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Introductory Chemistry - 1st Canadian Edition: Catalysis

Introductory Chemistry - 1st Canadian Edition
Catalysis
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Acknowledgments
  6. Dedication
  7. About BCcampus Open Education
  8. Chapter 1. What is Chemistry
    1. Some Basic Definitions
    2. Chemistry as a Science
  9. Chapter 2. Measurements
    1. Expressing Numbers
    2. Significant Figures
    3. Converting Units
    4. Other Units: Temperature and Density
    5. Expressing Units
    6. End-of-Chapter Material
  10. Chapter 3. Atoms, Molecules, and Ions
    1. Acids
    2. Ions and Ionic Compounds
    3. Masses of Atoms and Molecules
    4. Molecules and Chemical Nomenclature
    5. Atomic Theory
    6. End-of-Chapter Material
  11. Chapter 4. Chemical Reactions and Equations
    1. The Chemical Equation
    2. Types of Chemical Reactions: Single- and Double-Displacement Reactions
    3. Ionic Equations: A Closer Look
    4. Composition, Decomposition, and Combustion Reactions
    5. Oxidation-Reduction Reactions
    6. Neutralization Reactions
    7. End-of-Chapter Material
  12. Chapter 5. Stoichiometry and the Mole
    1. Stoichiometry
    2. The Mole
    3. Mole-Mass and Mass-Mass Calculations
    4. Limiting Reagents
    5. The Mole in Chemical Reactions
    6. Yields
    7. End-of-Chapter Material
  13. Chapter 6. Gases
    1. Pressure
    2. Gas Laws
    3. Other Gas Laws
    4. The Ideal Gas Law and Some Applications
    5. Gas Mixtures
    6. Kinetic Molecular Theory of Gases
    7. Molecular Effusion and Diffusion
    8. Real Gases
    9. End-of-Chapter Material
  14. Chapter 7. Energy and Chemistry
    1. Formation Reactions
    2. Energy
    3. Stoichiometry Calculations Using Enthalpy
    4. Enthalpy and Chemical Reactions
    5. Work and Heat
    6. Hess’s Law
    7. End-of-Chapter Material
  15. Chapter 8. Electronic Structure
    1. Light
    2. Quantum Numbers for Electrons
    3. Organization of Electrons in Atoms
    4. Electronic Structure and the Periodic Table
    5. Periodic Trends
    6. End-of-Chapter Material
  16. Chapter 9. Chemical Bonds
    1. Lewis Electron Dot Diagrams
    2. Electron Transfer: Ionic Bonds
    3. Covalent Bonds
    4. Other Aspects of Covalent Bonds
    5. Violations of the Octet Rule
    6. Molecular Shapes and Polarity
    7. Valence Bond Theory and Hybrid Orbitals
    8. Molecular Orbitals
    9. End-of-Chapter Material
  17. Chapter 10. Solids and Liquids
    1. Properties of Liquids
    2. Solids
    3. Phase Transitions: Melting, Boiling, and Subliming
    4. Intermolecular Forces
    5. End-of-Chapter Material
  18. Chapter 11. Solutions
    1. Colligative Properties of Solutions
    2. Concentrations as Conversion Factors
    3. Quantitative Units of Concentration
    4. Colligative Properties of Ionic Solutes
    5. Some Definitions
    6. Dilutions and Concentrations
    7. End-of-Chapter Material
  19. Chapter 12. Acids and Bases
    1. Acid-Base Titrations
    2. Strong and Weak Acids and Bases and Their Salts
    3. Brønsted-Lowry Acids and Bases
    4. Arrhenius Acids and Bases
    5. Autoionization of Water
    6. Buffers
    7. The pH Scale
    8. End-of-Chapter Material
  20. Chapter 13. Chemical Equilibrium
    1. Chemical Equilibrium
    2. The Equilibrium Constant
    3. Shifting Equilibria: Le Chatelier’s Principle
    4. Calculating Equilibrium Constant Values
    5. Some Special Types of Equilibria
    6. End-of-Chapter Material
  21. Chapter 14. Oxidation and Reduction
    1. Oxidation-Reduction Reactions
    2. Balancing Redox Reactions
    3. Applications of Redox Reactions: Voltaic Cells
    4. Electrolysis
    5. End-of-Chapter Material
  22. Chapter 15. Nuclear Chemistry
    1. Units of Radioactivity
    2. Uses of Radioactive Isotopes
    3. Half-Life
    4. Radioactivity
    5. Nuclear Energy
    6. End-of-Chapter Material
  23. Chapter 16. Organic Chemistry
    1. Hydrocarbons
    2. Branched Hydrocarbons
    3. Alkyl Halides and Alcohols
    4. Other Oxygen-Containing Functional Groups
    5. Other Functional Groups
    6. Polymers
    7. End-of-Chapter Material
  24. Chapter 17. Kinetics
    1. Factors that Affect the Rate of Reactions
    2. Reaction Rates
    3. Rate Laws
    4. Concentration–Time Relationships: Integrated Rate Laws
    5. Activation Energy and the Arrhenius Equation
    6. Reaction Mechanisms
    7. Catalysis
    8. End-of-Chapter Material
  25. Chapter 18. Chemical Thermodynamics
    1. Spontaneous Change
    2. Entropy and the Second Law of Thermodynamics
    3. Measuring Entropy and Entropy Changes
    4. Gibbs Free Energy
    5. Spontaneity: Free Energy and Temperature
    6. Free Energy under Nonstandard Conditions
    7. End-of-Chapter Material
  26. Appendix A: Periodic Table of the Elements
  27. Appendix B: Selected Acid Dissociation Constants at 25°C
  28. Appendix C: Solubility Constants for Compounds at 25°C
  29. Appendix D: Standard Thermodynamic Quantities for Chemical Substances at 25°C
  30. Appendix E: Standard Reduction Potentials by Value
  31. Glossary
  32. About the Authors
  33. Versioning History

Catalysis

Jessie A. Key

Learning Objectives

  • To gain an understanding of homogeneous, heterogeneous, and biological catalysts.

Catalysts are substances that lower the activation energy of a specific reaction by providing an alternate reaction pathway. Catalysts participate in a reaction, but are not permanently changed in the process, as they are regenerated to their original state. Many scientists classify catalysts into one of three categories: homogeneous catalysts, heterogeneous catalysts, and biological catalysts (enzymes).

Homogeneous Catalysts

A homogeneous catalyst is any catalyst that is present in the same phase as the reactant molecules. There are numerous examples of homogeneous catalysts, and we will examine one that is commonly used in textbooks.

The alkene 2-butene can exist as one of two isomers: cis where the methyl groups are located on the same side of the double bond, and trans where the methyl groups are located on opposite sides of the double bond (Figure 17.14 “Isomerization of 2-Butene”). To convert (isomerize) between the two structures, the carbon-carbon double bond must be broken and the molecule must rotate. This process has a relatively high activation energy of approximately 264 kJ/mol and is therefore fairly slow to occur at room temperature.

2-butene can exist as one of two isomers. This diagram shows the isomerization of 2-butene:

\chemname{\chemfig{(-[:120]H_3C)(-[:-120]H)=(-[:60]CH_3)-[:-60]H}}{cis-2-butene}\buildrel \text{isomerization} \over \longleftrightarrow \chemname{\chemfig{(-[:120]H_3C)(-[:-120]H)=(-[:60]H)-[:-60]CH_3}}{trans-2-butene}

A catalyst like iodine can be used to provide an alternate pathway for the reaction with a much lower activation energy of approximately 118 kJ/mol (see Figure 17.14 “Catalyzed and Uncatalyzed Reaction Pathways”).

Potential energy diagram of catalyzed vs uncatalyzed reaction pathway.
Figure 17.14 “Catalyzed and Uncatalyzed Reaction Pathways.” Potential energy diagrams of catalyzed and uncatalyzed reaction pathways.

In the catalyzed pathway, an iodine atom is generated that reacts with cis-2-butene to produce a reaction intermediate that has broken its carbon-carbon double bond and formed a new C-I bond and a radical (see Figure 17.15 “2-Butene Catalyzed Isomerization Steps”). The molecule can rotate more easily and the C-I breaks to reform the double bond.

2-butene catalyzed isomerization steps.
Figure 17.15 “2-Butene Catalyzed Isomerization Steps.” 2-butene catalyzed isomerization steps using iodine as the catalyst.

Heterogeneous Catalysts

Heterogeneous catalysts are those that are in a different phase from one or more of the reactants. Commonly solid metals and metal oxides are used to catalyze the reactions of gaseous or liquid reactants. Solid catalysts often serve as a surface on which reactions can occur, where one or more reactants will adsorb (bind to the surface) to the solid.

A common example of heterogeneous catalysis is the hydrogenation reaction of simple alkenes. The conversion of ethene (C2H4) to ethane (C2H6) can be performed with hydrogen gas in the presence of a metal catalyst such as palladium (Figure 17.16 “Conversion of Ethene to Ethane with Hydrogen and a Metal Catalyst”).

C2H4(g) + H2(g) → C2H6(g)

Heterogeneous catalysis mechanisms of reaction for ethene with hydrogen on a catalytic metal surface.
Figure 17.16 “Conversion of Ethene to Ethane with Hydrogen and a Metal Catalyst.” Heterogeneous catalysis mechanisms of reaction for ethene with hydrogen on a catalytic metal surface

Ethene and hydrogen adsorb onto the metal surface, where the H2 breaks into two individual hydrogen atoms bonded to the metal surface. A reaction occurs between adjacent ethene and hydrogen atoms on the metal surface, first to generate a C2H5 intermediate, then to generate ethane, C2H6, which desorbs from the surface.

Biological Catalysts

Catalysts within living things facilitate the vast and intricate system of chemical reactions required for life. There are two main types of naturally occurring catalytic biomolecules: ribozymes and enzymes.

Ribozymes are ribonucleic acid (RNA) molecules capable of catalyzing certain chemical reactions. Ribozymes are a relatively recent discovery, first reported in 1982, but their importance was demonstrated by the awarding of the 1989 Nobel Prize to the discoverers Sidney Altman and Thomas Cech. Research is ongoing to better understand these catalysts and develop new therapeutics and medicines using them.

Enzymes are protein molecules that catalyze biochemical reactions. They are remarkably specific for the reactants they can use, known as substrates, and many dramatically increase reaction rate by factors of 107 to 1014. A simple model often used to describe enzyme activity is known as the lock-and-key model (see Figure 17.17 “Lock-and-Key Model of Enzymatic Catalysis”). In this model, enzymes accelerate reactions by providing a tight-fitting area, known as the active site, where substrate molecules can react. Hydrophobicity and intermolecular forces such as hydrogen bonding, London-dispersion forces, and dipole-dipole interactions facilitate the binding of substrate molecules to the active site, forming an enzyme-substrate complex. When the reaction is completed at the active site, the product is released.

Lock-and-Key model of enzymatic catalysis.
Figure 17.17 “Lock-and-Key Model of Enzymatic Catalysis.” Lock-and-key model of enzymatic catalysis showing the tight-fitting area where substrate molecules react.

Key Takeaways

  • Catalysts provide an alternate, lower-energy reaction pathway.
  • A homogeneous catalyst is any catalyst that is present in the same phase as the reactant molecules.
  • Heterogeneous catalysts are in a different phase from one or more of the reactants, and often act as a surface on which the reaction can occur.
  • According to the lock-and-key model, enzymes accelerate reactions by providing a tight-fitting area, where substrate molecules can react.

Media Attributions

  • “Enzyme mechanism 1” © 2011 by Aejahnke is licensed under a CC BY-SA (Attribution-ShareAlike) license

Icon for the Creative Commons Attribution 4.0 International License

Catalysis by Jessie A. Key is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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Copyright © 2014

                                by Jessie A. Key

            Introductory Chemistry - 1st Canadian Edition by Jessie A. Key is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
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