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Introductory Chemistry - 1st Canadian Edition: Activation Energy and the Arrhenius Equation

Introductory Chemistry - 1st Canadian Edition
Activation Energy and the Arrhenius Equation
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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

Activation Energy and the Arrhenius Equation

Jessie A. Key

Learning Objectives

  • To gain an understanding of activation energy.
  • To determine activation energy graphically or algebraically.
Middle-aged man with a moustache and wearing a bowtie.
Figure 17.11 “Svante Arrhenius.” Swedish scientist Svante Arrhenius (1859–1927).

Earlier in the chapter, reactions were discussed in terms of effective collision frequency and molecule energy levels. In 1889, a Swedish scientist named Svante Arrhenius proposed an equation that relates these concepts with the rate constant:

k=Ae^{\frac{-E_a}{RT}}

where k represents the rate constant, Ea is the activation energy, R is the gas constant \left(\dfrac{8.3145\text{ J}}{\text{K mol}}\right), and T is the temperature expressed in Kelvin. A is known as the frequency factor, having units of L mol−1 s−1, and takes into account the frequency of reactions and likelihood of correct molecular orientation.

The Arrhenius equation allows us to calculate activation energies if the rate constant is known, or vice versa. As well, it mathematically expresses the relationships we established earlier: as activation energy term Ea increases, the rate constant k decreases and therefore the rate of reaction decreases.

Determining the Activation Energy

Graphically

We can graphically determine the activation energy by manipulating the Arrhenius equation to put it into the form of a straight line. Taking the natural logarithm of both sides gives us:

\ln k=-\dfrac{E_a}{RT}+\ln A

A slight rearrangement of this equation then gives us a straight line plot (y = mx + b) for ln k versus \dfrac{1}{T}, where the slope is -\dfrac{E_a}{R}:

\ln k=-\dfrac{E_a}{R}\left(\dfrac{1}{t}\right)+\ln A

Example 17.7

Using the data from the following table, determine the activation energy of the reaction:

Temperature (K)Rate Constant, k (s−1)
3751.68 × 10−5
4003.5 × 10−5
5004.2 × 10−4
6002.11 × 10−3

Solution

We can obtain the activation energy by plotting ln k versus \dfrac{1}{T}, knowing that the slope will be equal to -\dfrac{E_a}{R}.

First determine the values of ln k and \dfrac{1}{T}, and plot them in a graph:

\frac{1}{T}ln k
0.002667−10.9941
0.0025−10.2602
0.002−7.77526
0.001667−6.16107
Graphical determination of Ea example plot
Graphical determination of Ea example plot

    \begin{align*} \text{Slope}&=-\dfrac{E_a}{R} \\ \\ -4865\text{ K}&=-\dfrac{E_a}{8.3145\text{ J K}^{-1}\text{ mol}^{-1}} \\ \\ E_a&=4.0\times 10^4 \text{ J/mol} \end{align*}

Algebraically

The activation energy can also be calculated algebraically if k is known at two different temperatures:

    \begin{align*} \text{At temperature 1: }\ln k_1&=-\dfrac{E_a}{RT_1}+\ln A \\ \\ \text{At temperature 2: }\ln k_2&=-\dfrac{E_a}{RT_2}+\ln A \end{align*}

We can subtract one of these equations from the other:

\ln k_1-\ln k_2=\left(-\dfrac{E_a}{RT_1}+\ln A\right)-\left(-\dfrac{E_a}{RT_2}+\ln A\right)

This equation can then be further simplified to:

\ln \dfrac{k_1}{k_2}=\dfrac{E_a}{R}\left(\dfrac{1}{T_2}-\dfrac{1}{T_1}\right)

Example 17.8

Determine the value of Ea given the following values of k at the temperatures indicated:

    \begin{align*} 600\text{ K: }k&=2.75\times 10^{-8}\text{ L mol}^{-1}\text{s}^{-1} \\ 800\text{ K: }k&=1.95\times 10^{-7}\text{ L mol}^{-1}\text{s}^{-1} \end{align*}

Solution

Substitute the values stated into the algebraic method equation:

    \begin{align*} \ln \dfrac{k_1}{k_2}&=\dfrac{E_a}{R}\left(\dfrac{1}{T_2}-\dfrac{1}{T_1}\right) \\ \\ \ln \dfrac{2.75\times 10^{-8}\text{ L mol}^{-1}\text{s}^{-1}}{1.95\times10^{-7}\text{ L mol}^{-1}\text{s}^{-1}}&=\dfrac{E_a}{8.3145\text{ J K}^{-1}\text{mol}^{-1}}\left(\dfrac{1}{800\text{ K}}-\dfrac{1}{600\text{ K}}\right) \\ \\ 1.96&=\dfrac{E_a}{8.3145\text{ J K}^{-1}\text{mol}^{-1}}\left(-4.16\times10^{-4}\text{ K}^{-1}\right) \\ \\ 4.704\times 10^{-3}\text{ K}^{-1}&=\dfrac{E_a}{8.3145\text{ J K}^{-1}\text{mol}^{-1}} \\ \\ E_a&=3.92\times 10^4\text{ J/mol} \end{align*}

Key Takeaways

  • The activation energy can be graphically determined by manipulating the Arrhenius equation.
  • The activation energy can also be calculated algebraically if k is known at two different temperatures.

Media Attributions

  • “Arrhenius2” © 1909 by Meisenbach, Riffarth, & Co. Leipzig is licensed under a Public Domain license

Icon for the Creative Commons Attribution 4.0 International License

Activation Energy and the Arrhenius Equation 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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