8.2: Arrhenius Definition of Acids and Bases
- Page ID
- 290578
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Learning Objectives
- To recognize a compound as an Arrhenius acid or an Arrhenius base.
- To describe characteristics of acids and bases.
- To write equations of neutralization reactions.
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.
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 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.
| Acids | Bases | ||
|---|---|---|---|
| Formula | Name | Formula | Name |
| HCl(aq) | hydrochloric acid | NaOH(aq) | sodium hydroxide |
| HBr(aq) | hydrobromic acid | KOH(aq) | potassium hydroxide |
| HI(aq) | hydriodic acid | Mg(OH)2(aq) | magnesium hydroxide |
| H2S(aq) | hydrosulfuric acid | Ca(OH)2(aq) | calcium hydroxide |
| HC2H3O2(aq) | acetic acid | NH3(aq) | ammonia |
| HNO3(aq) | nitric acid | NaHCO3 (aq) | sodium bicarbonate |
| HNO2(aq) | nitrous acid | CaCO3 (aq) | calcium carbonate |
| 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 | ||
| H2CO3(aq) | carbonic acid | ||
Many bases and their aqueous solutions are named using the normal rules of ionic compounds that were presented previously; 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 \(\PageIndex{1}\) 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.
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.
Example \(\PageIndex{1}\)
Name each substance.
- HF(aq)
- Sr(OH)2(aq)
Solution
- 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.
- This base is named as an ionic compound between the strontium ion and the hydroxide ion: strontium hydroxide.
Exercise \(\PageIndex{1}\)
Name each substance.
- H2Se(aq)
- Ba(OH)2(aq)
- Answer
-
a. hydroselenic acid
b. barium hydroxide
Notice that one base listed in Table \(\PageIndex{1}\)—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:
\[NH_{3(aq)} + H_2O_{(ℓ)} \rightarrow NH^+_{4(aq)} + OH^−_{(aq)} \label{Eq1}\]
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.
Neutralization
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 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:
\[NaOH{(aq)} + HCl_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(ℓ)} \label{Eq2}\]
with coefficients all understood to be one. The neutralization reaction between sodium hydroxide and sulfuric acid is as follows:
\[2NaOH_{(aq)} + H_2SO_{4(aq)} \rightarrow Na_2SO_{4(aq)} + 2H_2O_{(ℓ)} \label{Eq3}\]
Once a neutralization reaction is properly balanced, we can use it to perform stoichiometry calculations, such as the ones we practiced earlier.
There are a number of examples of acid-base chemistry in everyday life. One example is the use of baking soda, or sodium bicarbonate in baking. NaHCO3 is a base. When it reacts with an acid such as lemon juice, buttermilk, or sour cream in a batter, bubbles of carbon dioxide gas are formed from decomposition of the resulting carbonic acid, and the batter “rises.” Baking powder is a combination of sodium bicarbonate, and one or more acid salts that react when the two chemicals come in contact with water in the batter.
\[ HCO_3^- (aq) + H^+ (aq) \rightarrow H_2 CO_3 (aq) \label{4.3.19}\]
\[ H_2 CO_3 (aq) \rightarrow CO_2 (g) + H_2 O(l) \]
Example \(\PageIndex{2}\)
Nitric acid [HNO3(aq)] can be neutralized by calcium hydroxide [Ca(OH)2(aq)].
Write a balanced chemical equation for the reaction between these two compounds and identify the salt it produces.
Solution
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.
Exercise \(\PageIndex{2}\)
Hydrocyanic acid [HCN(aq)] can be neutralized by potassium hydroxide [KOH(aq)].
Write a balanced chemical equation for the reaction between these two compounds and identify the salt it produces.
- Answer
- KOH(aq) + HCN(aq) → KCN(aq) + H2O(ℓ) KCN is the salt
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).
Stomach Antacids
Our stomachs contain a solution of roughly 0.03 M HCl, which helps us digest the food we eat. The burning sensation associated with heartburn is a result of the acid of the stomach leaking through the muscular valve at the top of the stomach into the lower reaches of the esophagus. The lining of the esophagus is not protected from the corrosive effects of stomach acid the way the lining of the stomach is, and the results can be very painful. When we have heartburn, it feels better if we reduce the excess acid in the esophagus by taking an antacid. As you may have guessed, antacids are bases. One of the most common antacids is calcium carbonate, CaCO3. The reaction,
\[CaCO_3(s)+2HCl(aq)⇌CaCl_2(aq)+H_2O(l)+CO_2(g)\]
not only neutralizes stomach acid, it also produces CO2(g), which may result in a satisfying belch.
Milk of Magnesia is a suspension of the sparingly soluble base magnesium hydroxide, Mg(OH)2. It works according to the reaction:
\[Mg(OH)_2(s)⇌Mg^{2+}(aq)+2OH^-(aq)\]
The hydroxide ions generated in this equilibrium then go on to react with the hydronium ions from the stomach acid, so that :
\[H_3O^+ + OH^- ⇌ 2H_2O(l)\]
This reaction does not produce carbon dioxide, but magnesium-containing antacids can have a laxative effect. Several antacids have aluminum hydroxide, Al(OH)3, as an active ingredient. The aluminum hydroxide tends to cause constipation, and some antacids use aluminum hydroxide in concert with magnesium hydroxide to balance the side effects of the two substances.
Concept Review Exercises
- Give the Arrhenius definitions of an acid and a base.
- What is neutralization?
Answers
- 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.
- 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
- Give two examples of Arrhenius acids.
- Give two examples of Arrhenius bases.
- List the general properties of acids.
- List the general properties of bases.
- Name each compound. (For acids, look up the name in Table 10.1.1. For bases, use the rules for naming ionic compounds from Chapter 3.)
a. HBr(aq)
b. Ca(OH)2(aq)
c. HNO3(aq)
d. Fe(OH)3(aq)
6. Name each compound.
a. HI(aq)
b. Cu(OH)2(aq)
c. H3PO4(aq)
d. CsOH(aq)
7. Write a balanced chemical equation for the neutralization of Ba(OH)2(aq) with HNO3(aq).
8. Write a balanced chemical equation for the neutralization of H2SO4(aq) with Cr(OH)3(aq).
9. Gastric juice, the digestive fluid produced in the stomach, contains hydrochloric acid, HCl. Milk of Magnesia, a suspension of solid Mg(OH)2 in an aqueous medium, is sometimes used to neutralize excess stomach acid. Write a complete balanced equation for the neutralization reaction.
10. Identify the salt produced in each acid-base reaction below. Then, balance the equation.
a. 2HCl + Sr(OH)2 → 2H2O + ??
b. KNO3; HNO3 + KOH → ?? + H2O
c. HF + Ca(OH)2 ---> ?? + H2O
Answers
- HCl and HNO3 (answers will vary)
- NaOH and Ca(OH)2 (answers will vary)
- sour taste, react with metals, react with bases, and turn litmus red
- bitter taste, feels slippery, react with acids and turn litmus blue
- a. hydrobromic acid
b. calcium hydroxide
c. nitric acid
d. iron(III) hydroxide
6. a. hydroiodic acid
b. cupric hydroxide
c. phosphoric acid
d. cesium hydroxide
7. 2HNO3(aq) + Ba(OH)2(aq) → Ba(NO3)2(aq) + 2H2O
8. 3H2SO4(aq) + 2Cr(OH)3(aq) → Cr2(SO4)3(aq) + 6H2O
9. Mg(OH)2 + 2HCl --> MgCl2 + 2H2O
10. a. SrCl2; 2HCl + Sr(OH)2 → 2H2O + SrCl2
b. KNO3; HNO3 + KOH → KNO3 + H2O
c. CaF2; 2HF + Ca(OH)2 → CaF2 + 2H2O
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