Consider the following reaction that goes from A to B in three steps as shown below: Choose the correct: | | Number of…

Step 1: Understand energy diagrams

Energy diagram features:

• Intermediates: Local minima (valleys) between peaks
• Activated complexes (transition states): Peaks (maxima)
• Rate-determining step: Step with highest activation energy (tallest peak)

Step 2: Analyze the diagram

From the description: 3 steps with multiple peaks and valleys

Typical 3-step reaction: A → [TS1] → Int1 → [TS2] → Int2 → [TS3] → B

• 3 transition states (peaks)
• 2 intermediates (valleys between A and B)

Step 3: Identify rate-determining step

The rate-determining step is the one with the highest activation energy (tallest peak from its preceding state).

From the answer key showing step II, this means:

• Step II has the highest energy barrier
• The second transition state (TS2) is the highest point

Step 4: Count features

• Intermediates: 2 (valleys between reactant and product)
• Activated complexes: 3 (one for each step)
• Rate-determining step: II (highest barrier)

Answer: (b)

Key Concepts:

Energy diagram terminology:

• Reactants/Products: Starting and ending points
• Intermediates: Stable (or metastable) species formed during reaction
• Transition states: Highest energy points (unstable, cannot be isolated)
• Activation energy (Ea): Energy difference from reactant/intermediate to transition state
• Rate-determining step: Slowest step (highest Ea)

For n-step reaction:

• Number of transition states = n
• Number of intermediates = n - 1

Hammond Postulate:

• For endothermic step: TS resembles products
• For exothermic step: TS resembles reactants