Reaction Energy Profile
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AP Chemistry › Reaction Energy Profile
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?
Reaction 2, because its curve is steeper near the reactant side.
Reaction 1, because its products are at lower potential energy than its reactants.
Reaction 1, because it has the smaller activation-energy barrier (lower peak above reactants).
Reaction 2, because its reactants are at higher potential energy than those of Reaction 1.
Reaction 2, because it has a larger overall energy change from reactants to products.
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.
Two single-step reactions, I and II, are carried out at the same temperature. Their reaction energy profiles are shown.
Potential energy
|
| ‡ (I)
| / \
| Reactants / ___ Products (I)
|
| ‡ (II)
| / \
| Reactants__/ ________ Products (II)
|
+--------------------------------------> Reaction coordinate
Based on the diagrams, which statement best compares the initial reaction rates of I and II?
Reaction II is slower because its products are at higher potential energy than its reactants
Reaction I is faster because its products are at lower potential energy than its reactants
Reaction I is faster because its curve is steeper on the way to the peak
The two reactions have the same rate because both diagrams show only one step
Reaction II is faster because it has a lower activation-energy barrier (lower peak above reactants)
Explanation
This question tests understanding of a reaction energy profile. Activation energy is determined by the vertical height from the reactants to the highest point on the curve, representing the transition state. A lower activation energy allows more reactant molecules to surmount the barrier, leading to a higher reaction rate at a given temperature. Comparing the two profiles, Reaction II has a shorter vertical rise to its peak, indicating lower activation energy and thus a faster rate. One tempting distractor is choice A, which confuses the overall energy change (exothermicity) with activation energy, but the net energy drop affects thermodynamics, not kinetics. Always remember that in reaction energy profiles, the height of the barrier, not the depth of products, determines rate.