AP Chemistry Quiz: Multistep Reaction Energy Profile

What this quiz covers

This quiz focuses on Multistep Reaction Energy Profile, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Step 1
2. Step 2 (correct answer)
3. Step 3
4. Step 3
5. Step 1

Explanation: This question tests the skill of analyzing a multistep reaction energy profile. The rate-determining step is identified by finding which step has the largest activation energy barrier—the vertical distance from the starting point of that step to its transition state. Even though the overall reaction is exergonic (large negative energy change), this doesn't affect which step is rate-determining; we must compare the three activation barriers: reactants to TS1, I1 to TS2, and I2 to TS3. The correct answer indicates that the I1 to TS2 barrier is the largest. A common error is confusing the overall energy change (ΔE) with activation energy. Always identify the rate-determining step by finding the tallest activation barrier, not the most exergonic step.

Question 2

1. Step 4
2. Step 3 (correct answer)
3. Step 2
4. Step 1
5. Step 5

Explanation: This query tests multistep reaction energy profiles. The rate-determining step is identified by the highest activation energy, which is the most significant energy hurdle in the pathway. This is the maximum vertical distance from an energy minimum to the subsequent transition state. With the third step having a visibly larger barrier than the first two, it controls the rate. One might pick Step 1 (choice D) due to the misconception that the initial step is always rate-limiting, ignoring actual barrier heights. The strategy to remember is that the tallest barrier controls the rate across the entire mechanism.

Question 3

1. Step 4
2. Step 1
3. Step 5
4. Step 2 (correct answer)
5. Step 3

Explanation: This question tests the skill of interpreting a multistep reaction energy profile. In a reaction energy profile, the rate-determining step is the slowest step in the mechanism, which is determined by the step with the highest activation energy barrier. The activation energy is the energy difference between the starting point of a step (reactants or intermediate) and the peak of the transition state for that step. Therefore, the step with the largest vertical rise from its starting valley to its transition state peak will be the rate-determining step, as it requires the most energy for molecules to overcome. A common misconception is to identify the rate-determining step as the one with the highest absolute transition state energy, but this confuses overall energy levels with activation energy barriers; for example, choice A (Step 1) might seem tempting if TS1 appears high, but the rise to TS2 is larger. Always remember that in multistep reactions, the tallest barrier relative to its own starting point controls the rate.

Question 4

1. Step 2 (correct answer)
2. Step 1
3. Step 3
4. Step 1
5. Step 3

Explanation: This problem involves analyzing a multistep reaction energy profile. The rate-determining step is identified by the highest activation energy barrier, measured from the starting point of each step to its transition state. The question notes that TS2 is the highest point on the diagram and that I2 is close in energy to TS3, with Step 2 being the correct answer, confirming that the I1 to TS2 barrier is the largest. Students might be distracted by the fact that I2 is close to TS3, thinking this makes Step 3 fast, but this is correct—a small barrier means a fast step. Focus on finding the largest activation barrier by measuring each valley-to-peak distance.