall change in potential energy from reactants to products on the energy profile—that net change (ΔH) tells you if the multistep reaction is endothermic or exothermic. If the product energy is higher than the reactant energy, ΔH is positive → overall endothermic. If product energy is lower, ΔH is negative → overall exothermic. You can also algebraically add the enthalpy changes of each elementary step (sum of step ΔH’s = overall ΔH). Activation energies, transition states, and which step is rate-determining affect kinetics but do NOT change the overall ΔH. Use the energy wells for intermediates and the initial and final vertical positions to read ΔH on a reaction coordinate diagram (CED keywords: activation energy, transition state, reaction intermediate, overall enthalpy change). For a quick refresher on drawing/reading multistep profiles, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ). For broader Unit 5 review and practice, check the unit page and practice set (https://library.fiveable.me/ap-chemistry/unit-5) (https://library.fiveable.me/practice/ap-chemistry).

Transition states = the peaks (energy maxima) on a reaction coordinate. Each corresponds to an activated complex for an elementary step, has the highest potential energy along that step, is transient (can’t be isolated), and is often drawn with a double-dagger (‡). Intermediates = the valleys (local minima) between peaks. They’re real species that exist briefly between steps, can sometimes be isolated or detected, and have lower energy than the surrounding transition states. On a multistep energy profile you use transition-state heights to read activation energies (Ea) for each elementary step; the largest Ea usually controls the overall rate (rate-determining step). Knowing the difference helps you show activation energy and overall ΔH on the profile (CED 5.10.A.1). For a quick refresher and practice, check the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and Unit 5 resources (https://library.fiveable.me/ap-chemistry/unit-5). For extra practice problems see (https://library.fiveable.me/practice/ap-chemistry).

You need multistep reactions because most real reactions don’t happen in one single step—they proceed by elementary steps with their own activation energies, transition states, and intermediates. Learning multistep energy profiles lets you: draw potential-energy diagrams for each elementary reaction, identify the rate-determining step (the highest energy barrier), show intermediates in energy wells, and compare the overall ΔH to the sum of step energies. That matters on the AP because Topic 5.10 asks you to represent activation energy and overall energy change for mechanisms (use terms like activation energy, transition state, reaction intermediate, reaction coordinate, Hammond postulate, catalyzed pathway). It also helps you predict how catalysts or a pre-equilibrium affect rates. For the AP exam, practice drawing and labeling these diagrams—see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and more Unit 5 review (https://library.fiveable.me/ap-chemistry/unit-5). For extra practice, try problems at (https://library.fiveable.me/practice/ap-chemistry).

A catalyst provides an alternative reaction pathway with lower activation energies for one or more elementary steps. On a multistep energy profile you’ll see one or more peaks (transition states) reduced in height—most importantly the peak for the rate-determining step—so the reaction rate increases. The catalyst does NOT change the overall enthalpy change (ΔH) between reactants and products, nor the position of equilibrium; it only changes the energy barriers and the relative heights of intermediate wells/transition states. Sometimes a catalyst changes which step is rate-determining. Use the energy profile to label activation energies for each step, the highest peak as the RDS, and overall ΔH (CED Topic 5.10 keywords: activation energy, transition state, reaction intermediate, rate-determining step, catalyzed pathway). For more examples and diagrams, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and Unit 5 overview (https://library.fiveable.me/ap-chemistry/unit-5). For extra practice, try problems at (https://library.fiveable.me/practice/ap-chemistry).

Yes—a multistep mechanism can have some elementary steps that are endothermic and others that are exothermic. On a multistep potential energy diagram you’ll see multiple peaks (transition states) and wells (reaction intermediates). Each step has its own ΔH (step enthalpy): some steps can lie above their reactants (endothermic) and others below (exothermic). The overall ΔHrxn is the sum of the step enthalpies (so the final products vs. initial reactants determine if the whole reaction is net exo- or endothermic). The rate-determining step is the step with the largest activation energy (largest peak) and controls the observed rate even if other steps are exothermic. When you draw or interpret these profiles, label activation energies, transition states, intermediates, and the overall energy change. For a clear AP-aligned example and practice, see the Topic 5.10 study guide (https://library.fiveable.me/ap-chemistry/unit-5/multistep-reaction-energy-profile/study-guide/f9jHiVcKKI9odrJrUITZ) and more Unit 5 resources (https://library.fiveable.me/ap-chemistry/unit-5). For extra practice, check the 1000+ AP problems at (https://library.fiveable.me/practice/ap-chemistry).