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AP Chemistry

AP Chemistry Help: Reaction Energy Profile

Review real example questions for Reaction Energy Profile in AP Chemistry.

Question 1

A single-step reaction energy profile diagram is shown below. A student claims the reaction must be fast because it is exothermic.

Which diagram-based statement best evaluates the student’s claim about reaction speed?

  1. The claim is supported because products below reactants means the activation-energy barrier is small.
  2. The claim is not supported because reaction speed depends on the peak height above reactants, not on whether products are lower than reactants.
  3. The claim is supported because a larger drop from reactants to products guarantees more successful collisions.
  4. The claim is not supported because the horizontal distance from reactants to products determines the rate.
  5. The claim is supported because the curve after the peak slopes downward toward products.
Explanation: This question tests evaluating claims about reaction rates from reaction energy profiles. The student's claim that an exothermic reaction must be fast is not supported by the diagram because reaction speed depends on the activation energy barrier (peak height above reactants), not on whether the reaction is exothermic or endothermic. A reaction can be highly exothermic (products much lower than reactants) yet still be very slow if it has a high activation barrier. Conversely, some endothermic reactions can be fast if their activation barriers are low. The common misconception here is confusing thermodynamic favorability (exothermic = favorable) with kinetic rate (low barrier = fast). The key principle is that the height of the energy barrier, not the depth of the energy well for products, determines how fast a reaction proceeds.

Question 2

Two different single-step reactions, Reaction 1 and Reaction 2, are carried out at the same temperature. Their reaction energy profile diagrams are shown below on the same axes.

Which reaction is expected to have the faster rate at that temperature?

  1. Reaction 1, because its products are at lower potential energy than its reactants.
  2. Reaction 2, because its reactants are at higher potential energy than those of Reaction 1.
  3. Reaction 1, because it has the smaller activation-energy barrier (lower peak above reactants).
  4. Reaction 2, because it has a larger overall energy change from reactants to products.
  5. Reaction 2, because its curve is steeper near the reactant side.
Explanation: This question tests comparing reaction rates using reaction energy profiles. When comparing two reactions at the same temperature, the reaction with the lower activation energy barrier will be faster because more molecular collisions will have sufficient energy to overcome that barrier. From the diagram, Reaction 1 has a lower peak height above its reactants compared to Reaction 2, meaning Reaction 1 has the smaller activation energy. Therefore, Reaction 1 will proceed faster at the same temperature. A tempting misconception is thinking that having products at lower energy (being more exothermic) makes a reaction faster, but thermodynamic favorability does not determine kinetic rate. The strategy for comparing rates is simple: identify which reaction has the lower peak above its reactants - that reaction will be faster regardless of where the products end up.

Question 3

A chemist proposes two different single-step pathways for converting the same reactants to the same products. The reaction energy profile diagram shows two curves: Pathway 1 has a lower peak than Pathway 2, while both start at the same reactant energy level and end at the same product energy level. Based on the diagram, which statement correctly compares the initial reaction rates at the same temperature (with all other conditions identical)?

  1. Pathway 2 is faster because its products are at the same energy as Pathway 1, so the rate must be the same.
  2. Pathway 1 is faster because it has the smaller activation-energy barrier (lower peak above the reactants).
  3. Pathway 2 is faster because its activation energy is larger, which increases the number of effective collisions.
  4. Pathway 1 is faster because its products are lower in energy than the reactants, so it is more spontaneous.
  5. Pathway 2 is faster because its curve is steeper on the way to the peak, indicating a faster rate.
Explanation: This question tests understanding of reaction energy profiles. The activation energy is the vertical distance from the reactants to the peak of the energy curve, which represents the minimum energy barrier that must be overcome for the reaction to proceed. Since Pathway 1 has a lower peak than Pathway 2 (both starting from the same reactant energy), Pathway 1 has the smaller activation energy. According to the Arrhenius equation, reactions with lower activation energies have faster rates at the same temperature because more molecules possess sufficient energy to overcome the barrier. Choice D incorrectly confuses thermodynamic favorability (product stability) with kinetic rate—a reaction can be thermodynamically favorable but still slow if the activation barrier is high. To determine reaction rate from an energy profile, always measure the height of the barrier from reactants to peak, not the final energy of products.

Question 4

A student claims that the reaction with the lower-energy products must always proceed faster. Two single-step reactions, 1 and 2, are shown on the reaction energy profile diagram. Reaction 2 has lower-energy products than reaction 1, but reaction 1 has the lower peak above its reactants. Which statement correctly evaluates the student's claim using the diagram?

  1. The student is correct; reaction 2 must be faster because its products are lower in energy.
  2. The student is correct; reaction 2 must be faster because its overall energy change is larger in magnitude.
  3. The student is incorrect; reaction 1 is expected to be faster because it has the smaller activation-energy barrier (lower peak above reactants).
  4. The student is incorrect; reaction 2 is expected to be faster because its peak is higher above the reactants.
  5. The student is correct; both reactions must have the same rate because both are single-step.
Explanation: This question addresses a common misconception about reaction energy profiles. The student's claim that lower-energy products mean faster reactions confuses thermodynamics with kinetics. Reaction rate is determined by activation energy (the height of the peak above reactants), not by product stability. Since Reaction 1 has a lower peak above its reactants, it has the smaller activation energy and will proceed faster, despite having higher-energy products than Reaction 2. This demonstrates that a reaction can be thermodynamically unfavorable (uphill in energy) yet kinetically fast (low barrier). The misconception likely stems from confusing spontaneity with speed—spontaneous reactions aren't necessarily fast. When predicting reaction rates from energy profiles, focus on the activation barrier height, not the relative energies of products and reactants.

Question 5

The reaction energy profile diagram shows a single-step reaction with one peak. A catalyst is introduced that provides an alternative single-step pathway (still one peak) between the same reactants and products. Which change must be shown on the diagram for the catalyzed pathway compared with the uncatalyzed pathway?

  1. The catalyzed pathway must have a higher peak because catalysts increase collision frequency.
  2. The catalyzed pathway must have reactants at a lower energy level than before.
  3. The catalyzed pathway must have products at a lower energy level than before.
  4. The catalyzed pathway must have the same peak height but a steeper curve.
  5. The catalyzed pathway must have a lower peak (smaller activation-energy barrier) while starting and ending at the same energy levels.
Explanation: This question tests understanding of how catalysts affect reaction energy profiles. A catalyst provides an alternative reaction pathway with a lower activation energy, which appears on the diagram as a lower peak between the same reactants and products. The catalyst does not change the energies of reactants or products (thermodynamics remains unchanged), only the height of the energy barrier that must be overcome. This lower activation energy allows more molecules to react at a given temperature, increasing the reaction rate. Choice A incorrectly suggests catalysts raise the peak—catalysts never increase activation energy, and they work by lowering the barrier, not by increasing collision frequency. When identifying catalyzed pathways on energy diagrams, look for the same starting and ending points but with a lower peak in between.