Question

1. Which of the following statements about the ratio of products formed by a reaction under kinetic control are true? (there can be more than one answer)

□ The ratio of products is determined by the relative energies of transition states The ratio of products is determined by the energies of products. The ratio of products is not determined by the energies of products. □ The ratio of products is not determined by the relative energies of transition states.

2. Which of the following statements relates the value and magnitude of ?Hf to stability of a product? (only one answer)

Answer 1

Concepts and reason

The ratio of the products depends on both thermodynamic reaction control and kinetic reaction control.

Under kinetic control, the composition of the products formed depends on the energy of transition state.

Under thermodynamic control, the composition of the products formed depend on the change in free energy of the reaction.

Fundamentals

Thermodynamics deals with the feasibility of the reaction. If $\Delta {\rm{G}} < 0$ , then the reaction is feasible. If $\Delta {\rm{G}} > 0$ , then the reaction is not feasible.

The kinetic study deals with the rate of reaction. Although the reaction is feasible under thermodynamics, it may take few hours or days to occur the reaction if the activation energy is high.

(1)

Under kinetic control, if the energy of transition state is low for one product, then it forms that product easily than the other which has higher energy transition state. Hence, the ratio of products is determined by the relative energies of transition states.

Under thermodynamic control, the ratio of products is determined by the energies of products.

Thus, the correct statements for the ratio of products formed under kinetic control are as follows:

The ratio of products is determined by the relative energies of transition states.

The ratio of products is not determined by the energies of products.

(2)

If product has lower energy than reactants, then the product is more stable than reactant.

If the product has higher energy than reactants, the product is less stable than reactant.

Hence, the correct statement is as follows:

A large negative value of $\Delta {{\rm{H}}_{\rm{f}}}$ indicates a stable product.

Ans: Part 1

The true statements for the ratio of products formed under kinetic control are as follows:

The ratio of products is determined by the relative energies of transition states.

The ratio of products is not determined by the energies of products.