Apparent Activation Energies in Complex Reaction Mechanisms: A Simple Relationship via Degrees of Rate Control

Mao, Zhongtian; Campbell, Charles T.

doi:10.1021/acscatal.9b02761

Title: Apparent Activation Energies in Complex Reaction Mechanisms: A Simple Relationship via Degrees of Rate Control

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Abstract

The apparent activation energy of chemical reactions previously played a central role in the field of chemical kinetics and has served as an important tool for analyzing and understanding reaction rates, mechanistic details of complex reaction mechanisms, elementary-step energetics, catalytic activity and reaction selectivity. In this work, we derive a general expression which shows that the apparent activation energy equals a weighted average of the standard-state enthalpies (relative to reactants) of all the species (intermediates, transition states and products) in the reaction mechanism, each weighted by its generalized degree of rate control (DRC). Since the DRC is zero for most of these species, even in very complex mechanisms, the weighted average includes only a few terms. This simplicity provides deep insight into the connection between the reaction energy diagram and the apparent activation energy. We prove both this and the quantitative validity of this equation by analysis of numerous reaction mechanisms. We also show the failures or weaknesses of previous equations for the apparent activation energy.

Authors:

Mao, Zhongtian ^[1];

^[1]

Univ. of Washington, Seattle, WA (United States)

Publication Date:: Tue Aug 27 00:00:00 EDT 2019

Research Org.:: Univ. of Washington, Seattle, WA (United States)

Sponsoring Org.:: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division

OSTI Identifier:: 1597967

Grant/Contract Number:: FG02-96ER14630

Resource Type:: Accepted Manuscript

Journal Name:: ACS Catalysis

Additional Journal Information:: Journal Volume: 9; Journal Issue: 10; Journal ID: ISSN 2155-5435

Publisher:: American Chemical Society (ACS)

Country of Publication:: United States

Language:: English

Subject:: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; kinetics; mechanism; catalyst; energy diagram; rate-controlling species

Citation Formats


                    Mao, Zhongtian, and Campbell, Charles T. Apparent Activation Energies in Complex Reaction Mechanisms: A Simple Relationship via Degrees of Rate Control.  United States: N. p., 2019. 
Web.  doi:10.1021/acscatal.9b02761.

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                    Mao, Zhongtian, & Campbell, Charles T. Apparent Activation Energies in Complex Reaction Mechanisms: A Simple Relationship via Degrees of Rate Control.  United States.  https://doi.org/10.1021/acscatal.9b02761

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                    Mao, Zhongtian, and Campbell, Charles T. Tue .  
"Apparent Activation Energies in Complex Reaction Mechanisms: A Simple Relationship via Degrees of Rate Control".  United States.  https://doi.org/10.1021/acscatal.9b02761.  https://www.osti.gov/servlets/purl/1597967.

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@article{osti_1597967,

  title        = {Apparent Activation Energies in Complex Reaction Mechanisms: A Simple Relationship via Degrees of Rate Control},

  author       = {Mao, Zhongtian and Campbell, Charles T.},

  abstractNote = {The apparent activation energy of chemical reactions previously played a central role in the field of chemical kinetics and has served as an important tool for analyzing and understanding reaction rates, mechanistic details of complex reaction mechanisms, elementary-step energetics, catalytic activity and reaction selectivity. In this work, we derive a general expression which shows that the apparent activation energy equals a weighted average of the standard-state enthalpies (relative to reactants) of all the species (intermediates, transition states and products) in the reaction mechanism, each weighted by its generalized degree of rate control (DRC). Since the DRC is zero for most of these species, even in very complex mechanisms, the weighted average includes only a few terms. This simplicity provides deep insight into the connection between the reaction energy diagram and the apparent activation energy. We prove both this and the quantitative validity of this equation by analysis of numerous reaction mechanisms. We also show the failures or weaknesses of previous equations for the apparent activation energy.},

  doi          = {10.1021/acscatal.9b02761},

  journal      = {ACS Catalysis},

  number       = 10,

  volume       = 9,

  place        = {United States},

  year         = {Tue Aug 27 00:00:00 EDT 2019},

  month        = {Tue Aug 27 00:00:00 EDT 2019}

}

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