11.1: Acids and Bases in Water

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Learning Objectives

To recognize a compound as an Arrhenius acid or an Arrhenius base.
To describe characteristics 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 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.

Lemon: acid, sour taste, turns blue litmus red, reacts with some metals to produce H2, dissolves carbonate salts (releasing CO2). Soap: Base, bitter taste, turns red litmus blue, slippery to the touch

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

Arrhenius Acids and Bases

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.

Table \(\PageIndex{1}\): Formulas and Names for Some Acids and Bases
Acids		Bases
Formula	Name	Formula	Name
HCl(aq)	hydrochloric acid	NaOH(aq)	sodium hydroxide
H₂S(aq)	hydrosulfuric acid	KOH(aq)	potassium hydroxide
HC₂H₃O₂(aq)	acetic acid	Mg(OH)₂(aq)	magnesium hydroxide
HNO₂(aq)	nitrous acid	Ca(OH)₂(aq)	calcium hydroxide
H₃PO₄(aq)	phosphoric acid	NH₃(aq)	ammonia
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, sodium hydroxide (NaOH) is both an ionic compound and a base. However, aqueous solutions of acids have their own naming rules which are not covered here. Table \(\PageIndex{1}\) lists some acids and bases and their names. Note that molecular formulas for 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.

Brønsted-Lowry Acids and Bases

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.

Ammonia (NH₃) increases the hydroxide ion concentration in aqueous solution by reacting with water rather than releasing hydroxide ions directly. In fact, the Arrhenius definitions of an acid and a base focus on hydrogen ions and hydroxide ions. Are there more fundamental definitions for acids and bases?

In 1923, the Danish scientist Johannes Brønsted and the English scientist Thomas Lowry independently proposed new definitions for acids and bases. Rather than considering both hydrogen and hydroxide ions, they focused on the hydrogen ion only. A Brønsted-Lowry acid is a compound that supplies a hydrogen ion in a reaction. A Brønsted-Lowry base, conversely, is a compound that accepts a hydrogen ion in a reaction. Thus, the Brønsted-Lowry definitions of an acid and a base focus on the movement of hydrogen ions in a reaction, rather than on the production of hydrogen ions and hydroxide ions in an aqueous solution.

Let us use the reaction of ammonia in water to demonstrate the Brønsted-Lowry definitions of an acid and a base. Ammonia and water molecules are reactants, while the ammonium ion and the hydroxide ion are products:

\[NH_{3}(aq) + H_2O(ℓ) \rightleftharpoons NH^+_{4}(aq) + OH^−(aq) \nonumber \]

What has happened in this reaction is that the original water molecule has donated a hydrogen ion to the original ammonia molecule, which in turn has accepted the hydrogen ion. We can illustrate this as follows:

NH3 + H2O <--> NH4^+ + OH^- An arrow points from the lone pair on NH3 to an H on water.

Figure \(\PageIndex{1}\): Reaction of ammonia and water. The curved arrow shows how the lone pair of valence electrons on the nitrogen becomes a bond between N and H. Because the ammonia gained H⁺ (a proton) and not a complete H atom, the product now has a positive charge. Water lost H⁺ but kept all the electrons, so it becomes OH^- with a negative charge.

Because the water molecule donates a hydrogen ion to the ammonia, it is the Brønsted-Lowry acid, while the ammonia molecule—which accepts the hydrogen ion—is the Brønsted-Lowry base. Thus, ammonia acts as a base in both the Arrhenius sense and the Brønsted-Lowry sense.

Is an Arrhenius acid like hydrochloric acid still an acid in the Brønsted-Lowry sense? Yes, but it requires us to understand what really happens when HCl is dissolved in water. Recall that the hydrogen atom is a single proton surrounded by a single electron. To make the hydrogen ion, we remove the electron, leaving a bare proton. Do we really have bare protons floating around in aqueous solution? No, we do not. What really happens is that the H⁺ ion attaches itself to H₂O to make H₃O⁺, which is called the hydronium ion. For most purposes, H⁺ and H₃O⁺ represent the same species, but writing H₃O⁺ instead of H⁺ shows that we understand that there are no bare protons floating around in solution. Rather, these protons are actually attached to solvent molecules.

A proton in aqueous solution may be surrounded by more than one water molecule, leading to formulas like H₅O₂⁺ or H₉O₄⁺ rather than H₃O⁺. It is simpler, however, to use H₃O⁺.

With this in mind, how do we define HCl as an acid in the Brønsted-Lowry sense? Consider what happens when HCl is dissolved in H₂O:

\[HCl(g) + H_2O(ℓ) \rightarrow H_3O^+(aq) + Cl^−(aq) \nonumber \]

We can depict this process using Lewis electron dot diagrams:

HCl + H2O --> Cl^- + H3O^+ An arrow points from one lone pair on water to the H on HCl

Figure \(\PageIndex{2}\): Reaction of HCl and water. The curved arrow shows how a lone pair of valence electrons on the oxygen atom becomes a bond between O and H. Because the water gained H⁺ (a proton) and not a complete H atom, the product now has a positive charge. The acid lost H⁺ but kept all the electrons, so it becomes H₃O⁺ with a positive charge.

Now we see that a hydrogen ion is transferred from the HCl molecule to the H₂O molecule to make chloride ions and hydronium ions. As the hydrogen ion donor, HCl acts as a Brønsted-Lowry acid; as a hydrogen ion acceptor, H₂O is a Brønsted-Lowry base. So HCl is an acid not just in the Arrhenius sense but also in the Brønsted-Lowry sense. Moreover, by the Brønsted-Lowry definitions, H₂O is a base in the formation of aqueous HCl. So the Brønsted-Lowry definitions of an acid and a base classify the dissolving of HCl in water as a reaction between an acid and a base—although the Arrhenius definition would not have labeled H₂O a base in this circumstance.

All Arrhenius acids and bases are Brønsted-Lowry acids and bases as well. However, not all Brønsted-Lowry acids and bases are Arrhenius acids and bases.

Example \(\PageIndex{1}\)

Aniline (C₆H₅NH₂) is slightly soluble in water. It has a nitrogen atom that can accept a hydrogen ion from a water molecule just like the nitrogen atom in ammonia does. Write the chemical equation for this reaction and identify the Brønsted-Lowry acid and base.

Solution

C₆H₅NH₂ and H₂O are the reactants. When C₆H₅NH₂ accepts a proton from H₂O, it gains an extra H and a positive charge and leaves an OH⁻ ion behind. The reaction is as follows:

C₆H₅NH₂(aq) + H₂O(ℓ) → C₆H₅NH₃⁺(aq) + OH⁻(aq)

Because C₆H₅NH₂ accepts a proton, it is the Brønsted-Lowry base. The H₂O molecule, because it donates a proton, is the Brønsted-Lowry acid.

Exercise \(\PageIndex{1}\)

Caffeine (C₈H₁₀N₄O₂) is a stimulant found in coffees and teas. When dissolved in water, it can accept a proton from a water molecule. Write the chemical equation for this process and identify the Brønsted-Lowry acid and base.

Answer

C₈H₁₀N₄O₂(aq) + H₂O(ℓ) → C₈H₁₁N₄O₂⁺(aq) + OH⁻(aq)

B-L base B-L acid

The Brønsted-Lowry definitions of an acid and a base can be applied to chemical reactions that occur in solvents other than water. The following example illustrates.

Example \(\PageIndex{2}\)

Sodium amide (NaNH₂) dissolves in methanol (CH₃OH) and separates into sodium ions and amide ions (NH₂⁻). The amide ions react with methanol to make ammonia and the methoxide ion (CH₃O⁻). Write a balanced chemical equation for this process and identify the Brønsted-Lowry acid and base.

Solution

The equation for the reaction is between NH₂⁻ and CH₃OH to make NH₃ and CH₃O⁻ is as follows:

NH₂⁻(solv) + CH₃OH(ℓ) → NH₃(solv) + CH₃O⁻(solv)

The label (solv) indicates that the species are dissolved in some solvent, in contrast to (aq), which specifies an aqueous (H₂O) solution. In this reaction, we see that the NH₂⁻ ion accepts a proton from a CH₃OH molecule to make an NH₃ molecule. Thus, as the proton acceptor, NH₂⁻ is the Brønsted-Lowry base. As the proton donor, CH₃OH is the Brønsted-Lowry acid.

Exercise \(\PageIndex{2}\)

Pyridinium chloride (C₅H₅NHCl) dissolves in ethanol (C₂H₅OH) and separates into pyridinium ions (C₅H₅NH⁺) and chloride ions. The pyridinium ion can transfer a hydrogen ion to a solvent molecule. Write a balanced chemical equation for this process and identify the Brønsted-Lowry acid and base.

Answer

C₅H₅NH⁺(solv) + C₂H₅OH(ℓ) → C₅H₅N(solv) + C₂H₅OH₂⁺(solv)

B-L acid B-L base

How to Recognize Acids and Bases

Acids and bases can be inorganic or organic. Inorganic acids and bases will often be represented by molecular formulas, as in Table \(\PageIndex{1}\). Organic acids and bases can be recognized based on their functional group when the structure is known.

Acids

As mentioned above, molecular formulas for acids start with H. This lets you know that the substance is an acid, and also indicates how many of the hydrogen atoms are acidic, that is how many can be lost as H⁺ in reactions. For example, the molecular formula for acetic acid is HC₂H₃O₂. Although there are four hydrogen atoms in the molecule, the format indicates that only one is acidic. Acetic acid can lose dissociate (separate) into H⁺ and C₂H₃O₂^-. The other three hydrogen atoms are not acidic; they are not lost during reactions. This is indicated by the fact that they are not written first in the molecular formula. Compare this to hydrosulfuric acid, H₂S, and phosphoric acid, H₃PO₄. In these molecules all the hydrogen atoms are acidic so they are all written first in the molecular formula.

For organic acids you may be provided with a picture of the structure (condensed structural formula or skeletal structure) instead of a molecular formula. In that case the presence of a carboxylic acid functional group will indicate that the substance is an acid.

C=O with the C also singly bonded to R and OH

Figure \(\PageIndex{2}\): Carboxylic acid functional group

The carboxylic acid functional group can also be abbreviated COOH as in CH₃CH₂COOH.

Bases

Inorganic bases are often metal hydroxides such as NaOH and Ca(OH)₂. Pay attention when you see OH in a formula! Metals bonded to hydroxide ions (OH^-) are bases, but the abbreviation COOH represents a carboxylic acid and other organic compounds containing an OH are alcohols which are neither an acid nor a base.

Amines are organic bases. Remember that the amine functional group contains a nitrogen that is not bonded to C=O. The number of hydrogen atoms bonded to the nitrogen in an amine can vary, so they should not be used for identification. For example, CH₃NHCH₃ and CH₃CH₂NH₂ are both amines.

There are additional inorganic bases with a variety of formulas. An easy one to recognize is ammonia, NH₃, because it is similar to an amine.

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.
Write a balanced chemical equation for the neutralization of Ba(OH)₂(aq) with HNO₃(aq).
Write a balanced chemical equation for the neutralization of H₂SO₄(aq) with Cr(OH)₃(aq).
Gastric juice, the digestive fluid produced in the stomach, contains hydrochloric acid, HCl. Milk of Magnesia, a suspension of solid Mg(OH)₂in an aqueous medium, is sometimes used to neutralize excess stomach acid. Write a complete balanced equation for the neutralization reaction.
Identify the salt produced in each acid-base reaction below. Then, balance the equation.
1. 2HCl + Sr(OH)₂ → 2H₂O + ??
2. KNO₃_;HNO₃ + KOH → ?? + H₂O
3. HF + Ca(OH)₂ ---> ?? + H₂O
Hydrazoic acid (HN₃) can be neutralized by a base. Write the balanced chemical equation for the reaction between hydrazoic acid and calcium hydroxide.
Citric acid (H₃C₆H₅O₇) has three hydrogen atoms that can form hydrogen ions in solution. Write the balanced chemical equation for the reaction between citric acid and sodium hydroxide.

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

5. 2HNO₃(aq) + Ba(OH)₂(aq) → Ba(NO₃)₂(aq) + 2H₂O

6. 3H₂SO₄(aq) + 2Cr(OH)₃(aq) → Cr₂(SO₄)₃(aq) + 6H₂O

7. Mg(OH)₂+ 2HCl --> MgCl₂ + 2H₂O

8. a. SrCl₂_;2HCl + Sr(OH)₂ → 2H₂O + SrCl₂

b. KNO₃_;HNO₃ + KOH → KNO₃ + H₂O

c. CaF₂_;2HF + Ca(OH)₂ → CaF₂ + 2H₂O

9. 2HN₃(aq) + Ca(OH)₂ → Ca(N₃)₂ + 2H₂O

10. H₃C₆H₅O₇(aq) + 3NaOH(aq) → Na₃C₆H₅O₇(aq) + 3H₂O

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