Chemistry Lecture 13: Rate Limiting Steps

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Questions and Answers

What are the reaction intermediates in the provided series of elementary steps?

  • HOI and OH- (correct)
  • Cl- and HOI (correct)
  • OCl- and HOCl
  • I- and CO2

Which step in the provided reaction series is likely to be the rate determining step?

  • The second step involving I- and HOCl (correct)
  • The first step involving OCl- and H2O
  • The third step involving OH- and HOI
  • There is no rate limiting step

Based on the identified rate limiting step, which rate law can be proposed?

  • Rate = k[OCl-][H2O]
  • Rate = k[Cl-][I-]
  • Rate = k[I-][HOCl] (correct)
  • Rate = k[OH-][HOI]

In the series of elementary steps, which of the following represents an elementary reaction?

<p>OCl- (aq) + H2O (l) → HOCl (aq) + OH- (aq) (D)</p> Signup and view all the answers

What role does water play in the elementary steps involving OCl-?

<p>It is a reactant (D)</p> Signup and view all the answers

What is the overall reaction represented in the provided content?

<p>NO2(g) + CO(g) → NO(g) + CO2(g) (B)</p> Signup and view all the answers

What is the molecularity of the rate law for step 1 in the provided reaction mechanisms?

<p>Bimolecular (B)</p> Signup and view all the answers

Which expression correctly represents the rate law for step 2 of the reaction?

<p>Rate = k[NO3][CO] (D)</p> Signup and view all the answers

What does the term 'reaction mechanism' refer to in the context of chemical reactions?

<p>The sequence of elementary steps leading to products (B)</p> Signup and view all the answers

In the provided context, which elementary step is the rate-determining step?

<p>The slowest elementary step that limits overall reaction rate (C)</p> Signup and view all the answers

Which of the following statements is true about the rate laws in chemical kinetics?

<p>They express the relationship between the rate and concentration of reactants. (B)</p> Signup and view all the answers

Given the reaction mechanisms, how are elementary steps related to rate laws?

<p>Each elementary step directly corresponds to a rate law. (B)</p> Signup and view all the answers

For the given reaction mechanisms, what role does the concentration of reactants play?

<p>It influences the rate according to the rate laws derived from elementary steps. (A)</p> Signup and view all the answers

What is the role of the rate-determining step in a reaction mechanism?

<p>It is the slowest step that limits the overall reaction rate. (A)</p> Signup and view all the answers

Which statement correctly describes termolecular reactions?

<p>They involve three reactant species at the same time. (D)</p> Signup and view all the answers

In the example provided, what is the observed rate law for the reaction 2NO2 (g) + F2 (g) → 2NO2F (g)?

<p>Rate = k[NO2][F] (A)</p> Signup and view all the answers

What characterizes intermediates in a reaction mechanism?

<p>They are short-lived species not present in the overall reaction. (D)</p> Signup and view all the answers

How is molecularity determined for an elementary step?

<p>By identifying the number of reactant species involved in that specific step. (B)</p> Signup and view all the answers

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

<p>It represents a single reaction event at the molecular level. (C)</p> Signup and view all the answers

Which of the following statements is true regarding bimolecular reactions?

<p>They involve the collision of two reactant species. (C)</p> Signup and view all the answers

What can be inferred when there are multiple elementary steps leading to a single reaction product?

<p>There may be a rate-determining step among them. (D)</p> Signup and view all the answers

Flashcards

Rate-determining step

The slowest elementary step in a reaction mechanism. The overall reaction rate is controlled by this step.

Elementary step

A single step in a reaction mechanism, involving a small number of reactant molecules.

Molecularity

The number of molecules that participate in an elementary step.

Intermediates

Species formed in one step and consumed in a later step. They do not appear in the overall reaction.

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Termolecular reaction

A reaction mechanism with three reactant molecules in the rate-determining step.

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Bimolecular reaction

A reaction mechanism where two reactant molecules participate in the rate-determining step

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Overall Reaction

Summing up all the elementary steps in a reaction mechanism; the reactants yield the products.

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Reaction Mechanism

A series of elementary reactions that describe the complete transformations from reactants to products.

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Rate-limiting Step

The slowest elementary step in a reaction mechanism, which determines the overall reaction rate.

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Rate Law

An equation that expresses the rate of a reaction based on the concentrations of reactants and the rate constant.

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Proposed rate law (based on rate-limiting step)

An equation that describes the rate of a reaction based only on the concentrations of the reactants in the slowest step.

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Unimolecular

An elementary step involving only one molecule.

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Bimolecular

An elementary step involving two molecules.

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Termolecular

An elementary step involving three molecules.

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Study Notes

Midterm I

  • Average score: ~69%
  • Total points possible: 160 pts
  • Score achieved: 41.3 pts

Lecture 13: Rate Limiting Steps, Mechanisms, and Catalysis

  • Topics covered include rate-limiting steps, reaction mechanisms, and catalysis.
  • Relevant concepts from this lecture include activation energy, temperature effects on reaction rates, and the Arrhenius equation.

Arrhenius Equation

  • k = A exp(-Ea/RT)
  • k: rate constant
  • A: frequency factor
  • Ea: activation energy
  • R: ideal gas constant
  • T: absolute temperature

Two-Point Arrhenius Equation

  • ln(k₂/k₁) = -Ea/R * (1/T₂ - 1/T₁)

Example: Hydroxylamine Decomposition

  • Rate constant at 0°C: 0.237 x 10⁻⁴ L mol⁻¹ s⁻¹
  • Rate constant at 25°C: 2.64 x 10⁻⁴ L mol⁻¹ s⁻¹
  • Reaction order: 2nd order
  • Activation energy (Ea): 6.52 x 10⁴ J mol⁻¹
  • Frequency factor (A): 7.1 x 10⁷ L mol⁻¹ s⁻¹

Reaction Mechanisms from Kinetics

  • Reactions proceed through a sequence of elementary steps.
  • Elementary steps define the molecularity of the reaction (unimolecular, bimolecular, termolecular).
  • Rate laws for each step are identified.
  • Overall reactions are determined by combining elementary steps.

Rate-Determining Step

  • The slowest elementary step in a reaction mechanism is the rate-determining step.
  • The observed rate law corresponds to this step.

Examples of Reaction Mechanisms and Rate-Determining Steps

  • Specific examples of multi-step reactions involving gas-phase and aqueous-phase reactions are provided.
  • Reaction intermediates are identified in each process.
  • Rate-laws for individual steps are noted.

Multi-steps Involving an Equilibrium

  • Reactions can proceed via series of steps involving reversible steps.
  • Equilibrium constants and rate constants are linked.

Examples of Reaction Mechanisms

  • Detailed mechanisms are provided for particular reactions such as the reaction of I−(aq) and OCl−(aq), decomposition of hydroxylamine, and the reaction of NO2 and F2.

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