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Reactions in aqueous solutions

Acids, Bases and Neutralization

reactions

ACIDS

Many acids and bases are industrial and household substances and

some are important components of biological fluids. Hydrochloric

acid (HCl), for example, is an important industrial chemical and the

main constituent of gastric juice in your stomach.

Acids and bases are also common electrolytes.

124 CHAPTER 4 Reactions in Aqueo

! FIGURE 4.5 Vinegar and lemon juice

are common household acids. Ammonia

and baking soda (sodium bicarbonate)

are common household bases.

**Acids are substances that ionize in aqueous solutions to form hydrogen ions H

(** aq ). As a hydrogen atom consists of a proton and an electron, H

is simply a proton. Thus, acids are also called proton donors.

ACIDS

Acetic acid (CH 3 COOH) is the primary component in vinegar and is a monoprotic acid. It has four hydrogens but only one of them, the H in the COOH group, is ionized in water. The three other hydrogens are bound to carbon and do not break their C-H bonds in water. CH 3 COOH ( aq ) ⟶ **H

(** aq) + CH 3 COO

- (aq) Acetic acid, CH 3 COOH Nitric acid, HNO 3 Hydrochloric acid, HCl ! FIGURE 4.6 Molecular models

three common acids.

BASES

**Bases are substances that accept H

ions. Bases produce hydroxide ions OH**

- ( aq ) when they dissolve in water. NH 3 ( aq ) + H 2 O ⟶ **NH 4

(** aq) + OH - (aq) Ionic hydroxide compounds, such as NaOH, KOH, and Ca(OH) 2 , are among the most common bases. When dissolved in water, they release OH

ions into the solution. Compounds that do not contain OH

ions can also be bases. For example, ammonia (NH 3 ) is a common base. When added to water, it accepts an H

ion from a water molecule and thereby produces an OH

ion. SECTION 4.3 Acids, Bases, and Neutralization Reactions 125 in component of citrus H 2 O NH 3 OH! " " NH 4

FIGURE 4.7 Hydrogen ion transfer.

An H 2 O molecule acts as a proton donor (acid), and NH 3 acts as a proton acceptor (base). Only a fraction of the NH 3 molecules

PRACTICE EXERCISE (Comparing Acid Strengths)

• The following diagrams represent aqueous solutions of acids HX, HY, and HZ. Rank the acids from strongest to weakest. Solution We can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most H+^ ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules. The order is HY > HZ > HX. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more H+^ ions and fewer molecules than HX, it is a stronger acid.
• Imagine a diagram showing 10 Na

• (^) ions and 10 OH- (^) ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction? Answer: The diagram would show10Na+^ ions, 2OH-ions, 8Y-^ ions and 8 H 2 O molecules.

126 CHAPTER 4 Reactions in Aqueous Solution

G I V E I T S O M E T H O U G H T Why isn’t Al(OH) 3 classified as a strong base? SAMPLE EXERCISE 4.5 Comparing Acid Strengths The following diagrams represent aqueous solutions of acids HX, HY, and HZ, with water molecules omitted for clarity. Rank the acids from strongest to weakest. SOLUTION Analyze We are asked to rank three acids from strongest to weakest, based on schematic drawings of their solutions. Plan We can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules. Solve The order is. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more ions and fewer molecules than HX, it is a stronger acid. PRACTICE EXERCISE Imagine a diagram showing 10 ions and 10 ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction? Answer: The diagram would show 10 ions, 2 ions, 8 ions, and 8 H 2 O molecules.

Identifying Strong and Weak Electrolytes

If we remember the common strong acids and bases (Table 4.2) and also remember that

NH 3 is a weak base, we can make reasonable predictions about the electrolytic strength of

a great number of water-soluble substances.! TABLE 4.3 summarizes our observations

about electrolytes. To classify a soluble substance as strong electrolyte, weak electrolyte,

or nonelectrolyte, we work our way down and across this table. We first ask whether the

substance is ionic or molecular. If it is ionic, it is a strong electrolyte. The second column

of Table 4.3 tells us that all ionic compounds are strong electrolytes. If the substance is

molecular, we ask whether it is an acid or a base. (It is an acid if it either has H first in the

chemical formula or contains a COOH group.) If it is an acid, we use Table 4.2 to deter-

mine whether it is a strong or weak electrolyte: All strong acids are strong electrolytes,

Na+ OH- Y- Na+ OH- H+ HY 7 HZ 7 HX H+

"^!

HX HY HZ

126 CHAPTER 4 Reactions in Aqueous Solution

G I V E I T S O M E T H O U G H T Why isn’t Al(OH) 3 classified as a strong base? SAMPLE EXERCISE 4.5 Comparing Acid Strengths The following diagrams represent aqueous solutions of acids HX, HY, and HZ, with water molecules omitted for clarity. Rank the acids from strongest to weakest. SOLUTION Analyze We are asked to rank three acids from strongest to weakest, based on schematic drawings of their solutions. Plan We can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules. Solve The order is. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more ions and fewer molecules than HX, it is a stronger acid. PRACTICE EXERCISE Imagine a diagram showing 10 ions and 10 ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction? Answer: The diagram would show 10 ions, 2 ions, 8 ions, and 8 H 2 O molecules.

Identifying Strong and Weak Electrolytes

If we remember the common strong acids and bases (Table 4.2) and also remember that

NH 3 is a weak base, we can make reasonable predictions about the electrolytic strength of

a great number of water-soluble substances.! TABLE 4.3 summarizes our observations

about electrolytes. To classify a soluble substance as strong electrolyte, weak electrolyte,

or nonelectrolyte, we work our way down and across this table. We first ask whether the

substance is ionic or molecular. If it is ionic, it is a strong electrolyte. The second column

of Table 4.3 tells us that all ionic compounds are strong electrolytes. If the substance is

molecular, we ask whether it is an acid or a base. (It is an acid if it either has H first in the

chemical formula or contains a COOH group.) If it is an acid, we use Table 4.2 to deter-

mine whether it is a strong or weak electrolyte: All strong acids are strong electrolytes,

Na+ OH- Y- Na+ OH- H+ HY 7 HZ 7 HX H+

"^!

HX HY HZ

126 CHAPTER 4 Reactions in Aqueous Solution

G I V E I T S O M E T H O U G H T Why isn’t Al(OH) 3 classified as a strong base? SAMPLE EXERCISE 4.5 Comparing Acid Strengths The following diagrams represent aqueous solutions of acids HX, HY, and HZ, with water molecules omitted for clarity. Rank the acids from strongest to weakest. SOLUTION Analyze We are asked to rank three acids from strongest to weakest, based on schematic drawings of their solutions. Plan We can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules. Solve The order is. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more ions and fewer molecules than HX, it is a stronger acid. PRACTICE EXERCISE Imagine a diagram showing 10 ions and 10 ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction? Answer: The diagram would show 10 ions, 2 ions, 8 ions, and 8 H 2 O molecules.

Identifying Strong and Weak Electrolytes

If we remember the common strong acids and bases (Table 4.2) and also remember that

NH 3 is a weak base, we can make reasonable predictions about the electrolytic strength of

a great number of water-soluble substances.! TABLE 4.3 summarizes our observations

about electrolytes. To classify a soluble substance as strong electrolyte, weak electrolyte,

or nonelectrolyte, we work our way down and across this table. We first ask whether the

substance is ionic or molecular. If it is ionic, it is a strong electrolyte. The second column

of Table 4.3 tells us that all ionic compounds are strong electrolytes. If the substance is

molecular, we ask whether it is an acid or a base. (It is an acid if it either has H first in the

chemical formula or contains a COOH group.) If it is an acid, we use Table 4.2 to deter-

mine whether it is a strong or weak electrolyte: All strong acids are strong electrolytes,

Na+ OH- Y- Na+ OH- H+ HY 7 HZ 7 HX H+

"^!

HX HY HZ

HX HY HZ

Strong and Weak Acids and Bases

Identifying Strong and Weak Electrolytes

It is possible to make predictions about the electrolytic strength of a great number of water-soluble substances. To classify a soluble substance as strong electrolyte, weak electrolyte, or non-electrolyte, we work our way down and across this table. We first ask whether the substance is ionic or molecular. If it is ionic, it is a strong electrolyte. Summary of the Electrolytic Behavior of Common Soluble Compounds Strong Electrolyte Weak Electrolyte Nonelectrolyte Ionic All None None Molecular Strong acids (see table) Weak acids, weak bases All other compounds

Neutralization Reactions and Salts

The properties of acidic solutions are quite different from those of basic solutions. Acids have a sour taste, whereas bases have a bitter taste. In addition, acidic and basic solutions differ in chemical properties in several other important ways. When an acid and a base are mixed, a neutralization reaction occurs. The products of the reaction have none of the characteristic properties of either the acidic solution or the basic solution. E.g. When hydrochloric acid is mixed with a solution of sodium hydroxide, the reaction is HCl ( aq ) + NaOH ( aq ) ⟶ H 2 O ( l ) + NaCl (aq) acid base water salt Water and table salt, NaCl, are the products of the reaction. By analogy to this reaction, the term salt has come to mean any ionic compound whose cation comes from a base (for example, Na

from NaOH) and whose anion comes from an acid (for example, Cl

from HCl). In general, a neutralization reaction between an acid and a metal hydroxide produces water and a salt.

PRACTICE EXERCISE (Writing Chemical Equations for a Neutralization Reaction)

• For the reaction between aqueous solutions of acetic acid (CH^3 COOH) and barium hydroxide, Ba(OH)^2 , write the balanced molecular equation Solution Neutralization reactions form two products: H 2 O and a salt. We examine the cation of the base and the anion of the acid to determine the composition of the salt. The salt contains the cation of the base (Ba2+) and the anion of the acid (CH 3 COO-). Thus, the salt formula is Ba(CH 3 COO) 2. This compound is soluble in water. The unbalanced molecular equation for the neutralization reaction is CH 3 COOH ( aq ) + Ba(OH) 2 ( aq ) ⟶ H 2 O ( l ) + Ba(CH 3 COO) 2 (aq) To balance this equation, we must provide two molecules of CH COOH to furnish the two CH COO- ions and to supply the two H ions needed to combine with the two OH ions of the base. The balanced molecular equation is 2 CH 3 COOH ( aq ) + Ba(OH) 2 ( aq ) ⟶ 2 H 2 O ( l ) + Ba(CH 3 COO) 2 (aq)
• For the reaction of phosphorous acid (H^3 PO^3 ) and potassium hydroxide (KOH), write the balanced molecular equation. Answers: (a)H 3 PO 3 ( aq ) + 3 KOH ( aq ) ⟶ 3 H 2 O (l ) + K 3 PO 3 ( aq )

Neutralization Reactions and Salts