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5.4: Neutralization Reactions and Gas Evolution

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    Learning Objectives
    • Predict the products of an acid base reaction
    • Write the net ionic equation of neutralization reaction

    An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to numerous natural and technological processes, ranging from the chemical transformations that take place within cells and the lakes and oceans, to the industrial-scale production of fertilizers, pharmaceuticals, and other substances essential to society. The subject of acid-base chemistry, therefore, is worthy of thorough discussion, and a full chapter is devoted to this topic later in the text.

    A neutralization reaction is a type of double displacement reaction where an acid reacts with a base to form water and salt. If the base is a metal hydroxide, then the general formula for the reaction of an acid with a base is described as follows:

    \[ acid+base\rightarrow salt+water\tag{5.4.1}\]

    For example, the reaction of equimolar amounts of HBr and NaOH to give water and a salt (NaBr) is a neutralization reaction:

    \[ \underset{acid}{HBr(aq)} + \underset{base}{NaOH(aq)} \rightarrow \underset{water}{H_2 O(l)} + \underset{salt}{NaBr(aq)} \tag{5.4.2}\]

    Note that in addition to water, this reaction produces a salt, sodium bromide.

    If we write the complete ionic equation for the reaction in Equation 5.4.1, we see that \(Na^+_{(aq)}\) and \(Br^−_{(aq)}\) are spectator ions and are not involved in the reaction:

    \( H^+ (aq) + \cancel{Br^- (aq)} + \cancel{Na^+ (aq)} + OH^- (aq) \rightarrow H_2 O(l) + \cancel{Na^+ (aq)} + \cancel{Br^- (aq)} \tag{5.4.3}\)

    The overall reaction is therefore simply the combination of H+(aq) and OH−(aq) to produce H2O, as shown in the net ionic equation:

    \(H^+(aq) + OH^-(aq) \rightarrow H_2O(l)\tag{5.4.4}\)

    The net ionic equation for the reaction of any strong acid with any strong base is identical to Equation 5.4.4.

    Acid/Base Neutralization Reactions & Net Ionic Equations: https://youtu.be/gDS93ySeF80

    The strengths of the acid and the base generally determine whether the reaction goes to completion. The reaction of any strong acid with any strong base goes essentially to completion, as does the reaction of a strong acid with a weak base, and a weak acid with a strong base. Examples of the last two are as follows:

    \( \underset{strong\: acid}{HCl(aq)} + \underset{weak\: base}{NH_3 (aq)} \rightarrow \underset{salt}{NH_4 Cl(aq)} \tag{5.4.5}\)

    \( \underset{weak\: acid} {CH_3 CO _2 H(aq)} + \underset{strong\: base}{NaOH(aq)} \rightarrow \underset{salt}{CH _3 CO _2 Na(aq)} + H_2 O(l) \tag{5.4.6}\)

    Sodium acetate is written with the organic component first followed by the cation, as is usual for organic salts. Most reactions of a weak acid with a weak base also go essentially to completion. One example is the reaction of acetic acid with ammonia:

    \( \underset{weak\: acid}{CH _3 CO _2 H(aq)} + \underset{weak\: base}{NH_3 (aq)} \rightarrow \underset{salt}{CH_3 CO_2 NH_4 (aq)} \tag{5.4.7}\)

    An example of an acid–base reaction that does not go to completion is the reaction of a weak acid or a weak base with water, which is both an extremely weak acid and an extremely weak base.

    In some cases, the reaction of an acid with an anion derived from a weak acid (such as HS−) produces a gas (in this case, H2S). Because the gaseous product escapes from solution in the form of bubbles, the reverse reaction cannot occur. Therefore, these reactions tend to be forced, or driven, to completion. Examples include reactions in which an acid is added to ionic compounds that contain the HCO3−, CN−, or S2− anions, all of which are driven to completion:

    \( HCO_3^- (aq) + H^+ (aq) \rightarrow H_2 CO_3 (aq) \tag{5.4.8}\)

    \( H_2 CO_3 (aq) \rightarrow CO_2 (g) + H_2 O(l) \)

    \( CN^- (aq) + H^+ (aq) \rightarrow HCN(g) \tag{5.4.9}\)

    \( S ^{2-} (aq) + H^+ (aq) \rightarrow HS^- (aq) \tag{5.4.10}\)

    \( HS^- (aq) + H^+ (aq) \rightarrow H_2 S(g) \)

    Example \(\PageIndex{1}\): Writing Equations for Acid-Base Reactions

    Calcium propionate is used to inhibit the growth of molds in foods, tobacco, and some medicines. Write a balanced chemical equation for the reaction of aqueous propionic acid (CH3CH2CO2H) with aqueous calcium hydroxide [Ca(OH)2] to give calcium propionate. Do you expect this reaction to go to completion, making it a feasible method for the preparation of calcium propionate?

    Solution

    Given: reactants and product

    Asked for: balanced chemical equation and whether the reaction will go to completion

    Strategy:

    Write the balanced chemical equation for the reaction of propionic acid with calcium hydroxide. Based on their acid and base strengths, predict whether the reaction will go to completion.

    Solution:

    Propionic acid is an organic compound that is a weak acid, and calcium hydroxide is an inorganic compound that is a strong base. The balanced chemical equation is as follows:

    \(2CH_3CH_2CO_2H(aq) + Ca(OH)_2(aq) \rightarrow (CH_3CH_2CO_2)_2Ca(aq) + 2H_2O(l)\)

    The reaction of a weak acid and a strong base will go to completion, so it is reasonable to prepare calcium propionate by mixing solutions of propionic acid and calcium hydroxide in a 2:1 mole ratio.

    Exercise

    Write a balanced chemical equation for the reaction of solid sodium acetate with dilute sulfuric acid to give sodium sulfate.

    Answer: \(2CH_3CO_2Na(s) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + 2CH_3CO_2H(aq)\)

    Exercise \(\PageIndex{1}\)

    Write balanced chemical equations for the acid-base reactions when a solution of barium hydroxide is neutralized with a solution of nitric acid

    Answer

    Ba(OH)2(aq)+2HNO3(aq)→Ba(NO3)2(aq)+2H2O(l)

    Summary

    Acid-base reactions involve the transfer of hydrogen ions between reactants.

    Glossary

    acid-base reaction
    reaction involving the transfer of a hydrogen ion between reactant species
    Neutralization reaction
    reaction between an acid and a base to produce salt and water
    salt
    ionic compound that can be formed by the reaction of an acid with a base that contains a cation and an anion other than hydroxide or oxide

    Contributors and Attributions

    5.4: Neutralization Reactions and Gas Evolution is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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