5.5 Neutralization Reactions

Learning Objectives

  1. Identify an acid and a base.
  2. Identify a neutralization reaction and predict its products.

In Chapter 3 “Atoms, Molecules, and Ions”, Section 3.5 “Acids”, we defined an acid as an ionic compound that contains H+ as the cation. This is slightly incorrect, but until additional concepts were developed, a better definition needed to wait. Now we can redefine an acid: an acid is any compound that increases the amount of hydrogen ion (H+) in an aqueous solution. The chemical opposite of an acid is a base. The equivalent definition of a base is that a base is a compound that increases the amount of hydroxide ion (OH) in an aqueous solution. These original definitions were proposed by Arrhenius (the same person who proposed ion dissociation) in 1884, so they are referred to as the Arrhenius definition of an acid and a base, respectively.

You may recognize that, based on the description of a hydrogen atom, an H+ ion is a hydrogen atom that has lost its lone electron; that is, H+ is simply a proton. Do we really have bare protons moving about in aqueous solution? No. What is more likely is that the H+ ion has attached itself to one (or more) water molecule(s). To represent this chemically, we define the hydronium ion H3O+(aq), a water molecule with an extra hydrogen ion attached to it. as H3O+, which represents an additional proton attached to a water molecule. We use the hydronium ion as the more logical way a hydrogen ion appears in an aqueous solution, although in many chemical reactions H+ and H3O+ are treated equivalently.

The reaction of an acid and a base is called a neutralization reaction. Although acids and bases have their own unique chemistries, the acid and base cancel each other’s chemistry to produce a rather innocuous substance—water. In fact, the general reaction between an acid and a base is

acid + base → water + salt

where the term salt is generally used to define any ionic compound (soluble or insoluble) that is formed from a reaction between an acid and a base. (In chemistry, the word salt refers to more than just table salt.) For example, the balanced chemical equation for the reaction between HCl(aq) and KOH(aq) is

HCl(aq) + KOH(aq) → H2O(ℓ) + KCl(aq)

where the salt is KCl. By counting the number of atoms of each element, we find that only one water molecule is formed as a product. However, in the reaction between HCl(aq) and Mg(OH)2(aq), additional molecules of HCl and H2O are required to balance the chemical equation:

2 HCl(aq) + Mg(OH)2(aq) → 2 H2O(ℓ) + MgCl2(aq)

Here, the salt is MgCl2. (This is one of several reactions that take place when a type of antacid—a base—is used to treat stomach acid.)

Example 11

Write the neutralization reactions between each acid and base.

  1. HNO3(aq) and Ba(OH)2(aq)
  2. H3PO4(aq) and Ca(OH)2(aq)

Solution

First, we will write the chemical equation with the formulas of the reactants and the expected products; then we will balance the equation.

  1. The expected products are water and barium nitrate, so the initial chemical reaction is

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

    To balance the equation, we need to realize that there will be two H2O molecules, so two HNO3 molecules are required:

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

    This chemical equation is now balanced.

  2. The expected products are water and calcium phosphate, so the initial chemical equation is

    H3PO4(aq) + Ca(OH)2(aq) → H2O(ℓ) + Ca3(PO4)2(s)

    According to the solubility rules, Ca3(PO4)2 is insoluble, so it has an (s) phase label. To balance this equation, we need two phosphate ions and three calcium ions; we end up with six water molecules to balance the equation:

    2 H3PO4(aq) + 3 Ca(OH)2(aq) → 6 H2O(ℓ) + Ca3(PO4)2(s)

    This chemical equation is now balanced.

Test Yourself

Write the neutralization reaction between H2SO4(aq) and Sr(OH)2(aq).

Answer

H2SO4(aq) + Sr(OH)2(aq) → 2 H2O(ℓ) + SrSO4(aq)

Neutralization reactions are one type of chemical reaction that proceeds even if one reactant is not in the aqueous phase. For example, the chemical reaction between HCl(aq) and Fe(OH)3(s) still proceeds according to the equation

3 HCl(aq) + Fe(OH)3(s) → 3 H2O(ℓ) + FeCl3(aq)

even though Fe(OH)3 is not soluble. When one realizes that Fe(OH)3(s) is a component of rust, this explains why some cleaning solutions for rust stains contain acids—the neutralization reaction produces products that are soluble and wash away. (Washing with acids like HCl is one way to remove rust and rust stains, but HCl must be used with caution!)

Complete and net ionic reactions for neutralization reactions will depend on whether the reactants and products are soluble, even if the acid and base react. For example, in the reaction of HCl(aq) and NaOH(aq),

HCl(aq) + NaOH(aq) → H2O(ℓ) + NaCl(aq)

the complete ionic reaction is

H+(aq) + Cl(aq) + Na+(aq) + OH(aq) → H2O(ℓ) + Na+(aq) + Cl(aq)

The Na+(aq) and Cl(aq) ions are spectator ions, so we can remove them to have

H+(aq) + OH(aq) → H2O(ℓ)

as the net ionic equation. If we wanted to write this in terms of the hydronium ion, H3O+(aq), we would write it as

H3O+(aq) + OH(aq) → 2H2O(ℓ)

With the exception of the introduction of an extra water molecule, these two net ionic equations are equivalent.

However, for the reaction between HCl(aq) and Cr(OH)2(s), because chromium(II) hydroxide is insoluble, we cannot separate it into ions for the complete ionic equation:

2 H+(aq) + 2 Cl(aq) + Cr(OH)2(s) → 2 H2O(ℓ) + Cr2+(aq) + 2 Cl(aq)

The chloride ions are the only spectator ions here, so the net ionic equation is

2 H+(aq) + Cr(OH)2(s) → 2 H2O(ℓ) + Cr2+(aq)

Example 12

Oxalic acid, H2C2O4(s), and Ca(OH)2(s) react very slowly. What is the net ionic equation between these two substances if the salt formed is insoluble? (The anion in oxalic acid is the oxalate ion, C2O42−.)

Solution

The products of the neutralization reaction will be water and calcium oxalate:

H2C2O4(s) + Ca(OH)2(s) → 2 H2O(ℓ) + CaC2O4(s)

Because nothing is dissolved, there are no substances to separate into ions, so the net ionic equation is the equation of the three solids and one liquid.

Test Yourself

What is the net ionic equation between HNO3(aq) and Ti(OH)4(s)?

Answer

4 H+(aq) + Ti(OH)4(s) → 4 H2O(ℓ) + Ti4+(aq)

Food and Drink App: Acids in Foods

Many foods and beverages contain acids. Acids impart a sour note to the taste of foods, which may add some pleasantness to the food. For example, orange juice contains citric acid, H3C6H5O7. Note how this formula shows hydrogen atoms in two places; the first hydrogen atoms written are the hydrogen atoms that can form H+ ions, while the second hydrogen atoms written are part of the citrate ion, C6H5O73−. Lemons and limes contain much more citric acid—about 60 times as much—which accounts for these citrus fruits being more sour than most oranges. Vinegar is essentially a ~5% solution of acetic acid (HC2H3O2) in water. Apples contain malic acid (H2C4H4O5; the name malic acid comes from the apple’s botanical genus name, malus), while lactic acid (HC3H5O3) is found in wine and sour milk products, such as yogurt and some cottage cheeses.

Table 5.2 “Various Acids Found in Food and Beverages” lists some acids found in foods, either naturally or as an additive. Frequently, the salts of acid anions are used as additives, such as monosodium glutamate (MSG), which is the sodium salt derived from glutamic acid. As you read the list, you should come to the inescapable conclusion that it is impossible to avoid acids in food and beverages.

Table 5.2 Various Acids Found in Food and Beverages
Acid Name Acid Formula Use and Appearance
acetic acid HC2H3O2 flavouring; found in vinegar
adipic acid H2C6H8O4 flavouring; found in processed foods and some antacids
alginic acid various thickener; found in drinks, ice cream, and weight loss products
ascorbic acid HC6H7O6 antioxidant, also known as vitamin C; found in fruits and vegetables
benzoic acid HC6H5CO2 preservative; found in processed foods
citric acid H3C6H5O7 flavouring; found in citrus fruits
dehydroacetic acid HC8H7O4 preservative, especially for strawberries and squash
erythrobic acid HC6H7O6 antioxidant; found in processed foods
fatty acids various thickener and emulsifier; found in processed foods
fumaric acid H2C4H2O4 flavouring; acid reactant in some baking powders
glutamic acid H2C5H7NO4 flavouring; found in processed foods and in tomatoes, some cheeses, and soy products
lactic acid HC3H5O3 flavouring; found in wine, yogurt, cottage cheese, and other sour milk products
malic acid H2C4H4O5 flavouring; found in apples and unripe fruit
phosphoric acid H3PO4 flavouring; found in some colas
propionic acid HC3H5O2 preservative; found in baked goods
sorbic acid HC6H7O2 preservative; found in processed foods
stearic acid HC18H35O2 anticaking agent; found in hard candies
succinic acid H2C4H4O4 flavouring; found in wine and beer
tartaric acid H2C4H4O6 flavouring; found in grapes, bananas, and tamarinds

Key Takeaways

  • The Arrhenius definition of an acid is a substance that increases the amount of H+ in an aqueous solution.
  • The Arrhenius definition of a base is a substance that increases the amount of OH in an aqueous solution.
  • Neutralization is the reaction of an acid and a base, which forms water and a salt.
  • Net ionic equations for neutralization reactions may include solid acids, solid bases, solid salts, and water.

 

Exercises

  1. What is the Arrhenius definition of an acid?

  2. What is the Arrhenius definition of a base?

  3. Predict the products of each acid-base combination listed. Assume that a neutralization reaction occurs.

a)  HCl and KOH

b)  H2SO4 and KOH

c)  H3PO4 and Ni(OH)2

 

4.  Predict the products of each acid-base combination listed. Assume that a neutralization reaction occurs.

a)  HBr and Fe(OH)3

b)  HNO2 and Al(OH)3

c)  HClO3 and Mg(OH)2

 

5.  Write a balanced chemical equation for each neutralization reaction in Exercise 3.

 

6.  Write a balanced chemical equation for each neutralization reaction in Exercise 4.

 

7.  Write a balanced chemical equation for the neutralization reaction between each given acid and base. Include the proper phase labels.

a)  HI(aq) + KOH(aq) → ?

b)  H2SO4(aq) + Ba(OH)2(aq) → ?

 

8.  Write a balanced chemical equation for the neutralization reaction between each given acid and base. Include the proper phase labels.

a)  HNO3(aq) + Fe(OH)3(s) → ?

b)  H3PO4(aq) + CsOH(aq) → ?

 

9.  Write the net ionic equation for each neutralization reaction in Exercise 7.

 

10.  Write the net ionic equation for each neutralization reaction in Exercise 8.

 

11.  Write the complete and net ionic equations for the neutralization reaction between HClO3(aq) and Zn(OH)2(s). Assume the salt is soluble.

 

12.  Write the complete and net ionic equations for the neutralization reaction between H2C2O4(s) and Sr(OH)2(aq). Assume the salt is insoluble.

 

13.  Explain why the net ionic equation for the neutralization reaction between HCl(aq) and KOH(aq) is the same as the net ionic equation for the neutralization reaction between HNO3(aq) and RbOH.

 

14.  Explain why the net ionic equation for the neutralization reaction between HCl(aq) and KOH(aq) is different from the net ionic equation for the neutralization reaction between HCl(aq) and AgOH.

 

15.  Write the complete and net ionic equations for the neutralization reaction between HCl(aq) and KOH(aq) using the hydronium ion in place of H+. What difference does it make when using the hydronium ion?

 

16.  Write the complete and net ionic equations for the neutralization reaction between HClO3(aq) and Zn(OH)2(s) using the hydronium ion in place of H+. Assume the salt is soluble. What difference does it make when using the hydronium ion?

Answers

  1. An Arrhenius acid increases the amount of H+ ions in an aqueous solution.

3.

a)  KCl and H2O

b)  K2SO4 and H2O

c)  Ni3(PO4)2 and H2O

5.

a)    HCl + KOH → KCl + H2O

b)  H2SO4 + 2 KOH → K2SO4 + 2 H2O

c)  2 H3PO4 + 3 Ni(OH)2 → Ni3(PO4)2 + 6 H2O

 

7.

a)  HI(aq) + KOH(aq) → KCl(aq) + H2O(ℓ)

b)  H2SO4(aq) + Ba(OH)2(aq) → BaSO4(s) + 2 H2O(ℓ)

 

9.

a)  H+(aq) + OH(aq) → H2O(ℓ)

b)  2 H+(aq) + SO42−(aq) + Ba2+(aq) + 2 OH(aq) → BaSO4(s) + 2 H2O(ℓ)

11.  Complete ionic equation:

 

2 H+(aq) + 2 ClO3(aq) + Zn2+(aq) + 2 OH(aq) → Zn2+(aq) + 2 ClO3(aq) + 2 H2O(ℓ)

Net ionic equation:

2 H+(aq) + 2 OH(aq) → 2 H2O(ℓ)

 

13.  Because the salts are soluble in both cases, the net ionic reaction is just H+(aq) + OH(aq) → H2O(ℓ).

 

15.  Complete ionic equation:

H3O+(aq) + Cl(aq) + K+(aq) + OH(aq) → 2 H2O(ℓ) + K+(aq) + Cl(aq)

Net ionic equation:

H3O+(aq) + OH(aq) → 2 H2O(ℓ)

The difference is simply the presence of an extra water molecule as a product.

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Introductory Chemistry – 1st Canadian / NSCC Edition Copyright © 2014 by David W. Ball and Jessie A. Key is an adapted version of the open textbook Introductory Chemistry – 1st Canadian and is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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