AP Chemistry Quiz: Elementary Reactions
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Elementary ReactionsQuestion 1 of 20

A researcher proposes the following elementary gas-phase step:

CO(g)+Cl2(g)→COCl2(g)\text{CO}(g)+\text{Cl}_2(g)\rightarrow \text{COCl}_2(g)

Which rate law must apply if the step is elementary?

rate=k[CO]2[Cl2]\text{rate}=k[\text{CO}]^2[\text{Cl}_2]
rate=k[COCl2]\text{rate}=k[\text{COCl}_2]
rate=k[CO][Cl2]\text{rate}=k[\text{CO}][\text{Cl}_2]
rate=k[CO]\text{rate}=k[\text{CO}]
rate=k[Cl2]\text{rate}=k[\text{Cl}_2]
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AP Chemistry Quiz

AP Chemistry Quiz: Elementary Reactions

Practice Elementary Reactions in AP Chemistry with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Elementary Reactions, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A researcher proposes the following elementary gas-phase step:

CO(g)+Cl2(g)→COCl2(g)\text{CO}(g)+\text{Cl}_2(g)\rightarrow \text{COCl}_2(g)

Which rate law must apply if the step is elementary?

  1. rate=k[CO]2[Cl2]\text{rate}=k[\text{CO}]^2[\text{Cl}_2]
  2. rate=k[COCl2]\text{rate}=k[\text{COCl}_2]
  3. rate=k[CO][Cl2]\text{rate}=k[\text{CO}][\text{Cl}_2] (correct answer)
  4. rate=k[CO]\text{rate}=k[\text{CO}]
  5. rate=k[Cl2]\text{rate}=k[\text{Cl}_2]

Explanation: This question examines understanding of elementary reactions. Elementary steps represent single collision events, and the rate law exponents equal the stoichiometric coefficients of the reactants. For CO(g)+Cl2(g)→COCl2(g)\text{CO}(g) + \text{Cl}_2(g) \rightarrow \text{COCl}_2(g), one CO molecule collides with one Cl2_2 molecule, giving rate=k[CO][Cl2]\text{rate} = k[\text{CO}][\text{Cl}_2]. Choice A incorrectly shows [CO]2[\text{CO}]^2, which would mean two CO molecules collide with one Cl2_2 molecule—this doesn't match the given elementary step. Always verify that your rate law exponents match the coefficients in the elementary step equation.

Question 2

In an aqueous solution, the following elementary step is proposed:

H+(aq)+OH−(aq)→H2O(l)\text{H}^+(aq)+\text{OH}^-(aq)\rightarrow \text{H}_2\text{O}(l)

Using the definition of an elementary step, which rate law is correct for this reaction?

  1. rate=k[H+][OH−]\text{rate}=k[\text{H}^+][\text{OH}^-] (correct answer)
  2. rate=k[H+]2[OH−]\text{rate}=k[\text{H}^+]^2[\text{OH}^-]
  3. rate=k[H2O]\text{rate}=k[\text{H}_2\text{O}]
  4. rate=k[OH−]\text{rate}=k[\text{OH}^-]
  5. rate=k[H+]+k[OH−]\text{rate}=k[\text{H}^+]+k[\text{OH}^-]

Explanation: This problem tests knowledge of elementary reactions. For elementary steps, the molecularity (number of molecules colliding) equals the sum of stoichiometric coefficients, and these coefficients become the exponents in the rate law. In H⁺(aq) + OH⁻(aq) → H₂O(l), one H⁺ ion collides with one OH⁻ ion, yielding rate = k[H⁺][OH⁻]. Choice C incorrectly uses the product concentration [H₂O], but rate laws for elementary steps depend only on reactant concentrations. The key strategy is to count the number of each reactant species in the elementary step and use those numbers as exponents.

Question 3

A chemist studying radical reactions proposes the following elementary step:

Cl(g)+O3(g)→ClO(g)+O2(g)\text{Cl}(g)+\text{O}_3(g)\rightarrow \text{ClO}(g)+\text{O}_2(g)

Which rate law follows directly from this step being elementary?

  1. rate=k[Cl]\text{rate}=k[\text{Cl}]
  2. rate=k[Cl][O3]\text{rate}=k[\text{Cl}][\text{O}_3] (correct answer)
  3. rate=k[O3]\text{rate}=k[\text{O}_3]
  4. rate=k[ClO][O2]\text{rate}=k[\text{ClO}][\text{O}_2]
  5. rate=k[Cl]2[O3]\text{rate}=k[\text{Cl}]^2[\text{O}_3]

Explanation: This question requires applying the concept of elementary reactions. In an elementary step, the rate law exponents match exactly the stoichiometric coefficients of the reactants because the step represents a single molecular collision event. For Cl(g)+O3(g)→ClO(g)+O2(g)\text{Cl}(g) + \text{O}_3(g) \rightarrow \text{ClO}(g) + \text{O}_2(g), one Cl atom collides with one O3\text{O}_3 molecule, giving rate=k[Cl][O3]\text{rate} = k[\text{Cl}][\text{O}_3]. Choice E incorrectly shows [Cl]2[\text{Cl}]^2, suggesting two Cl atoms collide simultaneously, which contradicts the given elementary step. Remember that only for elementary steps can you directly translate coefficients to rate law exponents.

Question 4

In a proposed gas-phase process, the following step is explicitly stated to be an elementary reaction:

2 NO2(g)→2 NO(g)+O2(g)2\,\text{NO}_2(g)\rightarrow 2\,\text{NO}(g)+\text{O}_2(g)

Which rate law is implied by this step being elementary?

  1. rate=k[NO2]\text{rate}=k[\text{NO}_2]
  2. rate=k[NO2]2\text{rate}=k[\text{NO}_2]^2 (correct answer)
  3. rate=k[NO]2[O2]\text{rate}=k[\text{NO}]^2[\text{O}_2]
  4. rate=k[NO][O2]\text{rate}=k[\text{NO}][\text{O}_2]
  5. rate=k[NO2]1/2\text{rate}=k[\text{NO}_2]^{1/2}

Explanation: This problem requires understanding of elementary reactions. For an elementary step, the rate law follows directly from the stoichiometric coefficients of the reactants. Since two NO₂ molecules must collide in this elementary reaction, the rate law is rate = k[NO₂]². The coefficient 2 becomes the exponent 2 because this represents an actual bimolecular collision between two NO₂ molecules. Choice C incorrectly uses the products NO and O₂ instead of the reactant NO₂, confusing the direction of the reaction. Always use reactant concentrations with coefficients as exponents for elementary steps.

Question 5

In an aqueous solution, the following step is stated to be an elementary reaction:

Br−(aq)+H2O2(aq)+H+(aq)→HOBr(aq)+H2O(l)\text{Br}^-(aq)+\text{H}_2\text{O}_2(aq)+\text{H}^+(aq)\rightarrow \text{HOBr}(aq)+\text{H}_2\text{O}(l)

Using the definition of an elementary step, which rate law is correct?

  1. rate=k[Br−][H+]\text{rate}=k[\text{Br}^-][\text{H}^+]
  2. rate=k[Br−][H2O2]\text{rate}=k[\text{Br}^-][\text{H}_2\text{O}_2]
  3. rate=k[Br−]1/2[H2O2]1/2[H+]\text{rate}=k[\text{Br}^-]^{1/2}[\text{H}_2\text{O}_2]^{1/2}[\text{H}^+]
  4. rate=k[Br−][H2O2][H+]\text{rate}=k[\text{Br}^-][\text{H}_2\text{O}_2][\text{H}^+] (correct answer)
  5. rate=k[H2O2][H+]2\text{rate}=k[\text{H}_2\text{O}_2][\text{H}^+]^2

Explanation: This problem tests knowledge of elementary reactions. For an elementary step, the rate law is constructed by raising each reactant concentration to the power of its stoichiometric coefficient. This elementary reaction shows one Br⁻, one H₂O₂, and one H⁺ ion colliding simultaneously, so the rate law is rate = k[Br⁻][H₂O₂][H⁺]. All three species must be included because they all participate in the elementary collision event. Choice A omits H⁺, incorrectly treating it like a catalyst rather than a reactant in the elementary step. The key strategy is that elementary steps allow you to write the rate law directly from the balanced equation without experimental data.

Question 6

In the gas phase, the following step is explicitly stated to be an elementary reaction:

2 SO2(g)+O2(g)→2 SO3(g)2\,\text{SO}_2(g)+\text{O}_2(g)\rightarrow 2\,\text{SO}_3(g)

According to the definition of an elementary step, which rate law is correct?

  1. rate=k[SO2][O2]\text{rate}=k[\text{SO}_2][\text{O}_2]
  2. rate=k[SO2]2\text{rate}=k[\text{SO}_2]^2
  3. rate=k[SO2]2[O2]\text{rate}=k[\text{SO}_2]^2[\text{O}_2] (correct answer)
  4. rate=k[SO2][O2]2\text{rate}=k[\text{SO}_2][\text{O}_2]^2
  5. rate=k[SO2]2/3[O2]1/3\text{rate}=k[\text{SO}_2]^{2/3}[\text{O}_2]^{1/3}

Explanation: This question involves applying knowledge of elementary reactions. In an elementary reaction, the rate law is determined by the molecularity - how many molecules must collide. This step requires two SO₂ molecules and one O₂ molecule to collide, giving rate = k[SO₂]²[O₂]. The exponents match the stoichiometric coefficients exactly because this represents the actual trimolecular collision event. Choice A incorrectly uses first-order dependence on SO₂, failing to account for the coefficient 2 in the elementary step. For elementary reactions, always use stoichiometric coefficients as exponents in the rate law.

Question 7

A gas-phase recombination is described by the following elementary step:

H(g)+H(g)→H2(g)\text{H}(g)+\text{H}(g)\rightarrow \text{H}_2(g)

Which rate law follows from the step being elementary?

  1. rate=k[H]\text{rate}=k[\text{H}]
  2. rate=k[H]2\text{rate}=k[\text{H}]^2 (correct answer)
  3. rate=k[H2]\text{rate}=k[\text{H}_2]
  4. rate=k[H]1/2\text{rate}=k[\text{H}]^{1/2}
  5. rate=k[H][H2]\text{rate}=k[\text{H}][\text{H}_2]

Explanation: This problem tests understanding of elementary reactions. For an elementary step, the rate law reflects the actual molecular collision event, with reactant concentrations raised to powers matching their coefficients. This reaction shows two H atoms colliding, so the rate law is rate = k[H]². The exponent 2 comes from needing two H atoms to collide simultaneously in this elementary step. Choice C incorrectly uses H₂ (the product) instead of H (the reactant), confusing products with reactants in the rate law. Remember that elementary steps allow direct translation from balanced equation to rate law using reactant coefficients.

Question 8

A solution-phase reaction step is stated to be an elementary process:

S2O82−(aq)+I−(aq)→SO42−(aq)+SO4∙−(aq)+I∙(aq)\text{S}_2\text{O}_8^{2-}(aq)+\text{I}^-(aq)\rightarrow \text{SO}_4^{2-}(aq)+\text{SO}_4^{\bullet-}(aq)+\text{I}^{\bullet}(aq)

Which rate law is consistent with an elementary step?

  1. rate=k[S2O82−]\text{rate}=k[\text{S}_2\text{O}_8^{2-}]
  2. rate=k[I−]\text{rate}=k[\text{I}^-]
  3. rate=k[S2O82−][I−]\text{rate}=k[\text{S}_2\text{O}_8^{2-}][\text{I}^-] (correct answer)
  4. rate=k[S2O82−]2[I−]\text{rate}=k[\text{S}_2\text{O}_8^{2-}]^2[\text{I}^-]
  5. rate=k[SO42−][I∙]\text{rate}=k[\text{SO}_4^{2-}][\text{I}^{\bullet}]

Explanation: This question tests understanding of elementary reactions. For an elementary step, the rate law is determined by the molecularity—the number of reactant particles that must collide. The elementary reaction S₂O₈²⁻(aq) + I⁻(aq) → SO₄²⁻(aq) + SO₄•⁻(aq) + I•(aq) shows one S₂O₈²⁻ ion colliding with one I⁻ ion, giving rate = k[S₂O₈²⁻][I⁻]. Choice E incorrectly uses products (SO₄²⁻ and I•) in the rate law, but elementary step rate laws depend only on reactant concentrations. When writing rate laws for elementary reactions, use only the reactants with their stoichiometric coefficients as exponents.

Question 9

A chemist identifies the following as a single elementary step in the gas phase:

NO(g)+NO3(g)→2NO2(g)\text{NO}(g)+\text{NO}_3(g)\rightarrow 2\text{NO}_2(g)

Which rate law must be true for this elementary step?

  1. rate=k[NO][NO3]\text{rate}=k[\text{NO}][\text{NO}_3] (correct answer)
  2. rate=k[NO]2[NO3]\text{rate}=k[\text{NO}]^2[\text{NO}_3]
  3. rate=k[NO2]2\text{rate}=k[\text{NO}_2]^2
  4. rate=k[NO]2\text{rate}=k[\text{NO}]^2
  5. rate=k[NO3]2\text{rate}=k[\text{NO}_3]^2

Explanation: This question tests understanding of elementary reactions. In an elementary reaction, the rate law reflects exactly how many molecules must collide for the reaction to occur. The elementary step NO(g) + NO₃(g) → 2NO₂(g) shows one NO molecule colliding with one NO₃ molecule, so the rate law must be rate = k[NO][NO₃]. Choice C incorrectly uses the product NO₂ in the rate law, but rate laws for elementary steps are based only on reactant concentrations and their stoichiometric coefficients. For elementary reactions, write the rate law directly from the balanced equation using reactant coefficients as exponents.

Question 10

In a mechanism study, one step is identified and explicitly labeled as an elementary reaction:

Cl(g)+O3(g)→ClO(g)+O2(g)\text{Cl}(g)+\text{O}_3(g)\rightarrow \text{ClO}(g)+\text{O}_2(g)

Which rate law is consistent with this elementary step?

  1. rate=k[ClO][O2]\text{rate}=k[\text{ClO}][\text{O}_2]
  2. rate=k[Cl]2[O3]\text{rate}=k[\text{Cl}]^2[\text{O}_3]
  3. rate=k[Cl][O3]\text{rate}=k[\text{Cl}][\text{O}_3] (correct answer)
  4. rate=k[O3]2\text{rate}=k[\text{O}_3]^2
  5. rate=k[Cl]12[O3]\text{rate}=k[\text{Cl}]^\tfrac{1}{2}[\text{O}_3]

Explanation: This question tests understanding of elementary reactions. For an elementary step, the rate law is determined by the molecularity—the number of molecules that must collide for the reaction to occur. The elementary reaction Cl(g) + O₃(g) → ClO(g) + O₂(g) shows one Cl atom colliding with one O₃ molecule, giving a rate law of rate = k[Cl][O₃]. Choice C incorrectly uses products (ClO and O₂) in the rate law, but rate laws for elementary steps depend only on reactant concentrations. When dealing with elementary reactions, always write the rate law using only the reactants with their stoichiometric coefficients as exponents.

Question 11

In the stratosphere, the following elementary step is considered:

NO(g)+O(g)→NO2(g)\text{NO}(g)+\text{O}(g)\rightarrow \text{NO}_2(g)

Which rate law is required by the step being elementary?

  1. rate=k[O]2\text{rate}=k[\text{O}]^2
  2. rate=k[NO][O]\text{rate}=k[\text{NO}][\text{O}] (correct answer)
  3. rate=k[NO]2[O]\text{rate}=k[\text{NO}]^2[\text{O}]
  4. rate=k[NO2]\text{rate}=k[\text{NO}_2]
  5. rate=k[NO]\text{rate}=k[\text{NO}]

Explanation: This problem examines elementary reactions. Elementary steps describe single collision events, and the rate law exponents equal the stoichiometric coefficients of the reactants. For NO(g)+O(g)→NO2(g)\text{NO}(g) + \text{O}(g) \rightarrow \text{NO}_2(g), one NO molecule collides with one O atom, yielding rate=k[NO][O]\text{rate} = k[\text{NO}][\text{O}]. Choice B incorrectly shows [NO]2[O][\text{NO}]^2[\text{O}], suggesting two NO molecules collide with one O atom, which doesn't match the given elementary step. The key insight is that elementary steps provide a direct path from balanced equation to rate law.

Question 12

In the gas phase, the following step is explicitly labeled as an elementary reaction:

2H(g)+O2(g)→H2O2(g)2\text{H}(g)+\text{O}_2(g)\rightarrow \text{H}_2\text{O}_2(g)

Which rate law corresponds to this elementary step?

  1. rate=k[H][O2]\text{rate}=k[\text{H}][\text{O}_2]
  2. rate=k[H]2[O2]\text{rate}=k[\text{H}]^2[\text{O}_2] (correct answer)
  3. rate=k[H2O2]\text{rate}=k[\text{H}_2\text{O}_2]
  4. rate=k[O2]2\text{rate}=k[\text{O}_2]^2
  5. rate=k[H]2[O2]2\text{rate}=k[\text{H}]^2[\text{O}_2]^2

Explanation: This question tests understanding of elementary reactions. In an elementary reaction, the molecularity (number of molecules that must collide) determines the rate law directly from the balanced equation. The elementary step 2H(g) + O₂(g) → H₂O₂(g) requires two H atoms and one O₂ molecule to collide simultaneously, giving rate = k[H]²[O₂]. Choice A incorrectly uses [H] to the first power, missing that 2 H atoms must collide as shown by the coefficient 2 in the balanced equation. For elementary reactions, always match the exponents in the rate law to the stoichiometric coefficients of the reactants.

Question 13

A single step in a proposed mechanism is explicitly stated to be an elementary reaction in aqueous solution:

S2O82−(aq)+2 I−(aq)→2 SO42−(aq)+I2(aq)\text{S}_2\text{O}_8^{2-}(aq)+2\,\text{I}^-(aq)\rightarrow 2\,\text{SO}_4^{2-}(aq)+\text{I}_2(aq)

Which rate law follows directly from this step being elementary?

  1. rate=k[S2O82−][I−]\text{rate}=k[\text{S}_2\text{O}_8^{2-}][\text{I}^-]
  2. rate=k[S2O82−]2[I−]\text{rate}=k[\text{S}_2\text{O}_8^{2-}]^2[\text{I}^-]
  3. rate=k[S2O82−][I−]2\text{rate}=k[\text{S}_2\text{O}_8^{2-}][\text{I}^-]^2 (correct answer)
  4. rate=k[SO42−]2[I2]\text{rate}=k[\text{SO}_4^{2-}]^2[\text{I}_2]
  5. rate=k[S2O82−]1/2[I−]\text{rate}=k[\text{S}_2\text{O}_8^{2-}]^{1/2}[\text{I}^-]

Explanation: This question tests understanding of elementary reactions. In an elementary step, the rate law directly reflects the molecularity—how many particles must collide simultaneously. This reaction shows one S₂O₈²⁻ ion colliding with two I⁻ ions, so the rate law must be rate = k[S₂O₈²⁻][I⁻]². Choice A incorrectly uses [I⁻]¹ instead of [I⁻]², missing that the coefficient 2 in front of I⁻ becomes the exponent in the rate law. For elementary reactions only, the stoichiometric coefficients become the reaction orders.

Question 14

A mechanism includes the following elementary step (termolecular collision): 2NO(g)+O2(g)→2NO2(g)\mathrm{2NO(g) + O_2(g) \rightarrow 2NO_2(g)} Which rate law is consistent with the stoichiometric coefficients of reactants in an elementary step?

  1. Rate=k[NO][O2]\text{Rate}=k[\mathrm{NO}][\mathrm{O_2}]
  2. Rate=k[NO2]2\text{Rate}=k[\mathrm{NO_2}]^2
  3. Rate=k[NO]2[O2]\text{Rate}=k[\mathrm{NO}]^2[\mathrm{O_2}] (correct answer)
  4. Rate=k[O2]2\text{Rate}=k[\mathrm{O_2}]^2
  5. Rate=k[NO]2\text{Rate}=k[\mathrm{NO}]^2

Explanation: The concept being tested is rate laws for elementary reactions. In elementary steps, the rate law is written directly from the reactant coefficients because molecularity dictates the order. This termolecular step involves two NO and one O2, so Rate=k[NO]2[O2]\text{Rate} = k [\mathrm{NO}]^2 [\mathrm{O_2}]. The rate depends on the frequency of three-molecule collisions. Choice B, Rate=k[NO2]2\text{Rate} = k [\mathrm{NO_2}]^2, is a distractor that wrongly uses products and ignores the elementary step's reactants. A transferable approach is to derive rate laws solely from elementary steps' reactant coefficients, not from overall stoichiometry.

Question 15

The following single reaction step is described as elementary: N2O5(g)→NO2(g)+NO3(g)\mathrm{N_2O_5(g) \rightarrow NO_2(g) + NO_3(g)} Which rate law corresponds to this elementary unimolecular decomposition?

  1. Rate=k[N2O5]\text{Rate}=k[\mathrm{N_2O_5}] (correct answer)
  2. Rate=k[NO2][NO3]\text{Rate}=k[\mathrm{NO_2}][\mathrm{NO_3}]
  3. Rate=k[N2O5]2\text{Rate}=k[\mathrm{N_2O_5}]^2
  4. Rate=k[NO2]\text{Rate}=k[\mathrm{NO_2}]
  5. Rate=k[NO3]\text{Rate}=k[\mathrm{NO_3}]

Explanation: This question is about elementary reactions and unimolecular decompositions. For elementary reactions, the rate law corresponds directly to the molecularity, with the order equaling the number of molecules involved. This unimolecular step means one N2O5 molecule decomposes, giving Rate=k[N2O5]\text{Rate} = k [\mathrm{N_2O_5}]. No other reactants are involved, so it's first-order. Choice B, Rate=k[NO2][NO3]\text{Rate} = k [\mathrm{NO_2}][\mathrm{NO_3}], is incorrect because it includes products instead of the reactant in the rate law. When dealing with elementary steps, always base the rate law on the reactants' coefficients to ensure accuracy.

Question 16

In a gas-phase kinetics study, the following reaction step is explicitly stated to be an elementary step:

2 NO(g)+O2(g)→2 NO2(g)2\,\mathrm{NO}(g)+\mathrm{O_2}(g)\rightarrow 2\,\mathrm{NO_2}(g)

Based on the definition of an elementary step (molecularity equals the stoichiometric coefficients of reactants in that step), which rate law is correct for this step?

  1. Rate=k[NO]2[O2]\text{Rate}=k[\mathrm{NO}]^2[\mathrm{O_2}] (correct answer)
  2. Rate=k[NO][O2]\text{Rate}=k[\mathrm{NO}][\mathrm{O_2}]
  3. Rate=k[NO]2\text{Rate}=k[\mathrm{NO}]^2
  4. Rate=k[NO2]2\text{Rate}=k[\mathrm{NO_2}]^2
  5. Rate=k[NO]2[O2]2\text{Rate}=k[\mathrm{NO}]^2[\mathrm{O_2}]^2

Explanation: This question tests the skill of elementary reactions. For an elementary reaction, the rate law is determined directly from the balanced equation of that step, where the order with respect to each reactant equals its stoichiometric coefficient. In this case, the reaction involves two NO molecules and one O2 molecule colliding, making it termolecular, so the rate law incorporates [NO]^2 and [O2]^1. This direct relationship holds because elementary steps represent single collision events, and the molecularity defines the reaction order. A tempting distractor is choice D, Rate = k[NO2]^2, which is incorrect due to the misconception of basing the rate law on products instead of reactants. Always remember that only elementary steps allow coefficients to directly define rate laws; for overall reactions, experimental data is needed.

Question 17

A gas-phase process includes the following step, which is explicitly stated to be an elementary termolecular reaction:

2 SO2(g)+O2(g)→2 SO3(g)2\,\mathrm{SO_2}(g)+\mathrm{O_2}(g)\rightarrow 2\,\mathrm{SO_3}(g)

Which rate law is consistent with this elementary step?

  1. Rate=k[SO2][O2]\text{Rate}=k[\mathrm{SO_2}][\mathrm{O_2}]
  2. Rate=k[SO2]2\text{Rate}=k[\mathrm{SO_2}]^2
  3. Rate=k[SO2]2[O2]\text{Rate}=k[\mathrm{SO_2}]^2[\mathrm{O_2}] (correct answer)
  4. Rate=k[SO3]2\text{Rate}=k[\mathrm{SO_3}]^2
  5. Rate=k[SO2]2[O2]2\text{Rate}=k[\mathrm{SO_2}]^2[\mathrm{O_2}]^2

Explanation: This question tests the skill of elementary reactions. For elementary reactions, especially termolecular ones, the rate law uses coefficients as orders for all reactants involved. This step has two SO2 and one O2, so rate=k[SO2]2[O2]rate = k[\mathrm{SO_2}]^2[\mathrm{O_2}], reflecting the rare three-molecule collision. The molecularity directly informs the rate law because it's a single mechanistic step. A tempting distractor is choice D, Rate=k[SO3]2Rate = k[\mathrm{SO_3}]^2, which is wrong due to the misconception of basing the rate on products instead of reactants. Always remember that only elementary steps allow coefficients to directly define rate laws; for overall reactions, experimental data is needed.

Question 18

A proposed mechanism includes the following single step, which is explicitly identified as an elementary reaction:

Br(g)+H2(g)→HBr(g)+H(g)\mathrm{Br}(g)+\mathrm{H_2}(g)\rightarrow \mathrm{HBr}(g)+\mathrm{H}(g)

What is the rate law for this elementary step?

  1. Rate=k[Br][H2]2\text{Rate}=k[\mathrm{Br}][\mathrm{H_2}]^2
  2. Rate=k[Br][H2]\text{Rate}=k[\mathrm{Br}][\mathrm{H_2}] (correct answer)
  3. Rate=k[HBr][H]\text{Rate}=k[\mathrm{HBr}][\mathrm{H}]
  4. Rate=k[H2]\text{Rate}=k[\mathrm{H_2}]
  5. Rate=k[Br]2[H2]\text{Rate}=k[\mathrm{Br}]^2[\mathrm{H_2}]

Explanation: This question tests the skill of elementary reactions. The rate law for an elementary step mirrors the molecularity, with exponents matching the number of reactant molecules involved in the collision. Here, one Br and one H2 molecule react, indicating a bimolecular process, so the rate is first-order in each. This is because elementary reactions proceed via a single step, and the rate depends on the frequency of those specific collisions. A tempting distractor is choice E, Rate=k[Br]2[H2]\text{Rate} = k[\mathrm{Br}]^2[\mathrm{H_2}], which is wrong because of the misconception of doubling the coefficient for Br without basis in the step. Always remember that only elementary steps allow coefficients to directly define rate laws; for overall reactions, experimental data is needed.

Question 19

The following step is specified to be elementary in a reaction mechanism: Cl(g)+O3(g)→ClO(g)+O2(g)\mathrm{Cl(g) + O_3(g) \rightarrow ClO(g) + O_2(g)} Which rate law matches an elementary bimolecular collision?

  1. Rate=k[Cl]2[O3]\text{Rate}=k[\mathrm{Cl}]^2[\mathrm{O_3}]
  2. Rate=k[Cl][O3]\text{Rate}=k[\mathrm{Cl}][\mathrm{O_3}] (correct answer)
  3. Rate=k[ClO]\text{Rate}=k[\mathrm{ClO}]
  4. Rate=k[O3]2\text{Rate}=k[\mathrm{O_3}]^2
  5. Rate=k[O2]\text{Rate}=k[\mathrm{O_2}]

Explanation: This question evaluates understanding of elementary reactions in mechanisms. The rate law for an elementary step directly follows from its molecularity, as it represents a single collision event. Here, the bimolecular collision between Cl and O3 gives Rate=k[Cl][O3]\text{Rate} = k [\mathrm{Cl}][\mathrm{O_3}], with each reactant to the first power. The exponents are identical to the coefficients in the balanced elementary equation. Choice A, Rate=k[Cl]2[O3]\text{Rate} = k [\mathrm{Cl}]^2 [\mathrm{O_3}], is incorrect because it assumes a termolecular collision instead of the given bimolecular one. Focus on the given elementary step's stoichiometry to derive rate laws, ignoring overall reactions unless specified.

Question 20

A textbook lists the following as an elementary step in the gas phase: H(g)+HBr(g)→H2(g)+Br(g)\mathrm{H(g) + HBr(g) \rightarrow H_2(g) + Br(g)} Which rate law is correct for this elementary step?

  1. Rate=k[HBr]2\text{Rate}=k[\mathrm{HBr}]^2
  2. Rate=k[H]\text{Rate}=k[\mathrm{H}]
  3. Rate=k[HBr]\text{Rate}=k[\mathrm{HBr}]
  4. Rate=k[H][HBr]\text{Rate}=k[\mathrm{H}][\mathrm{HBr}] (correct answer)
  5. Rate=k[H2][Br]\text{Rate}=k[\mathrm{H_2}][\mathrm{Br}]

Explanation: This problem examines rate laws derived from elementary reactions. Elementary steps have rate laws that match their molecularity, using coefficients as exponents for reactants. The bimolecular collision here between H and HBr yields Rate=k[H][HBr]\text{Rate} = k [\mathrm{H}][\mathrm{HBr}]. Each reactant appears to the first power, reflecting the single collision. Choice E, Rate=k[H2][Br]\text{Rate} = k [\mathrm{H_2}][\mathrm{Br}], is wrong as it mistakenly uses products rather than reactants. Remember, only elementary reactions permit direct use of stoichiometric coefficients for rate law orders.