A laboratory chemist wishes to produce compound D from simple reagents P and Q
CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry
Problem
A laboratory chemist wishes to produce compound D from simple reagents P and Q. Three elementary reactions that can occur in the reaction vessel are listed with their standard Gibbs free energy changes at 298 K.
(1) P+Q→RΔG1∘=+22kJmol−1
(2) R→SΔG2∘=+15kJmol−1
(3) S→DΔG3∘=−60kJmol−1
Through appropriate choice of catalysts, the chemist can promote or suppress any of these steps. The goal is to design a pathway that yields D from P and Q with the most favorable overall thermodynamics.
Answer the following questions.
(1) Part 1: (a) Propose a sequence of the elementary reactions (choose any subset, in any order, and specify stoichiometric coefficients if a reaction must occur more than once) that converts P+Q into D while eliminating all intermediates.
(2) Part 2: (b) For your proposed pathway, calculate the net standard Gibbs free energy change. Show all reasoning, including any stoichiometric scaling you performed.
(3) Part 3: (c) Justify why your chosen sequence is thermodynamically preferable to any alternative combination that also yields D from P+Q using the given elementary reactions.
(4) Part 4: (d) If experimental results show that intermediate S accumulates instead of being fully converted to D, explain how this observation affects the actual free-energy profile of the pathway and suggest one practical method to overcome the issue.