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21.1: Neutralization

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Lesson Objectives

The student will:

  • explain what is meant by a neutralization reaction and give an example of one.
  • write a balanced equation for the reaction that occurs when an acid reacts with a base.
  • describe the formation of a salt in terms of the Arrhenius definitions of acids and bases.
  • predict the salt that will be produced from the neutralization reaction between a given acid and base.
  • identify acidic, basic, and neutral salts from a neutralization reaction.

Vocabulary

  • acidic salt
  • basic salt
  • neutral salt
  • neutralization

Introduction

Neutralization is a reaction between an acid and a base that produces water and a salt. The general reaction is shown below:

acid + base  \rightarrow salt + water

After reviewing the concept of ionic compounds, we will examine neutralization reactions and look at the different types of salts that can be formed from acids and bases as they react with one another.

Neutralization Reactions

Acids are a combination of a hydrogen cation and a nonmetal anion. Examples include \mathrm{HCl}, \mathrm{HNO}_3, and \mathrm{HC}_2\mathrm{H}_3\mathrm{O}_2. Many bases are a combination of metal cations and nonmetal anions. Examples include \mathrm{NaOH}, \mathrm{KOH}, and \mathrm{Mg(OH)}_2. According to the Arrhenius definitions of acids and bases, the acid will contribute an \mathrm{H}^+ ion that will react to neutralize the \mathrm{OH}^- ion contributed by the base, producing neutral water molecules.

All acid-base reactions produce salts. The anion from the acid will combine with the cation from the base to form the ionic salt. Examples are shown below.

\mathrm{HClO}_{4(aq)} + \mathrm{NaOH}_{(aq)} \rightarrow \mathrm{NaClO}_{4(aq)} + \mathrm{HOH}_{(l)}
\mathrm{H}_2\mathrm{SO}_{4(aq)} + 2 \mathrm{KOH}_{(aq)} \rightarrow \mathrm{K}_2\mathrm{SO}_{4(aq)} + 2 \mathrm{HOH}_{(l)}
(Note: \mathrm{HOH} = \mathrm{H}_2\mathrm{O})

No matter what the acid or the base may be, the products of this type of reaction will always be a salt and water. Aside from the fact that the \mathrm{H}^+ ion will neutralize the \mathrm{OH}^- ion to form water, we also know that these reactions are double displacement reactions, because they consist of cations exchanging anions.

We can write this as a total ionic equation:

\mathrm{H}^+_{(aq)} + \mathrm{ClO}^-_{4(aq)} + \mathrm{Na}^+_{(aq)} + \mathrm{OH}^-_{(aq)} \rightarrow \mathrm{Na}^+_{(aq)} + \mathrm{ClO}^-_{4(aq)} + \mathrm{H}_2\mathrm{O}_{(l)}

However, since \mathrm{Na}^+ and \mathrm{ClO}_4^- are spectator ions, the net ionic equation is:

\mathrm{H}^+_{(aq)} + \mathrm{OH}^-_{(aq)} \rightarrow \mathrm{H}_2\mathrm{O}_{(l)}

This is the net ionic equation for all neutralization reactions.

Salt Hydrolysis

When a salt is dissolved in water, it is possible for the solution to be neutral, acidic, or basic. If a solution is to be acidic, it must contain more hydrogen ions than hydroxide ions. For the solution to be basic, it must contain more hydroxide ions than hydrogen ions. Consider the solution produced when the salt \mathrm{KBr} dissolves in water. There will be four ions present in the solution.

\mathrm{K}^+ + \mathrm{Br}^- + \mathrm{H}^+ + \mathrm{OH}^-

When potassium ions in the solution come into contact with hydroxide ions, the KOH ion pair could potentially form. However, since KOH is a strong base, it would immediately dissociate back into ions. Similarly, if the bromide ions come into contact with hydrogen ions, the molecule formed would be \mathrm{HBr}, a strong acid, so they would immediately dissociate back into ions. Thus, having potassium and bromide ions in a water solution would not cause a reaction.

Now consider the solution produced when the salt \mathrm{NH}_4\mathrm{Cl} is dissolved in water. There will be four ions present in the solution.

\mathrm{NH}_4^+ + \mathrm{Cl}^- + \mathrm{H}^+ + \mathrm{OH}^-

When hydrogen ions come into contact with chloride ions, if they join together, the resultant molecule would be \mathrm{HCl}, which is a strong acid. Therefore, the \mathrm{HCl} would immediately dissociate back into the ions. When \mathrm{NH}_4^+ ions come into contact with \mathrm{OH}^- ions, however, the resultant molecule would be \mathrm{NH}_4\mathrm{OH}, which is a weak base and does not dissociate very much. Therefore, when ammonium chloride is dissolved in water, a reaction occurs.

\mathrm{NH}_4^+ + \mathrm{Cl}^- + \mathrm{H}^+ + \mathrm{OH}^- \rightarrow \mathrm{NH}_4\mathrm{OH}_{(aq)} + \mathrm{Cl}^- + \mathrm{H}^+

The ammonium hydroxide dissociates very little, so we would have mostly un-dissociated ammonium hydroxide molecules in solution with hydrogen and chloride ions. The hydrogen ions in this final solution would cause the solution to be acidic. Thus, dissolving ammonium chloride in water produces an acidic solution.

By a similar process, dissolving sodium acetate, \mathrm{NaC}_2\mathrm{H}_3\mathrm{O}_2, in water will produce a basic solution. When the sodium acetate is dissolved in water, four ions will be present in the solution.

\mathrm{Na}^+ + \mathrm{C}_2\mathrm{H}_3\mathrm{O}_2^- + \mathrm{H}^+ + \mathrm{OH}^-

If sodium ions contact hydroxide ions, the substance formed would be a strong base which would immediately dissociate. If hydrogen ions contact acetate ions, however, the molecule formed would be acetic acid, which is a weak acid and the ions would NOT dissociate. Therefore, when sodium acetate is dissolved in water, a reaction will occur as shown below.

\mathrm{Na}^+ + \mathrm{C}_2\mathrm{H}_3\mathrm{O}^-_2 + \mathrm{H}^+ + \mathrm{OH}^- \rightarrow \mathrm{HC}_2\mathrm{H}_3\mathrm{O}_{2(aq)} + \mathrm{Na}^+ + \mathrm{OH}^-

The resultant solution has excess of hydroxide ions, so it is basic. Dissolving sodium acetate in water produces a basic solution.

Neutral, Acidic, and Basic Salts

Table below shows all of the strong acids and bases that we have encountered so far. You can assume that any other acids and bases we will look at are weak.

Strong Acids and Bases
Strong Acid Formula Strong Base Formula
Hydrochloric Acid \mathrm{HCl} Lithium Hydroxide \mathrm{LiOH}
Hydrobromic Acid \mathrm{HBr} Sodium Hydroxide \mathrm{NaOH}
Hydroiodic Acid \mathrm{HI} Potassium Hydroxide \mathrm{KOH}
Nitric Acid \mathrm{HNO}_3 Rubidium Hydroxide \mathrm{RbOH}
Perchloric Acid \mathrm{HClO}_4 Cesium Hydroxide \mathrm{CsOH}
Sulfuric Acid \mathrm{H}_2\mathrm{SO}_4 Calcium Hydroxide \mathrm{Ca(OH)}_2
Strontium Hydroxide \mathrm{Sr(OH)}_2
Barium Hydroxide \mathrm{Ba(OH)}_2

The information in the table helps us to determine what type of salt is formed in an acid-base reaction. For example, a reaction between a strong acid and a strong base will form a neutral salt. It is like a power struggle between the acid and the base. Since both are strong, neither overpower the other, so the salt ends up being neutral. If, however, we have a reaction between a weak acid and a strong base, the result would be a basic salt.

\mathrm{HC}_2\mathrm{H}_3\mathrm{O}_{2(aq)} + \mathrm{NaOH}_{(aq)} \rightarrow \mathrm{NaC}_2\mathrm{H}_3\mathrm{O}_{2(aq)} + \mathrm{H}_2\mathrm{O}_{(l)}

Acetic acid is a weak acid and sodium hydroxide is a strong base. Therefore, the salt formed, sodium acetate, is a basic salt.

When the basic salt is dissolved in water, a reaction takes places in which extra hydroxide ions, \mathrm{OH}^-, are produced from the salt and the water molecules. A similar situation will occur when we have a strong acid reacting with a weak base. When a strong acid reacts with a weak base, an acidic salt is formed.

\mathrm{HCl}_{(aq)} + \mathrm{NH}_4\mathrm{OH}_{(aq)} \rightarrow \mathrm{NH}_4\mathrm{Cl}_{(aq)} + \mathrm{H}_2\mathrm{O}_{(l)}

Hydrochloric acid is a strong acid and ammonium hydroxide is a weak base. Therefore, the salt formed, ammonium chloride, is an acidic salt. The salt will react with water molecules to produce hydrogen ions, \mathrm{H}^+, so it is referred to as an acidic salt.

Example:

Complete the following neutralization reactions and identify the type of salt produced.

  1. \mathrm{H}_2\mathrm{SO}_4 + \mathrm{Ba(OH)}_2 \rightarrow ?
  2. \mathrm{HCOOH} + \mathrm{Ca(OH)}_2 \rightarrow ?

Solution:

  1. \mathrm{H}_2\mathrm{SO}_4 + \mathrm{Ba(OH)}_2 \rightarrow \mathrm{BaSO}_4 + 2 \ \mathrm{H}_2\mathrm{O}; A strong acid reacts with a strong base and produces a neutral salt.
  2. 2 \ \mathrm{HCOOH} + \mathrm{Ca(OH)}_2 \rightarrow \mathrm{Ca(HCOO)}_2 + 2 \ \mathrm{H}_2\mathrm{O}; A weak acid reacts with a strong base and produces a basic salt.

By determining which acid and base were used to form the salt, you can figure out if the salt is acidic, basic, or neutral. For example, let's say you are looking at calcium nitrate, \mathrm{Ca(NO}_3)_2. Remember that there is a double displacement reaction that forms the salt, so we can write out the reaction:

Therefore, the neutralization reaction would have been:

2 \ \mathrm{HNO}_3 + \mathrm{Ca(OH)}_2 \rightarrow \mathrm{Ca(NO}_3)_2 + 2 \ \mathrm{HOH}

This salt would have been produced from a strong acid-strong base reaction, so it is neutral.

Let’s try another. Consider the salt copper(II) chloride (\mathrm{CuCl}_2).

The copper would have come from the base, \mathrm{Cu(OH)}_2, which is a weak base. The chloride would have come from the acid, \mathrm{HCl}, which is a strong acid.

2 \ \mathrm{HCl} + \mathrm{Cu(OH)}_2 \rightarrow \mathrm{CuCl}_2 + 2 \ \mathrm{HOH}

The reaction is a strong acid weak base reaction, so the salt should be acidic.

This video is an overview of endothermic and exothermic reactions and includes a demonstration of an exothermic reaction (7b): http://www.youtube.com/watch?v=sAyPDTQHEeE (2:31).

Lesson Summary

  • A neutralization reaction between an acid and a base will produce a salt and water.
  • In a neutralization reaction, the acid will produce H+ ions that react to neutralize the OH- ions produced by the base, forming neutral water. The other product will be an ionic salt.
  • A strong acid + a strong base in an acid/base neutralization reaction will form a neutral salt.
  • A strong acid + a weak base in an acid/base neutralization reaction will form an acidic salt.
  • A weak acid + a strong base in an acid/base neutralization reaction will form a basic salt.

Further Reading / Supplemental Links

Review Questions

  1. How do an acid and a base fit the definition of an ionic compound? Use examples in your answer.
  2. Explain neutralization reactions in terms of Arrhenius theory. Use an example in your answer.
  3. Which salt will form a basic solution when dissolved in water?
    1. \mathrm{KNO}_3
    2. \mathrm{CaCl}_2
    3. \mathrm{NaClO}_4
    4. \mathrm{NaNO}_2
  4. Which salt will form an acidic solution when dissolved in water?
    1. copper(II) sulfate
    2. sodium acetate
    3. potassium chloride
    4. sodium cyanide
  5. Milk of magnesia is a common over-the-counter antacid that has magnesium hydroxide as its main ingredient. It is used by the public to relieve acid indigestion. Acid indigestion is caused by excess stomach acid being present. Write the balanced chemical equation for the reaction between milk of magnesia and hydrochloric acid. What type of reaction is this? What type of salt is formed?
  6. Complete the following neutralization reactions and identify the type of salt produced.
    1. \mathrm{H}_2\mathrm{SO}_{4(aq)} + \mathrm{NaOH}_{(aq)} \rightarrow
    2. \mathrm{HNO}_{3(aq)} + \mathrm{NH}_4\mathrm{OH}_{(aq)} \rightarrow
    3. \mathrm{HF}_{(aq)} + \mathrm{NH}_4\mathrm{OH}_{(aq)} \rightarrow
    4. \mathrm{CH}_3\mathrm{COOH}_{(aq)} + \mathrm{KOH}_{(aq)} \rightarrow
    5. \mathrm{HCl}_{(aq)} + \mathrm{KOH}_{(aq)} \rightarrow

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