Elementary steps Knowledge-Based Download PDF

Questions

1. 1.

   Consider the gas-phase elementary step $\text{A(g)} + \text{B(g)} \rightarrow \text{C(g)}$. By what factor does the rate of this elementary step increase if the partial pressure of $\text{A}$ is doubled and the partial pressure of $\text{B}$ is tripled at constant temperature?

   • A) 2
   • B) 3
   • C) 5
   • D) 6
2. 2.

   Which of the following characteristics is impossible for a single elementary step in a proposed reaction mechanism?

   • A) Requiring a positive activation energy to reach the transition state
   • B) Proceeding through a bimolecular collision between two different molecules
   • C) Having a rate constant that increases when the temperature is raised
   • D) Exhibiting fractional or negative reaction orders in its rate law
3. 3.

   Why are termolecular elementary steps exceptionally rare in proposed chemical reaction mechanisms?

   • A) They always proceed with negative activation energies that prevent the formation of an activated complex.
   • B) The probability of three independent particles colliding simultaneously with sufficient energy and proper orientation is extremely low.
   • C) Termolecular elementary steps violate the law of mass action because their reaction orders cannot be deduced from stoichiometry.
   • D) Quantum mechanical selection rules strictly forbid three chemical species from exchanging electrons during a single step.
4. 4.

   An elementary reaction step in a mechanism is given by $\text{O}_3\text{(g)} + \text{NO(g)} \rightarrow \text{NO}_2\text{(g)} + \text{O}_2\text{(g)}$. What is the molecularity of this elementary step, and what is its corresponding rate law?

   • A) Bimolecular, with a rate law of $\text{Rate} = k[\text{O}_3][\text{NO}]$
   • B) Bimolecular, with a rate law that cannot be deduced without empirical overall kinetic data
   • C) Termolecular, with a rate law of $\text{Rate} = k[\text{O}_3]^2[\text{NO}]$
   • D) Unimolecular, with a rate law of $\text{Rate} = k[\text{O}_3]$
5. 5.

   Which of the following statements correctly describes an elementary step in a reaction mechanism?

   • A) Its rate law can be written directly from its stoichiometric coefficients because it occurs in a single molecular event.
   • B) Its overall order is determined strictly by initial rates experiments rather than its stoichiometric coefficients.
   • C) It always proceeds through multiple intermediate transition states before forming stable products.
   • D) Its molecularity can be zero, fractional, or negative depending on reaction conditions.