Worksheet: Elementary and Rate-Determining Steps

In this worksheet, we will practice using the orders of reaction in a rate law to identify the rate-determining step in a multi-step reaction.

Q1:

Chlorine radicals from the decomposition of chlorofluoromethanes, such as CClF22, catalyze the decomposition of ozone (O3) in the atmosphere. One decomposition pathway involves three elementary reactions:

Reaction 1: OO+O32, rate constant =𝑘.

Reaction 2: O+ClO+ClO32, rate constant =𝑘.

Reaction 3: O+ClOO+Cl2, rate constant =𝑘.

Write a balanced chemical equation for the overall reaction.

  • A2O+2Cl3O+2ClO32
  • BO+ClO+ClO32
  • CO2O32
  • DO+2ClO+2ClO32
  • E2O3O32

Explain why chlorine radicals are catalysts in this reaction.

  • AThe reaction of O3 with chlorine radicals is faster than the decomposition reaction 1.
  • BChlorine radicals react in the rate-determining step, reaction 2.
  • CReaction 2 converts chlorine radicals to ClO radicals, which are necessary to consume the O radicals produced in Reaction 1.
  • DChlorine radicals are consumed in reaction 2 but regenerated in reaction 3, so there is no net change in [Cl].
  • EReaction 2 converts chlorine radicals to ClO radicals, which are less stable than the O radicals produced in Reaction 1.

Q2:

A reaction proceeds by the following two-step mechanism. A+2BC+DC+A2E The first elementary reaction is much slower than the second. What is the rate equation for the overall reaction?

  • ARate = 𝑘[A][B]22
  • BRate = 𝑘[A]2
  • CRate = 𝑘[A][B]2
  • DRate = 𝑘[B]2
  • ERate = 𝑘[A][B]2

Q3:

A reaction proceeds by the following three-step mechanism. AB+CCD+EA+D2B The first elementary reaction is much slower than the second and the third. What is the rate equation for the overall reaction?

  • ARate = 𝑘[A][B]22
  • BRate = 𝑘[A]
  • CRate = 𝑘[A][B]2
  • DRate = 𝑘[A][B]2
  • ERate = 𝑘[A][B]

Q4:

An elementary reaction has the shown equation. A+2BC What is the rate equation for this reaction?

  • ARate = 𝑘[C][A][B]2
  • BRate = 𝑘[A][B]2
  • CRate = 𝑘([A]+2[B])
  • DRate = 𝑘[A][B]
  • ERate = 𝑘[C]([A]+2[B])

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