10.1 Arrhenius Definition of Acids and Bases

One way to define a class of compounds is by describing the various characteristics its members have in common. In the case of acids, the common characteristics include a sour taste, the ability to change the color of litmus to red, and the ability to react with certain metals, eroding the metal and producing hydrogen gas and an ionic compound. For 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 in neutralization reactions, forming water and ionic compounds, also known as salts.

Indicators are compounds that exhibit different colors when placed in solutions with different acid/base (pH) balances.  As described in the previous paragraph, litmus, a compound obtained from lichens, is an indicator because it is blue when in a basic solutions and red when in an acidic solution. Juice from purple cabbage is another natural indicator, and it exhibits a wide range of colors based on the pH it experiences as seen in Figure 10.1 below.  Other indicators include phenolphthalein and  bromothymol blue.  Indicaotrs are often used in titration, a measured form of  neutralization reaction, to visually indicate that the moles of acid and moles of base are balanced at the equivalence point.

Figure 10.1 Purple Cabbage Juice Indicator at Various pH Levels.  Attribution: By Indikator-Blaukraut.JPG: Supermartlderivative work: Haltopub (Indikator-Blaukraut.JPG) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Chemists define acids and bases in terms of their chemical properties, but the definitions differ in models developed at different times.  The Swedish chemist Svante Arrhenius developed the first chemical definitions of acids and bases in the late 1800s. Arrhenius defined an acid 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 base 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”.  For example, the name sodium hydroxide (NaOH) is used as the name of the ionic compound and for an aqueous solution of the compound.

However, aqueous solutions of binary acids, compounds with hydrogen and one other element in their formula, have one name for the compound alone and a different name when the compound is dissolved in water. Such a compound alone is named hydrogen                        ide with the blank being filled with the element’s root name.  For example, HCl not dissolved in water is hydrogen chloride.But when hydrogen chloride dissolves in water, its name changes to hydrochloric acid, following the pattern hydro                   ic acid, again with the blank being replaced by the element’s root name.

Acids composed of more than two elements, typically one to three hydrogen atoms with a polyatomic ion. The name of an oxyacid is based on the name of the polyatomic ion.  If the polyatomic ion name ends with -ate, name of its acid will end in -ic acid.  If the polyatomic ion mane ends in -ite, the name of the acid ends in -ous acid.   For example HNO₃ is nitric acid because it contains the nitrate polyatomic ion.  HNO₂ is nitrous acid because it contains the polyatomic ion nitrite.

. 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 bases that have the negative hydroxide ion have it written last.

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 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 neutralization. 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, it can be used to perform stoichiometry calculations, such as the ones  in Chapter 5 “Introduction to Chemical Reactions” and Chapter 6 “Quantities in Chemical Reactions”.