29: Chemical Kinetics II: Reaction Mechanisms
- Page ID
- 62259
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 29.1: A Mechanism is a Sequence of Elementary Reactions
- This page discusses reaction mechanisms as sequences of steps that transition reactants to products via short-lived intermediates. Each step, called an elementary reaction, affects the overall reaction rate. The interplay between the overall reaction rate and the rates of elementary steps is illustrated using two examples of two-step reactions.
- 29.2: The Principle of Detailed Balance
- This page discusses the principle of detailed balance, which states that at equilibrium, the rates of direct and reverse processes are equal. Introduced by Lewis in 1925, it highlights the need for equal conversion processes at equilibrium. Ter Haar reinforces this idea, while Boltzmann applied it to collisions. Wegscheider further adapted it to chemical kinetics, showing that irreversible cycles are impossible in reaction systems and establishing relationships between kinetic constants.
- 29.3: Multiple Mechanisms are often Indistinguishable
- This page discusses chemical kinetics and its role in understanding reaction mechanisms and rate laws. It highlights that different mechanisms can predict varying reaction orders, exemplifying a scenario with reactants A and B. By comparing observed rate laws, certain mechanisms can be dismissed, though further evidence is necessary for conclusive validation.
- 29.4: The Steady-State Approximation
- This page discusses the steady state approximation, a method for analyzing the concentration of short-lived intermediates that remain constant over time. It enables the calculation of the rate of formation of products in reaction mechanisms, leading to the establishment of specific rate laws. By comparing predicted rates with observed data, researchers can validate or reject potential mechanisms.
- 29.5: Rate Laws Do Not Imply Unique Mechanism
- This page discusses reaction mechanisms focusing on the role of intermediates. It highlights that the conversion of intermediate \(C\) to product \(D\) can lead to a simplified rate law through the steady-state approximation. Three cases are presented: Case IIa, where intermediate \(C\) forms quickly, yielding a first-order rate; Case IIb, involving equilibrium; and Case IIc, which allows for a non-negligible concentration of \(C\) but results in a similar rate.
- 29.6: The Lindemann Mechanism
- This page discusses the Lindemann mechanism, which outlines reactions via elementary steps and the role of activated intermediates, leading to variable reaction orders under different conditions. It highlights that bimolecular activation and unimolecular reactions yield insights into kinetics, demonstrated by experiments such as \(N_2O_5\) decomposition.
- 29.7: Some Reaction Mechanisms Involve Chain Reactions
- This page discusses the mechanism of a radical chain reaction, emphasizing initiation, propagation, and termination steps, illustrated by the reaction of hydrogen and bromine forming hydrobromic acid. It details rate equations and utilizes the steady state approximation to derive the concentration of intermediates. The final rate expression aligns with experimental observations, validating the theoretical model.
- 29.8: A Catalyst Affects the Mechanism and Activation Energy
- This page covers the role of catalysts in chemical reactions, highlighting their ability to lower activation energy and increase reaction rates without being consumed. It distinguishes between homogeneous and heterogeneous catalysts and details mechanisms like Langmuir-Hinshelwood and Eley-Rideal in gas-phase reactions on solid surfaces. The importance of surface area is discussed, along with equations for reaction rates and concentrations.
- 29.9: The Michaelis-Menten Mechanism for Enzyme Catalysis
- This page discusses enzymes, specialized proteins that serve as biological catalysts in living organisms. It traces the historical development of enzyme study from the 19th century through the understanding of their structures in the 1920s. It explains Michaelis-Menten kinetics, which describes enzyme-substrate interactions, and introduces key parameters like the Michaelis constant (K_M) that influence reaction rates.