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5.5: Neutralization Reactions

  • Page ID
    282118
  • Learning Objectives

    • Define the Arhennius acid/base and Bronsted-Lowry acid/base and differentiate between them
    • Recognize if an acid or base is strong or weak.
    • Identify the acid and base in chemical reaction
    • Write molecular, complete ionic, and net ionic equations for acid-base neutralization reactions

    Acids and Bases

    What are acids and bases?

    We will cover two definitions, the Arrhenius and Bronsted-Lowry definitions.

    Arrhenius Acid/Base

    Arrhenius Acid - Increases Hydronium ion to that of neutral water

    Arrhenius Base - Increases hydroxide ion to that of neutral water.

    This definition is based on the behavior of acids and bases when they are added to water. First we need to look at the autoionization of water. When two
    water molecules bump into each other a proton can be transferred from one to the other forming hydronium and hydroxide ions (eq. 3.5.1).

    \[H_2O(l) + H_2O(l) \rightarrow H_3O^+(aq) + OH^-(aq)\]

    Arrhenius Acid

    These are compounds that donate a proton to water.

    \[HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq)+Cl^-(aq)\]

    This is often written with the waters omitted, that is H+(aq) represents hydronium ion and not a proton.

    \[HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)\]

    Arrhenius Base

    These are compounds that increase the hydroxide concentration. They can be soluble ionic compounds that have hydroxide ions, or compounds that remove a proton from water forming hydroxide, and the later can be molecules.

    Sodium hydroxide is an ionic compound that is a strong base (you can it is soluble because of solubility rule IA)

    \[NaOH(aq) \rightarrow Na^+(aq)+OH^-(aq)\]

    Ammonia is a molecule that is a weak base, and it removes a proton from water forming ammonium and hydroxide. But it does not react much, so we use a two way arrow.

    \[NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)\]

    In the later case the ammonia took a proton from the water and this reaction is similar to the autoinoization of water, except that it is an ammonia and not another water molecule that acquires the second proton.

    NOTE: Stong Acids and Bases are strong electrolytes

    Bronsted-Lowry Acid/Base

    The Bronsted-Lowry definition is broader than the Arrhenius. If you look at the Arrhenius Acid, it donates a proton to water, so the Bronstead expands that to donating a proton to anything, not just water. Likewise, the Arrhenius accepts a water from water, and the Bronstead expands that to accepting a Proton from anything.

    Bronstead-Lowry Acid: A Proton Donor. Note the HCl gave a proton to the water molecule in the above example

    Bronstead-Lowry Base: A Proton Acceptor. Note the ammonia above accepted a proton from the water.

    Extending this to a reaction that does not involve water:

    \[HCl + NH_3 \rightarrow NH_4Cl\]

    Where HCl is the acid (it donates a proton to ammonia and forms ammonium) while ammonia is the proton acceptor (becoming the ammonium ion).

     

    Video \(\PageIndex{1}\): YouTube Visualization of Acid and Base behavior in water uploaded by Resa Kelly (https://youtu.be/Q7Z5jijT-Ek)

    Exercise \(\PageIndex{8}\)

    Identify the following as a Bronsted-Lowry acid, base, both, or neither

    1. H3PO4
    2. CaCl2
    3. SO3-2
    4. NaOH
    Answer a

    Bronsted-Lowry acid - has 3 protons (H+) to donate

    Answer b

    neither - ionic compound that is charge balanced. Neither gives nor accepts protons

    Answer c

    Bronsted-Lowry base - is an anion and so can accept a positive proton (in fact two of them).

    Answer d

    Bronsted-Lowry base - the hydroxide ion accepts a proton and so functions as a bronstead base

    Neutralization Reactions

    A reaction between an acid and a base is called a neutralization reaction, and these can be considered to be a type of displacement reaction, where the proton of the acid is being displaced as it is given to another species. You may have heard that a neutralization reaction is when an acid and a base react to form a salt plus water. That is not always true, but they do always form a salt, as the hydrogen is being "displaced" from the acid in the formation of the salt.

    Neutralization reactions always give off heat, that is they are "exothermic" (a concept we will study in Chapter 5), but when a neutralization reactions forms salt and a soluble salt, the only observation may be the heat evolved.

    There are 4 types of neutralization reactions, depending on whether the acid and base are strong or weak.

    1. Strong Acids and Strong Bases
    2. Strong Acids and Weak Bases
    3. Weak Acids and Strong Bases
    4. Weak Acids and Weak Bases

    To identify which type reaction is going on, you need to know if the acid or base is strong. need to know the following strong acids and strong bases. There are others, and if a problem calls it a strong acid, treat it as a strong acid. Strong acids were covered in section 3.4.1.2.1 Acids and strong bases are covered by the solubility rules Ia and IIIa of section 3.4.2.1. Table \(\PageIndex{1}\) reviews them for you.

    Strong Acids Strong Bases
    • HCl, HBr, HI
    • HClO3, HClO4
    • HNO3
    • H2SO4
    • others as specified
    • Soluble alkali metal hydroxides (solubility rule 1a)
      • LiOH, NaOH, KOH, RbOH, CsOH
    • Soluble heavier alkaline earth hydroxides (solubility rule 3a)
      • Sr(OH)2, Ba(OH)2​ ​​​​​​
      • Ca(OH)2 should probably be considered as a strong base even though we are not calling it a soluble salt in rule 3a. A saturated solution has is 1.8 g/L) and thus strong, and we will cover these "border cases" in gen chem 2 when we get to solubility constants.

    You need to know the general (molecular), total (complete) and net ionic equations for acid base neutralization reactions

    Strong Acid and Strong Base

    The result is a salt plus water; the anion of the acid and the cation of the base are spectator ions. Example: HCl and NaOH

    General Molecular Equation:

    \[HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l) \]

    Total Ionic Equation
    \[H^+(aq) + Cl^-(aq)+ Na^+(aq) + OH^-(aq) \rightarrow Na^+(aq) + Cl^-(aq) + H_2O(l)\]

    Net Ionic Equation

    \[H^+(aq) + OH^-(aq) \rightarrow H_2O(l)\]

    Video \(\PageIndex{1}\): 1'35" YouTube predicting products and writing aqueous equations for a strong acid and a strong base, (https://youtu.be/-NfFjKwCDp4). Digital corrigendum: "all" salts of sodium (not chloride) are soluble

    Strong Acid and Weak Base

    The cation of the strong acid is a spectator ion. Example: HCl & NH3.

    General Equation: \[HCl(aq) + NH_3(aq) \rightarrow NH_4Cl(aq)\]

    Total Ionic Equation: \[H^+(aq) + Cl^-(aq)+ NH_3(aq) \rightarrow NH_4^+(aq) + Cl^-(aq)\]

    Net Ionic Equation: \[H^+(aq) + NH_3(aq)\rightarrow NH_4^+(aq)\]

    Video \(\PageIndex{2}\): 1'06" Youtube on the reactions of strong acid and weak base (https://youtu.be/Yf2YQ6QvMEc).

    Weak Acid and Strong Base

    In the reaction of a Weak Acid and a Strong base the cation of the base is a spectator ion. Example: HF + NaOH.

    General Equation

    \[HF(aq)+NaOH(aq) \rightarrow NaF(aq) +H_2O\]

    Total Ionic Equation
    \[HF(aq) + Na^+(aq) + OH^-(aq) \rightarrow Na^+(aq) + F^-(aq) + H_2O(l)\]

    Net Ionic Equation

    \[HF(aq)+ OH^-(aq) \rightarrow F^-(aq) + H_2O(l)\]

    Video \(\PageIndex{3}\): 1'19" Youtube on the reactions of weak acid and strong base (https://youtu.be/bI0tolYG_s0).

    Weak Acid and Weak Base

    In the reaction of a weak acid and a weak base there is no spectator ion. Since there are no spectator ions, the total ionic and the net ionic are exactly the same. Example HF and NH3.

    General Equation

    \[HF(aq) + NH_3(aq) \rightarrow NH_4F(aq)\]

    Total Ionic Equation
    \[HF(aq) + NH_3 (aq) \rightarrow NH_4^+(aq) + F^-(aq)\]

    Net Ionic Equation
    \[HF(aq) + NH_3 (aq) \rightarrow NH_4^+(aq) + F^-(aq)\]

    Video \(\PageIndex{4}\): 0'57" Youtube on the reactions of weak acid and strong base (https://youtu.be/561IAx1lzKk)

     

    Contributors

    Robert E. Belford (University of Arkansas Little Rock; Department of Chemistry). The breadth, depth and veracity of this work is the responsibility of Robert E. Belford, rebelford@ualr.edu. You should contact him if you have any concerns. This material has both original contributions, and content built upon prior contributions of the LibreTexts Community and other resources, including but not limited to:

    • November Palmer, Ronia Kattoum & Emily Choate (UALR)
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