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 C C l F 2 2 , catalyze the decomposition of ozone ( O 3 ) in the atmosphere. One decomposition pathway involves three elementary reactions:

Reaction 1: O O + O 3 2 , rate constant = 𝑘 1 .

Reaction 2: O + C l O + C l O 3 2 , rate constant = 𝑘 2 .

Reaction 3: O + C l O O + C l 2 , rate constant = 𝑘 3 .

Write a balanced chemical equation for the overall reaction.

  • A O + C l O + C l O 3 2
  • B O 2 O 3 2
  • C 2 O + 2 C l 3 O + 2 C l O 3 2
  • D 2 O 3 O 3 2
  • E O + 2 C l O + 2 C l O 3 2

Explain why chlorine radicals are catalysts in this reaction.

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

Q2:

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

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

Q3:

A reaction proceeds by the following three-step mechanism: 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 ]
  • BRate = 𝑘 [ A ] [ B ] 2
  • CRate = 𝑘 [ A ] [ B ] 2 2
  • DRate = 𝑘 [ A ]
  • ERate = 𝑘 [ A ] [ B ] 2

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