Day 35 Podia Question

Day 35 Pre-class Podia Problem: Activated Complex (Transition State)

Consider the gas-phase decomposition of cyclobutane, C4H8, to ethylene, C2H4, which occurs via a unimolecular elementary reaction:

In this figure, structural formulas are used to illustrate a chemical reaction. On the left, a structural formula for cyclobutane is shown. This structure is composed of 4 C atoms connected with single bonds in a square shape. Each C atom is bonded to two other C atoms in the structure, leaving two bonds for H atoms pointing outward above, below, left, and right. An arrow points right to two identical ethane molecules with a plus symbol between them. Each of these molecules contains two C atoms connected with a double bond oriented vertically between them. The C atom at the top of these molecules has H atoms bonded above to the right and left. Similarly, the lower C atom has two H atoms bonded below to the right and left.

1. Write the rate law for this reaction. Express the rate of reaction in terms of concentration. (Don’t just write “rate =”, use Δ[A]/Δt.)

2. Describe in words which bonds must break and which bonds must form when this reaction occurs.

3. Based on your description in part 2, suggest a structure for the activated complex (transition state) for this reaction. Use dotted lines to indicate existing bonds that are partially broken or new bonds that are partially formed.

4. At the temperature of the kinetic study the reaction energy change, Δr = – 300 kJ/mol. The activation energy, Ea = 261 kJ/mol. Use this information to draw a reaction energy diagram. Make the vertical axis scale as accurate as possible. Draw the structures of the reactant, the transition state, and the product molecules at appropriate positions on the diagram.

Two days before the next whole-class session, this Podia question will become live on Podia, where you can submit your answer.

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.