A chemist studies the gas-phase isomerization of compound R into two possible…
CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry
Problem
A chemist studies the gas-phase isomerization of compound R into two possible products, P1 and P2. A verbal description of the potential-energy profile is as follows:
• Reactant R is taken as the zero of potential energy.
• The transition state leading to P1 is 55kJ mol−1 above R, whereas the transition state leading to P2 is 75kJ mol−1 above R.
• The Gibbs free energies of the isolated products at 298K are ΔG∘(P1)=−10kJ mol−1 and ΔG∘(P2)=−30kJ mol−1, respectively.
Assume the reaction is elementary in each direction and that no reverse reaction occurs during short reaction times at low temperature.
(a) Define what is meant by (i) kinetic control and (ii) thermodynamic control of a reaction.
(b) On the basis of the numerical data provided, identify which product is the kinetic product and which is the thermodynamic product. Justify your choices.
(c) The reaction mixture is held at −20∘C for 2min and then rapidly quenched to 0∘C. Predict which product will be present in the higher mole fraction and explain your reasoning without performing a rate calculation.
(d) A separate sample of R is heated to 150∘C and maintained for 4h before cooling. Predict which product will now predominate and provide a thermodynamic argument for your prediction.
(e) Describe qualitatively how adding a catalyst that lowers both activation barriers by the same amount would affect the product distribution for a reaction conducted at 25∘C for 30s.