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Introductory Chemistry - 1st Canadian Edition: Strong and Weak Acids and Bases and Their Salts

Introductory Chemistry - 1st Canadian Edition
Strong and Weak Acids and Bases and Their Salts
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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

Strong and Weak Acids and Bases and Their Salts

Learning Objectives

  1. Define a strong and a weak acid and base.
  2. Recognize an acid or a base as strong or weak.
  3. Determine if a salt produces an acidic or a basic solution.

Except for their names and formulas, so far we have treated all acids as equals, especially in a chemical reaction. However, acids can be very different in a very important way. Consider HCl(aq). When HCl is dissolved in H2O, it completely dissociates into H+(aq) and Cl−(aq) ions; all the HCl molecules become ions:

HCl → H+(aq) + Cl−(aq) (100%)

Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid. HC2H3O2 is an example of a weak acid:

HC2H3O2→ H+(aq) + C2H3O2−(aq) (~5%)

Because this reaction does not go 100% to completion, it is more appropriate to write it as an equilibrium:

HC2H3O2 ⇄ H+(aq) + C2H3O2−(aq)

As it turns out, there are very few strong acids, which are given in Table 12.1 “Strong Acids and Bases”. If an acid is not listed here, it is a weak acid. It may be 1% ionized or 99% ionized, but it is still classified as a weak acid.

The issue is similar with bases: a strong base is a base that is 100% ionized in solution. If it is less than 100% ionized in solution, it is a weak base. There are very few strong bases (see Table 12.1); any base not listed is a weak base. All strong bases are OH– compounds. So a base based on some other mechanism, such as NH3 (which does not contain OH− ions as part of its formula), will be a weak base.

Table 12.1 Strong Acids and Bases
AcidsBases
HClLiOH
HBrNaOH
HIKOH
HNO3RbOH
H2SO4CsOH
HClO3Mg(OH)2
HClO4Ca(OH)2
Sr(OH)2
Ba(OH)2

Example 12.2

Identify each acid or base as strong or weak.

  1. HCl
  2. Mg(OH)2
  3. C5H5N

Solution

  1. Because HCl is listed in Table 12.1, it is a strong acid.
  2. Because Mg(OH)2 is listed in Table 12.1, it is a strong base.
  3. The nitrogen in C5H5N would act as a proton acceptor and therefore can be considered a base, but because it does not contain an OH compound, it cannot be considered a strong base; it is a weak base.

Test Yourself
Identify each acid or base as strong or weak.

  1. RbOH
  2. HNO2

Answers

  1. strong base
  2. weak acid

Example 12.3

Write the balanced chemical equation for the dissociation of Ca(OH)2 and indicate whether it proceeds 100% to products or not.

Solution
This is an ionic compound of Ca2+ ions and OH− ions. When an ionic compound dissolves, it separates into its constituent ions:

Ca(OH)2 → Ca2+(aq) + 2OH−(aq)

Because Ca(OH)2 is listed in Table 12.1, this reaction proceeds 100% to products.

Test Yourself
Write the balanced chemical equation for the dissociation of hydrazoic acid (HN3) and indicate whether it proceeds 100% to products or not.

Answer
The reaction is as follows:

HN3 → H+(aq) + N3−(aq)

It does not proceed 100% to products because hydrazoic acid is not a strong acid.

Certain salts will also affect the acidity or basicity of aqueous solutions because some of the ions will undergo hydrolysis, just like NH3 does to make a basic solution. The general rule is that salts with ions that are part of strong acids or bases will not hydrolyze, while salts with ions that are part of weak acids or bases will hydrolyze.

Consider NaCl. When it dissolves in an aqueous solution, it separates into Na+ ions and Cl− ions:

NaCl → Na+(aq) + Cl−(aq)

Will the Na+(aq) ion hydrolyze? If it does, it will interact with the OH− ion to make NaOH:

Na+(aq) + H2O → NaOH + H+(aq)

However, NaOH is a strong base, which means that it is 100% ionized in solution:

NaOH → Na+(aq) + OH−(aq)

The free OH−(aq) ion reacts with the H+(aq) ion to remake a water molecule:

H+(aq) + OH−(aq) → H2O

The net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Na+(aq) ion. What about the Cl− ion? Will it hydrolyze? If it does, it will take an H+ ion from a water molecule:

Cl−(aq) + H2O → HCl + OH−

However, HCl is a strong acid, which means that it is 100% ionized in solution:

HCl → H+(aq) + Cl−(aq)

The free H+(aq) ion reacts with the OH−(aq) ion to remake a water molecule:

H+(aq) + OH−(aq) → H2O

The net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Cl−(aq) ion. Because neither ion in NaCl affects the acidity or basicity of the solution, NaCl is an example of a neutral salt.

Things change, however, when we consider a salt like NaC2H3O2. We already know that the Na+ ion won’t affect the acidity of the solution. What about the acetate ion? If it hydrolyzes, it will take an H+ from a water molecule:

C2H3O2−(aq) + H2O → HC2H3O2 + OH−(aq)

Does this happen? Yes, it does. Why? Because HC2H3O2 is a weak acid. Any chance a weak acid has to form, it will (the same with a weak base). As some C2H3O2− ions hydrolyze with H2O to make the molecular weak acid, OH− ions are produced. OH− ions make solutions basic. Thus NaC2H3O2 solutions are slightly basic, so such a salt is called a basic salt.

There are also salts whose aqueous solutions are slightly acidic. NH4Cl is an example. When NH4Cl is dissolved in H2O, it separates into NH4+ ions and Cl− ions. We have already seen that the Cl− ion does not hydrolyze. However, the NH4+ ion will:

NH4+(aq) + H2O → NH3(aq) + H3O+(aq)

Recall from the section “Arrhenius Acids and Bases” that H3O+ ion is the hydronium ion, the more chemically proper way to represent the H+ ion. This is the classic acid species in solution, so a solution of NH4+(aq) ions is slightly acidic. NH4Cl is an example of an acid salt. The molecule NH3 is a weak base, and it will form when it can, just like a weak acid will form when it can.

So there are two general rules:

  1. If an ion derives from a strong acid or base, it will not affect the acidity of the solution.
  2. If an ion derives from a weak acid, it will make the solution basic; if an ion derives from a weak base, it will make the solution acidic.

Example 12.4

Identify each salt as acidic, basic, or neutral.

  1. KCl
  2. KNO2
  3. NH4Br

Solution

  1. The ions from KCl derive from a strong acid (HCl) and a strong base (KOH). Therefore, neither ion will affect the acidity of the solution, so KCl is a neutral salt.
  2. Although the K+ ion derives from a strong base (KOH), the NO2− ion derives from a weak acid (HNO2). Therefore the solution will be basic, and KNO2 is a basic salt.
  3. Although the Br− ions derive from a strong acid (HBr), the NH4+ ion derives from a weak base (NH3), so the solution will be acidic, and NH4Br is an acidic salt.

Test Yourself
Identify each salt as acidic, basic, or neutral.

  1. (C5H5NH)Cl
  2. Na2SO3

Answers

  1. acidic
  2. basic

Some salts are composed of ions that come from both weak acids and weak bases. The overall effect on an aqueous solution depends on which ion exerts more influence on the overall acidity. We will not consider such salts here.

Key Takeaways

  • Strong acids and bases are 100% ionized in aqueous solution.
  • Weak acids and bases are less than 100% ionized in aqueous solution.
  • Salts of weak acids or bases can affect the acidity or basicity of their aqueous solutions.

Exercises

Questions

  1. Differentiate between a strong acid and a weak acid.
  2. Differentiate between a strong base and a weak base.
  3. Identify each as a strong acid or a weak acid. Assume aqueous solutions.
    1. HF
    2. HCl
    3. HC2O4
  4. Identify each as a strong base or a weak base. Assume aqueous solutions.
    1. NaOH
    2. Al(OH)3
    3. C4H9NH2
  5. Write a chemical equation for the ionization of each acid and indicate whether it proceeds 100% to products or not.
    1. HNO3
    2. HNO2
    3. HI3
  6. Write a chemical equation for the ionization of each base and indicate whether it proceeds 100% to products or not.
    1. NH3
    2. (CH3)3N
    3. Mg(OH)2
  7. Write the balanced chemical equation for the reaction of each acid and base pair.
    1. HCl + C5H5N
    2. H2C2O4 + NH3
    3. HNO2 + C7H9N
  8. Write the balanced chemical equation for the reaction of each acid and base pair.
    1. H3C5H5O7 + Mg(OH)2
    2. HC3H3O3 + (CH3)3N
    3. HBr + Fe(OH)3
  9. Identify each salt as neutral, acidic, or basic.
    1. NaBr
    2. Fe(NO3)2
    3. Fe(NO3)3
  10. Identify each salt as neutral, acidic, or basic.
    1. NH4I
    2. C2H5NH3Cl
    3. KI
  11. Identify each salt as neutral, acidic, or basic.
    1. NaNO2
    2. NaNO3
    3. NH4NO3
  12. Identify each salt as neutral, acidic, or basic.
    1. KC2H3O2
    2. KHSO4
    3. KClO3
  13. Write the hydrolysis reaction that occurs, if any, when each salt dissolves in water.
    1. K2SO3
    2. KI
    3. NH4ClO3
  14. Write the hydrolysis reaction that occurs, if any, when each salt dissolves in water.
    1. NaNO3
    2. CaC2O4
    3. C5H5NHCl
  15. When NH4NO2 dissolves in H2O, both ions hydrolyze. Write chemical equations for both reactions. Can you tell if the solution will be acidic or basic overall?
  16. When pyridinium acetate (C5H5NHC2H3O2) dissolves in H2O, both ions hydrolyze. Write chemical equations for both reactions. Can you tell if the solution will be acidic or basic overall?
  17. A lab technician mixes a solution of 0.015 M Mg(OH)2. Is the resulting OH− concentration greater than, equal to, or less than 0.015 M? Explain your answer.
  18. A lab technician mixes a solution of 0.55 M HNO3. Is the resulting H+ concentration greater than, equal to, or less than 0.55 M? Explain your answer.

Answers

  1. A strong acid is 100% ionized in aqueous solution, whereas a weak acid is not 100% ionized.
    1. weak acid
    2. strong acid
    3. weak acid
    1. HNO3(aq) → H+(aq) + NO3−(aq); proceeds 100%
    2. HNO2(aq) → H+(aq) + NO2−(aq); does not proceed 100%
    3. HI3(aq) → H+(aq) + I3−(aq); does not proceed 100%
    1. HCl + C5H5N → Cl− + C5H5NH+
    2. H2C2O4 + 2NH3 → C2O42− + 2NH4+
    3. HNO2 + C7H9N → NO2− + C7H9NH+
    1. neutral
    2. acidic
    3. acidic
    1. basic
    2. neutral
    3. acidic
    1. SO32− + H2O → HSO3− + OH−
    2. no reaction
    3. NH4+ + H2O → NH3 + H3O+
  1. NH4+ + H2O → NH3 + H3O+; NO2− + H2O → HNO2 + OH−; it is not possible to determine whether the solution will be acidic or basic.
  1. Greater than 0.015 M, because there are two OH− ions per formula unit of Mg(OH)2.

Annotate

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Brønsted-Lowry Acids and Bases
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Chemistry

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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