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10.1 Arrhenius Definition of Acids and Bases

Learning Objective

  1. Recognize a compound as an Arrhenius acid or an Arrhenius base.

One way to define a class of compounds is by describing the various characteristics its members have in common. In the case of the compounds known as acids, the common characteristics include a sour taste, the ability to change the color of the vegetable dye litmus to red, and the ability to dissolve certain metals and simultaneously produce hydrogen gas. For the compounds called bases, the common characteristics are a slippery texture, a bitter taste, and the ability to change the color of litmus to blue. Acids and bases also react with each other to form compounds generally known as salts.

Note

Although we include their tastes among the common characteristics of acids and bases, we never advocate tasting an unknown chemical!

Chemists prefer, however, to have definitions for acids and bases in chemical terms. The Swedish chemist Svante Arrhenius developed the first chemical definitions of acids and bases in the late 1800s. Arrhenius defined an acidA compound that increases the concentration of hydrogen ion (H+) in aqueous solution. as a compound that increases the concentration of hydrogen ion (H+) in aqueous solution. Many acids are simple compounds that release a hydrogen cation into solution when they dissolve. Similarly, Arrhenius defined a baseA compound that increases the concentration of hydroxide ion (OH) in aqueous solution. as a compound that increases the concentration of hydroxide ion (OH) in aqueous solution. Many bases are ionic compounds that have the hydroxide ion as their anion, which is released when the base dissolves in water.

Many bases and their aqueous solutions are named using the normal rules of ionic compounds that were presented in Chapter 3 "Ionic Bonding and Simple Ionic Compounds", Section 3.4 "Ionic Nomenclature"; that is, they are named as hydroxide compounds. For example, the base sodium hydroxide (NaOH) is both an ionic compound and an aqueous solution. However, aqueous solutions of acids have their own naming rules. The names of binary acids (compounds with hydrogen and one other element in their formula) are based on the root of the name of the other element preceded by the prefix hydro- and followed by the suffix -ic acid. Thus, an aqueous solution of HCl [designated “HCl(aq)”] is called hydrochloric acid, H2S(aq) is called hydrosulfuric acid, and so forth. Acids composed of more than two elements (typically hydrogen and oxygen and some other element) have names based on the name of the other element, followed by the suffix -ic acid or -ous acid, depending on the number of oxygen atoms in the acid’s formula. Other prefixes, like per- and hypo-, also appear in the names for some acids. Unfortunately, there is no strict rule for the number of oxygen atoms that are associated with the -ic acid suffix; the names of these acids are best memorized. Table 10.1 "Formulas and Names for Some Acids and Bases" lists some acids and bases and their names. Note that acids have hydrogen written first, as if it were the cation, while most bases have the negative hydroxide ion, if it appears in the formula, written last.

Note

The name oxygen comes from the Latin meaning “acid producer” because its discoverer, Antoine Lavoisier, thought it was the essential element in acids. Lavoisier was wrong, but it is too late to change the name now.

Table 10.1 Formulas and Names for Some Acids and Bases

Formula Name
Acids
HCl(aq) hydrochloric acid
HBr(aq) hydrobromic acid
HI(aq) hydriodic acid
H2S(aq) hydrosulfuric acid
HC2H3O2(aq) acetic acid
HNO3(aq) nitric acid
HNO2(aq) nitrous acid
H2SO4(aq) sulfuric acid
H2SO3(aq) sulfurous acid
HClO3(aq) chloric acid
HClO4(aq) perchloric acid
HClO2(aq) chlorous acid
H3PO4(aq) phosphoric acid
H3PO3(aq) phosphorous acid
Bases
NaOH(aq) sodium hydroxide
KOH(aq) potassium hydroxide
Mg(OH)2(aq) magnesium hydroxide
Ca(OH)2(aq) calcium hydroxide
NH3(aq) ammonia

Example 1

Name each substance.

  1. HF(aq)
  2. Sr(OH)2(aq)

Solution

  1. This acid has only two elements in its formula, so its name includes the hydro- prefix. The stem of the other element’s name, fluorine, is fluor, and we must also include the -ic acid ending. Its name is hydrofluoric acid.
  2. This base is named as an ionic compound between the strontium ion and the hydroxide ion: strontium hydroxide.

Skill-Building Exercise

    Name each substance.

  1. H2Se(aq)

  2. Ba(OH)2(aq)

Notice that one base listed in Table 10.1 "Formulas and Names for Some Acids and Bases"—ammonia—does not have hydroxide as part of its formula. How does this compound increase the amount of hydroxide ion in aqueous solution? Instead of dissociating into hydroxide ions, ammonia molecules react with water molecules by taking a hydrogen ion from the water molecule to produce an ammonium ion and a hydroxide ion:

NH3(aq) + H2O(ℓ) → NH4+(aq) + OH(aq)

Because this reaction of ammonia with water causes an increase in the concentration of hydroxide ions in solution, ammonia satisfies the Arrhenius definition of a base. Many other nitrogen-containing compounds are bases because they too react with water to produce hydroxide ions in aqueous solution.

As we noted previously, acids and bases react chemically with each other to form salts. A salt is a general chemical term for any ionic compound formed from an acid and a base. In reactions where the acid is a hydrogen ion containing compound and the base is a hydroxide ion containing compound, water is also a product. The general reaction is as follows:

acid + base → water + salt

The reaction of acid and base to make water and a salt is called neutralizationThe reaction of acid and base to make water and a salt.. Like any chemical equation, a neutralization chemical equation must be properly balanced. For example, the neutralization reaction between sodium hydroxide and hydrochloric acid is as follows:

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

with coefficients all understood to be one. The neutralization reaction between sodium hydroxide and sulfuric acid is as follows:

2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(ℓ)

Once a neutralization reaction is properly balanced, we can use it to perform stoichiometry calculations, such as the ones we practiced in Chapter 5 "Introduction to Chemical Reactions" and Chapter 6 "Quantities in Chemical Reactions".

Example 2

Nitric acid [HNO3(aq)] can be neutralized by calcium hydroxide [Ca(OH)2(aq)].

  1. Write a balanced chemical equation for the reaction between these two compounds and identify the salt it produces.
  2. For one reaction, 16.8 g of HNO3 is present initially. How many grams of Ca(OH)2 are needed to neutralize that much HNO3?
  3. In a second reaction, 805 mL of 0.672 M Ca(OH)2 is present initially. What volume of 0.432 M HNO3 solution is necessary to neutralize the Ca(OH)2 solution?

Solution

  1. Because there are two OH ions in the formula for Ca(OH)2, we need two moles of HNO3 to provide H+ ions. The balanced chemical equation is as follows:

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

    The salt formed is calcium nitrate.

  2. This calculation is much like the calculations we did in Chapter 6 "Quantities in Chemical Reactions". First we convert the mass of HNO3 to moles using its molar mass of 1.01 + 14.00 + 3(16.00) = 63.01 g/mol; then we use the balanced chemical equation to determine the related number of moles of Ca(OH)2 needed to neutralize it; and then we convert that number of moles of Ca(OH)2 to the mass of Ca(OH)2 using its molar mass of 40.08 + 2(1.01) + 2(16.00) = 74.10 g/mol.

    16.8 g HNO3×1 mol  HNO 363.01 g  HNO 3×1 mol  Ca(OH) 22 mol  HNO 3×74.10 g Ca(OH)21 mol  Ca(OH) 2=9.88 g Ca(OH)2 needed
  3. Having concentration information allows us to employ the skills we developed in Chapter 9 "Solutions". First, we use the concentration and volume data to determine the number of moles of Ca(OH)2 present. Recognizing that 805 mL = 0.805 L,

    0.672 M Ca(OH)2=mol Ca(OH)20.805 L soln (0.672 M CaOH)2 × (0.805 L soln) = mol Ca(OH)2 = 0.541 mol Ca(OH)2

    We combine this information with the proper ratio from the balanced chemical equation to determine the number of moles of HNO3 needed:

    0.541 mol Ca(OH)2×2 mol HNO31  mol Ca(OH) 2=1.08 mol HNO3

    Now, using the definition of molarity one more time, we determine the volume of acid solution needed:

    0.432 M HNO3=1.08 mol HNO3volume of HNO3 volume of HNO3=1.08 mol HNO30.432 M HNO3=2.50 L=2.50×103 mL HNO3

Skill-Building Exercise

    Hydrocyanic acid [HCN(aq)] can be neutralized by potassium hydroxide [KOH(aq)].

  1. Write a balanced chemical equation for the reaction between these two compounds and identify the salt it produces.

  2. For one reaction, 37.5 g of HCN is present initially. How many grams of KOH are needed to neutralize that much HCN?

  3. In a second reaction, 43.0 mL of 0.0663 M KOH is present initially. What volume of 0.107 M HCN solution is necessary to neutralize the KOH solution?

Note

Hydrocyanic acid (HCN) is one exception to the acid-naming rules that specify using the prefix hydro- for binary acids (acids composed of hydrogen and only one other element).

Concept Review Exercises

  1. Give the Arrhenius definitions of an acid and a base.

  2. What is neutralization?

Answers

  1. Arrhenius acid: a compound that increases the concentration of hydrogen ion (H+) in aqueous solution; Arrhenius base: a compound that increases the concentration of hydroxide ion (OH) in aqueous solution.

  2. the reaction of an acid and a base

Key Takeaway

  • An Arrhenius acid increases the H+ ion concentration in water, while an Arrhenius base increases the OH ion concentration in water.

Exercises

  1. Give two examples of Arrhenius acids.

  2. Give two examples of Arrhenius bases.

  3. List the general properties of acids.

  4. List the general properties of bases.

  5. Name each compound.

    1. HBr(aq)
    2. Ca(OH)2(aq)
    3. HNO3(aq)
    4. Fe(OH)3(aq)
  6. Name each compound.

    1. HI(aq)
    2. Cu(OH)2(aq)
    3. H3PO4(aq)
    4. CsOH(aq)
  7. Propose a name for water (H2O) using the rules for naming acids.

  8. Propose a name for hydrogen peroxide (H2O2) using the rules for naming acids.

  9. Write a balanced chemical equation for the neutralization of Ba(OH)2(aq) with HNO3(aq).

  10. Write a balanced chemical equation for the neutralization of H2SO4(aq) with Cr(OH)3(aq).

  11. How many moles of sodium hydroxide (NaOH) are needed to neutralize 0.844 mol of acetic acid (HC2H3O2)? (Hint: begin by writing a balanced chemical equation for the process.)

  12. How many moles of perchloric acid (HClO4) are needed to neutralize 0.052 mol of calcium hydroxide [Ca(OH)2]? (Hint: begin by writing a balanced chemical equation for the process.)

  13. Hydrazoic acid (HN3) can be neutralized by a base.

    1. Write the balanced chemical equation for the reaction between hydrazoic acid and calcium hydroxide.
    2. How many milliliters of 0.0245 M Ca(OH)2 are needed to neutralize 0.564 g of HN3?
  14. Citric acid (H3C6H5O7) has three hydrogen atoms that can form hydrogen ions in solution.

    1. Write the balanced chemical equation for the reaction between citric acid and sodium hydroxide.
    2. If an orange contains 0.0675 g of H3C6H5O7, how many milliliters of 0.00332 M NaOH solution are needed to neutralize the acid?
  15. Magnesium hydroxide [Mg(OH)2] is an ingredient in some antacids. How many grams of Mg(OH)2 are needed to neutralize the acid in 158 mL of 0.106 M HCl(aq)? It might help to write the balanced chemical equation first.

  16. Aluminum hydroxide [Al(OH)3] is an ingredient in some antacids. How many grams of Al(OH)3 are needed to neutralize the acid in 96.5 mL of 0.556 M H2SO4(aq)? It might help to write the balanced chemical equation first.

Answers

  1. HCl and HNO3 (answers will vary)

  2. sour taste, react with metals, react with bases, and turn litmus red

    1. hydrobromic acid
    2. calcium hydroxide
    3. nitric acid
    4. iron(III) hydroxide
  3. perhaps hydroxic acid

  4. 2HNO3(aq) + Ba(OH)2(aq) → Ba(NO3)2(aq) + 2H2O

  5. 0.844 mol

    1. 2HN3(aq) + Ca(OH)2 → Ca(N3)2 + 2H2O
    2. 268 mL
  6. 0.488 g