11.2: Reactions of Acids and Bases

 Learning Objectives

1. Recognize a compound as a Brønsted-Lowry acid or a Brønsted-Lowry base.
2. Illustrate the proton transfer process that defines a Brønsted-Lowry acid-base reaction.

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.

\[ HCO_3^- (aq) + H^+ (aq) \rightarrow H_2 CO_3 (aq) \label{4.3.19} \]

\[ H_2 CO_3 (aq) \rightarrow CO_2 (g) + H_2 O(l) \nonumber \]

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

HNO₃ is the acid, so it loses H⁺. In acid-base neutralization reactions any metals will always be spectator ions. In this case the positive calcium ion will be attracted to the nitrate ion that remains when nitric acid loses H⁺. This ionic compound is the salt and it has the formula Ca(OH)₂ because calcium ions have a +2 charge while hydroxide ions have a -1 charge. The base is OH^- and as a base its job is to accept H⁺. The number of H atoms increases by 1 and the charge increases by 1 because OH^- is gaining an H⁺, not an H atom. OH^- + H⁺ form H₂O. 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)

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.

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,

\[CaCO_3(s)+2HCl(aq)⇌CaCl_2(aq)+H_2O(l)+CO_2(g) \nonumber \]

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:

\[Mg(OH)_2(s)⇌Mg^{2+}(aq)+2OH^-(aq) \nonumber \]

The hydroxide ions generated in this equilibrium then go on to react with the hydronium ions from the stomach acid, so that :

\[H_3O^+ + OH^- ⇌ 2H_2O(l) \nonumber \]

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.

Conjugate Acid-Base Pairs

According to the Bronsted-Lowry theory of acids and bases, an acid is a proton donor and a base is a proton acceptor. Once an acid has given up a proton, the part that remains is called the acid's conjugate base. This species is a base because it can accept a proton (to re-form the acid). The conjugate base of HF (first example below) is fluoride ion, F^-.

\(\mathrm{{\color{Red} Acid} = H^+ + {\color{Blue} Conjugate\: base\: of\: Acid}^-}\)

\({\color{Red} \mathrm{HF}} \rightleftharpoons \mathrm{H^+} + {\color{Blue} \mathrm{F^-}}\)

\({\color{Red} \mathrm{H_2O}} \rightleftharpoons \mathrm{H^+} + {\color{Blue} \mathrm{OH^-}}\)

\({\color{Red} \mathrm{NH_4^+}} \rightleftharpoons \mathrm{H^+} + {\color{Blue} \mathrm{NH_3}}\)

Similarly, the part of the base that remains after a base accepts a proton is called the base's conjugate acid. This species is an acid because it can give up a proton (and thus re-form the base). The conjugate acid of fluoride ion, F^- (first example below) is HF.

\(\mathrm{H^+ + {\color{Blue} Base} = {\color{Red} Conjugate\: acid\: of\: Base}^+}\)

\(\mathrm{H^+ + {\color{Blue} F^- } \rightleftharpoons {\color{Red} HF}}\)

\(\mathrm{H^+ + {\color{Blue} OH^- } \rightleftharpoons {\color{Red} H_2O}}\)

\(\mathrm{H^+ + {\color{Blue} H_2O } \rightleftharpoons {\color{Red} H_3O^+}}\)

\(\mathrm{H^+ + {\color{Blue} NH_3 } \rightleftharpoons {\color{Red} NH_4^+}}\)

To summarize, the conjugate base of HF is fluoride ion, F^-, and the conjugate acid of fluoride ion, F^-, is HF. The HF and F^- pair is referred to as a conjugate acid-base pair. The difference in the formulas of a conjugate acid-base pair is H⁺. The table below lists conjugate acid-base pairs for your reference so that you can figure out the strategy of identifying them by looking at the pattern in the examples. For any given acid or base, you should be able to give its conjugate base or conjugate acid. The formula of an acid's conjugate base is generated by removing a proton (H_⁺) from the acid formula. The formula of the base's conjugate acid is formed by adding a proton (H_⁺) to the formula of the base.

Table \(\PageIndex{1}\). Conjugate acid-base pairs.

Conjugate Acid	Conjugate Base
\(\ce{H3O+}\)	\(\ce{H2O}\)
\(\ce{H2O}\)	\(\ce{OH-}\)
\(\ce{H2SO4}\)	\(\ce{HSO4-}\)
\(\ce{HSO4-}\)	\(\ce{SO4^2-}\)
\(\ce{NH4+}\)	\(\ce{NH3}\)
\(\ce{NH3}\)	\(\ce{NH2-}\)
\(\ce{CH3COOH}\)	\(\ce{CH3COO-}\)
\(\ce{CH3NH3+}\)	\(\ce{CH3NH2}\)

Example \(\PageIndex{3}\) : Conjugate Pairs

Write the formula of the conjugate base of (a) HCl and (b) HCO₃^– .

Write the formula of the conjugate acid of (c) CH₃NH₂ and (d) OH^–.

Solution:

A conjugate base is formed by removing a proton (H^₊). A conjugate acid is formed by accepting a proton (H^₊).

1. After HCl donates a proton, a Cl^– ion is produced, and so Cl^– is the conjugate base.
2. After hydrogen carbonate ion, HCO₃^–, donates a proton, its conjugate base, CO₃^2– is produced.
3. After accepting a proton (H^₊), CH₃NH₂ is converted to CH₃NH₃⁺, its conjugate acid.
4. After accepting a proton (H^₊), OH^– is converted to H₂O, its conjugate acid.

Exercise \(\PageIndex{3}\): Conjugate Pairs

Write the formula of the conjugate base of (a) HNO₂ and (b) H₂CO₃.

Write the formula of the conjugate acid of (c) C₆H5NH₂ and (d) HCO₃^–.

Answer

a. NO₂^– is the conjugate base of HNO₂.

b. HCO₃^– is the conjugate base of H₂CO₃

c. C₆H5NH₃⁺ is the conjugate acid of C₆H5NH_2.

d. H₂CO₃ is the conjugate acid of HCO₃^–

In the reaction of ammonia with water to give ammonium ions and hydroxide ions, ammonia acts as a base by accepting a proton from a water molecule, which in this case means that water is acting as an acid. In the reverse reaction, an ammonium ion acts as an acid by donating a proton to a hydroxide ion, and the hydroxide ion acts as a base. The conjugate acid–base pairs for this reaction are \(NH_4^+/NH_3\) and \(H_2O/OH^−\). This means that the conjugate acid of the base NH₃ is NH₄⁺ while the conjugate base of the acid NH₄⁺ is NH_3. Similarly, the conjugate base of the acid H₂O is OH^-, and the conjugate acid of the base OH^- is H₂O.

 

 

 

In the forward reaction, the parent acid is H₂O and and the parent base is NH₃ (shown in the illustration below). The acid H₂O loses a proton (H^₊) to form its conjugate base OH^-. The base NH₃ gains a proton, to produce its conjugate acid NH₄⁺. In the reverse reaction, the acid NH₄⁺ loses a proton (H^₊) to form its conjugate base NH₃. The base OH^- gains a proton, to produce its conjugate acid H₂O.

When hydrogen fluoride (HF) dissolves in water and ionizes, protons are transferred from hydrogen fluoride (parent acid) molecules to water (parent base) molecules, yielding hydronium ions (conjugate acid of water) and fluoride ions (conjugate base of HF):

\({\color{Red} \mathrm{HF}} + {\color{Blue} H_2O } \rightleftharpoons {\color{Blue} H_3O^+} + {\color{Red} \mathrm{F^-}}\)

Example \(\PageIndex{3}\)

Identify the conjugate acid-base pairs in this equilibrium.

\[\ce{CH3CO2H + H2O <=> H3O^{+} + CH3CO2^{-}} \nonumber \]

Solution

Similarly, in the reaction of acetic acid with water, acetic acid donates a proton to water, which acts as the base. In the reverse reaction, \(H_3O^+\) is the acid that donates a proton to the acetate ion, which acts as the base.

Once again, we have two conjugate acid–base pairs:

• the parent acid and its conjugate base (\(CH_3CO_2H/CH_3CO_2^−\)) and
• the parent base and its conjugate acid (\(H_3O^+/H_2O\)).

Example \(\PageIndex{4}\)

Identify the conjugate acid-base pairs in this equilibrium.

\[(CH_{3})_{3}N + H_{2}O\rightleftharpoons (CH_{3})_{3}NH^{+} + OH^{-} \nonumber \]

Solution

One pair is H₂O and OH⁻, where H₂O has one more H⁺ and is the conjugate acid, while OH⁻ has one less H⁺ and is the conjugate base.

The other pair consists of (CH₃)₃N and (CH₃)₃NH⁺, where (CH₃)₃NH⁺ is the conjugate acid (it has an additional proton) and (CH₃)₃N is the conjugate base.

Exercise \(\PageIndex{4}\)

Identify the conjugate acid-base pairs in this equilibrium.

\[\ce{NH2^{-} + H2O\rightleftharpoons NH3 + OH^{-}} \nonumber \]

Answer:

H₂O (acid) and OH⁻ (base); NH₂⁻ (base) and NH₃ (acid)

The use of conjugate acid-base pairs allows us to make a very simple statement about relative strengths of acids and bases. The stronger an acid, the weaker its conjugate base, and, conversely, the stronger a base, the weaker its conjugate acid.

To Your Health: Brønsted-Lowry Acid-Base Reactions in Pharmaceuticals

There are many interesting applications of Brønsted-Lowry acid-base reactions in the pharmaceutical industry. For example, drugs often need to be water soluble for maximum effectiveness. However, many complex organic compounds are not soluble or are only slightly soluble in water. Fortunately, those drugs that contain proton-accepting nitrogen atoms (and there are a lot of them) can be reacted with dilute hydrochloric acid [HCl(aq)]. The nitrogen atoms—acting as Brønsted-Lowry bases—accept the hydrogen ions from the acid to make an ion, which is usually much more soluble in water. The modified drug molecules can then be isolated as chloride salts:

\[ RN(sl\: aq) + HCl(aq) \rightarrow RNH^+(aq) + Cl^-(aq) \rightarrow RNHCl(s) \nonumber \]

where RN represents some organic compound containing nitrogen. The label (sl aq) means “slightly aqueous,” indicating that the compound RN is only slightly soluble. Drugs that are modified in this way are called hydrochloride salts. Examples include the powerful painkiller codeine, which is commonly administered as codeine hydrochloride. Acids other than hydrochloric acid are also used. Hydrobromic acid, for example, gives hydrobromide salts. Dextromethorphan, an ingredient in many cough medicines, is dispensed as dextromethorphan hydrobromide. The accompanying figure shows another medication (lidocaine) as a hydrochloride salt.

Figure used with permission from Wikipedia.

 

Concept Review Exercises

1. What is neutralization?

Answers

1. The reaction of an acid and a base

Exercises

1. Write a balanced chemical equation for the neutralization of Ba(OH)₂(aq) with HNO₃(aq).
2. Write a balanced chemical equation for the neutralization of H₂SO₄(aq) with Cr(OH)₃(aq).
3. 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.
4. Identify the salt produced in each acid-base reaction below. Then, balance the equation.
   1. 2HCl + Sr(OH)₂ → 2H₂O + ??
   2. KNO_3; HNO₃ + KOH → ?? + H₂O
   3. HF + Ca(OH)₂ ---> ?? + H₂O
5. Hydrazoic acid (HN₃) can be neutralized by a base. Write the balanced chemical equation for the reaction between hydrazoic acid and calcium hydroxide.
6. 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

1. 2HNO₃(aq) + Ba(OH)₂(aq) → Ba(NO₃)₂(aq) + 2H₂O
2. 3H₂SO₄(aq) + 2Cr(OH)₃(aq) → Cr₂(SO₄)₃(aq) + 6H₂O
3. Mg(OH)₂ + 2HCl --> MgCl₂ + 2H₂O
4. a. SrCl_2; 2HCl + Sr(OH)₂ → 2H₂O + SrCl₂

    b. KNO_3; HNO₃ + KOH → KNO₃ + H₂O

    c. CaF_2; 2HF + Ca(OH)₂ → CaF₂ + 2H₂O

1. 2HN₃(aq) + Ca(OH)₂ → Ca(N₃)₂ + 2H₂O
2. H₃C₆H₅O₇(aq) + 3NaOH(aq) → Na₃C₆H₅O₇(aq) + 3H₂O