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Elementary steps Knowledge-Based Practice

Chemistry · Chemical Kinetics · Chemistry · Reaction Mechanisms & Catalysis

5 Chemistry practice questions on Elementary steps. Questions that assess core concepts and understanding.

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Questions

  1. 1.

    For the reversible elementary reaction A(g)+B(g)⇌kfkrAB(g)\text{A}(\text{g}) + \text{B}(\text{g}) \underset{k_{\text{r}}}{\overset{k_{\text{f}}}{\rightleftharpoons}} \text{AB}(\text{g}), which expression correctly represents the equilibrium constant KcK_{\text{c}} in terms of the rate constants of the elementary steps?

    • A) Kc=kf×krK_{\text{c}} = k_{\text{f}} \times k_{\text{r}}
    • B) Kc=kf−krK_{\text{c}} = k_{\text{f}} - k_{\text{r}}
    • C) Kc=kfkrK_{\text{c}} = \frac{k_{\text{f}}}{k_{\text{r}}}
    • D) Kc=krkfK_{\text{c}} = \frac{k_{\text{r}}}{k_{\text{f}}}
  2. 2.

    Why can an elementary step in a reaction mechanism never have a fractional reaction order with respect to any of its reactants?

    • A) Because the activation energy of an elementary step is quantized as an integer multiple of single bond energies.
    • B) Because elementary reactions must always establish rapid pre-equilibrium before products can form.
    • C) Because elementary steps represent discrete molecular events involving whole, indivisible chemical entities.
    • D) Because the rate-determining step always forces all participating elementary steps to adopt first-order kinetics.
  3. 3.

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

    • A) The simultaneous collision of three distinct reactant species with correct orientation and sufficient energy is statistically highly improbable.
    • B) Termolecular elementary steps inherently violate the conservation of mass during the formation of the activated complex.
    • C) The activation energy of any three-body collision is mathematically zero, which prevents the formation of stable reaction intermediates.
    • D) Collisions involving three molecules always result in purely elastic rebounds that preclude electronic rearrangement.
  4. 4.

    Which statement correctly describes an essential characteristic of an elementary step in a chemical reaction mechanism?

    • A) An elementary step proceeds through multiple distinct transition states and forms stable, isolable reaction products within that single step.
    • B) The molecularity of an elementary step reflects the actual number of colliding reactant particles and is restricted to small positive integers.
    • C) Termolecular elementary steps occur far more frequently than bimolecular steps because a larger number of particles increases overall collision frequency.
    • D) The rate law of an elementary step cannot be predicted from its stoichiometry and must always be established exclusively through empirical initial-rate experiments.
  5. 5.

    For the proposed elementary step A+2B→C\text{A} + 2\text{B} \rightarrow \text{C}, which statement correctly identifies its molecularity and corresponding rate law?

    • A) It is bimolecular with a rate law of Rate=k[A][B]\text{Rate} = k[\text{A}][\text{B}].
    • B) It is termolecular with a rate law of Rate=k[A][B]\text{Rate} = k[\text{A}][\text{B}].
    • C) It is bimolecular with a rate law of Rate=k[A][B]2\text{Rate} = k[\text{A}][\text{B}]^2.
    • D) It is termolecular with a rate law of Rate=k[A][B]2\text{Rate} = k[\text{A}][\text{B}]^2.

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