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Introductory Chemistry - 1st Canadian Edition: Concentration–Time Relationships: Integrated Rate Laws

Introductory Chemistry - 1st Canadian Edition
Concentration–Time Relationships: Integrated Rate Laws
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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

Concentration–Time Relationships: Integrated Rate Laws

Jessie A. Key

Learning Objectives

  • To gain an understanding of graphical methods used to determine rate laws.
  • To gain an understanding of half-life with respect to first-order reactions.

An alternate way to determine a rate law is to monitor the concentration of reactants or products in a single trial over a period of time and compare that to what is expected mathematically for a first-, second-, or zero-order reaction.

First-Order Reactions

We have seen earlier that the rate law of a generic first-order reaction where A → B can be expressed in terms of the reactant concentration:

\text{Rate of reaction}=-\dfrac{\Delta \ [A]}{\Delta \ t}=\text{k}[A]^1

This form of the rate law is sometimes referred to as the differential rate law. We can perform a mathematical procedure known as an integration to transform the rate law to another useful form known as the integrated rate law:

\ln \dfrac{[A]_t}{[A]_0}=-kt

where “ln” is the natural logarithm, [A]0 is the initial concentration of A, and [A]t is the concentration of A at another time.

The process of integration is beyond the scope of this textbook, but is covered in most calculus textbooks and courses. The most useful aspect of the integrated rate law is that it can be rearranged to have the general form of a straight line (y = mx + b).

    \begin{align*} \ln [A]_t&=-kt+\ln [A]_0 \\ y&=mx+b \end{align*}

Therefore, if we were to graph the natural logarithm of the concentration of a reactant (ln) versus time, a reaction that has a first-order rate law will yield a straight line, while a reaction with any other order will not yield a straight line (Figure 17.7 “Concentration vs. Time, First-Order Reaction”). The slope of the straight line corresponds to the negative rate constant, –k, and the y-intercept corresponds to the natural logarithm of the initial concentration.

Plot of natural logarithm of concentration versus time for a first-order reaction.
Figure 17.7. “Concentration vs. Time, First-Order Reaction.” This graph shows the plot of the natural logarithm of concentration versus time for a first-order reaction.

Example 17.4

The decomposition of a pollutant in water at 15°C occurs with a rate constant of 2.39 y−1, following first-order kinetics. If a local factory spills 6,500 moles of this pollutant into a lake with a volume of 2,500 L, what will the concentration of pollutant be after two years, assuming the lake temperature remains constant at 15°C?

Solution

We are given the rate constant and time and can determine an initial concentration from the number of moles and volume given.

[\text{Pollutant}]_0=\dfrac{6500 \text{ mol}}{2500\text{ L}}=2.6\text{ M}

We can substitute this data into the integrated rate law of a first-order equation and solve for the concentration after 2.0 years:

    \begin{align*} \ln [\text{Pollutant}]_{2 \ y}&=-kt+\ln [\text{Pollutant}]_0 \\ &=-(2.39y_{-1})(2.0y)+\ln (2.6\text{ M}) \\ &=-4.78+0.955=-3.82 \\ [\text{Pollutant}]_{2 \ y}&=e^{-3.82}=0.022\text{ M} \end{align*}

Second-Order Reactions

The rate for second-order reactions depends either on two reactants raised to the first power or a single reactant raised to the second power. We will examine a reaction that is the latter type: C → D. The differential rate law can be written:

\text{Rate of reaction}=-\dfrac{\Delta \ [C]}{\Delta \ t}=k[C]^2

The integrated rate law can be written in the form of a straight line as:

\dfrac{1}{[C]_t}=kt+\dfrac{1}{[C]_0}

Therefore, if the reaction is second order, a plot of \frac{1}{[C]_t} versus t will produce a straight line with a slope that corresponds to the rate constant, k, and a y-intercept that corresponds to the inverse of the initial concentration, \frac{1}{[C]_0} (Figure 17.8 “\frac{1}{[C]_t} vs. Time, Second-Order Reaction”).

Plot of 1/[C] versus time for a second-order reaction.
Figure 17.8 “\frac{1}{[C]_t} vs. Time, Second-Order Reaction.” The graph shows a plot of \frac{1}{[C]_t} versus time for a second-order reaction.

Zero-Order Reactions

Zero-order reaction rates occur when the rate of reactant disappearance is independent of reactant concentrations. The differential rate law for the hypothetical zero-order reaction E → F could be written as:

\text{Rate of reaction}=-\dfrac{\Delta \ [E]}{\Delta \ t}=k

The integrated rate law can be written in the form of a straight line as:

[E]_{t}=-kt + [E]_{0}

 Therefore, if the reaction is zero order, a plot of [E] versus t will produce a straight line with a slope that corresponds to the negative of the product of the rate constant and time, −kt, and a y-intercept that corresponds to the initial concentration, [E]0 (Figure 17.9 “Concentration vs. Time, Zero-Order Reaction”).

Plot of concentration versus time for a zero-order reaction.
Figure 17.9 “Concentration vs. Time, Zero-Order Reaction.” The graph shows the plot of concentration versus time for a zero-order reaction.

Graphical Methods for Determining Reaction Order—A Summary

We have just seen that first-, second-, and zero-order reactions all have unique, integrated rate-law equations that allow us to plot them as a straight line (y = mx + b) (Table 17.1 “Integrated Rate Law Summary”). When presented with experimental concentration–time data, we can determine the order by simply plotting the data in different ways to obtain a straight line.

Table 17.1 Integrated Rate Law Summary
Reaction OrderIntegrated Rate LawCharacteristic Kinetic PlotSlope of Kinetic PlotUnits of Rate Constant
Zero[E]t = −kt + [E]0[E] vs. t−ktmol L−1s−1
Firstln [A]t = −kt + ln [A]0ln [A] vs. t−ks−1
Second\frac{1}{[C]_t}=kt+\frac{1}{[C]_0}\frac{1}{[C]} vs. tkL mol−1s−1

Example 17.5

The following data were obtained for the reaction 3A → 2B:

Time, s05101520
[A], M0.2000.02820.01560.01060.008

Determine the order of the reaction.

Solution

We can plot the characteristic kinetic plots of zero-, first-, and second-order reactions to determine which will give a straight line.

Time, s[A], mol L−1ln [A]\frac{1}{[A]}, L mol−1
00.200−1.615.00
50.0282−3.5735.5
100.0156−4.1664.1
150.0106−4.5594.3
200.008−4.83125
Example Kinetics Plots

The reaction is second order since \dfrac{1}{[A]_t} versus t gives a straight line.

Half-Life

The half-life of a reaction, t_{\frac{1}{2}}, is the duration of time required for the concentration of a reactant to drop to one-half of its initial concentration.

[A]_{t_{\frac{1}{2}}}=\dfrac{1}{2}[A]_{0}

Half-life is typically used to describe first-order reactions and serves as a metric to discuss the relative speeds of reactions. A slower reaction will have a longer half-life, while a faster reaction will have a shorter half-life.

To determine the half-life of a first-order reaction, we can manipulate the integrated rate law by substituting t_{\frac{1}{2}} for t and [A]_{t_{\frac{1}{2}}}=[A]_0 for [A]_t, then solve for t_{\frac{1}{2}}:

    \begin{align*} \ln&=-kt+\ln\hspace{2em}\text{(integrated rate law for a first-order reaction)} \\ \\ \ln \dfrac{1}{2}[A]_0&=-kt_{\frac{1}{2}}+\ln [A]_0 \\ \\ \ln \dfrac{\frac{1}{2}[A]_0}{[A]_0}&=-kt_{\frac{1}{2}} \\ \\ \ln \dfrac{1}{2}&=-kt_{\frac{1}{2}} \\ \\ t_{\frac{1}{2}}&=-\dfrac{\ln \frac{1}{2}}{k}=\dfrac{0.693}{k} \end{align*}

Since the half-life equation of a first-order reaction does not include a reactant concentration term, it does not rely on the concentration of reactant present. In other words, a half-life is independent of concentration and remains constant throughout the duration of the reaction. Consequently, plots of kinetic data for first-order reactions exhibit a series of regularly spaced t_{\frac{1}{2}} intervals (Figure 17.10 “Generic First-Order Reaction Kinetics Plot”).

A kinetics plot of a generic first-order reaction showing repeating half-lives.
Figure 17.10 “Generic First-Order Reaction Kinetics Plot.” This graph shows repeating half-lives on a kinetics plot of a generic first-order reaction.

Example 17.6

A reaction having a first-order rate has a rate constant of 4.00 × 10−3 s−1.

  1. Determine the half-life.
  2. How long will it take for a sample of reactant at 1.0 M to decrease to 0.25 M?
  3. What concentration of the 1.0 M sample of reactant would you expect to be present after it has reacted for 500 s?

Solution

  1. t_{\frac{1}{2}}=\dfrac{0.693}{k}=\dfrac{0.693}{4.00\times 10^{-3}\text{ s}^{-1}}=173\text{ s}
  2. A simple way to calculate this is to determine how many half-lives it will take to go from 1.00 M to 0.250 M and use the half-life calculated in part 1.

        \begin{align*} 1\text{ half-life}&=0.500\text{ M} \\ 2\text{ half-lives}&=0.250\text{ M} \end{align*}

    Therefore, it will take 2 × 173 s = 346 s.

  3. We can use the rate-constant value in the integrated rate law to determine the concentration remaining.

        \begin{align*} \ln \dfrac{[A]_t}{[A]_0}&=-kt \\ \\ \ln \dfrac{[A]_t}{1.0\text{ M}}&=-(4.00\times 10^{-3}\text{ s}^{-1})(500\text{ s}) \\ \\ \ln \dfrac{[A]_t}{1.0\text{ M}}&=-2 \\ \\ \dfrac{[A]_t}{1.0\text{ M}}&=e^{-2}=0.135 \\ \\ [A]_t&=0.14\text{ M} \end{align*}

Key Takeaways

  • The reaction rate may be determined by monitoring the concentration of reactants or products in a single trial over a period of time and comparing it to what is expected mathematically for a first-, second-, or zero-order reaction.
  • The half-life of a reaction is the duration of time required for the concentration of a reactant to drop to one-half of its initial concentration.

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Concentration–Time Relationships: Integrated Rate Laws 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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