10. Reactions in Aqueous Solutions I

For instance, your stomach “digestive juice” contains approximately 0.10 mole of [blank_start]hydrochloric acid (HCl)[blank_end] per liter. Human blood and the aqueous components of most cells are slightly [blank_start]basic[blank_end]. The liquid in your car battery is approximately 40% [blank_start]sulfuric acid (H2SO4)[blank_end] by mass. Baking soda (NaHCO3) is a salt of [blank_start]carbonic acid (H2CO3)[blank_end]. [blank_start]Sodium hydroxide (NaOH)[blank_end], a base, is used in the manufacture of soaps, paper, and many other chemicals. “[blank_start]Drano[blank_end]” is solid NaOH that contains some aluminum chips. [blank_start]Sodium chloride (NaCl)[blank_end] is used to season food and as a food preservative. [blank_start]Calcium chloride (CaCl2)[blank_end] is used to melt ice on highways and in the emergency treatment of cardiac arrest. Several [blank_start]ammonium salts[blank_end] are used as fertilizers. Many organic acids ([blank_start]carboxylic acids[blank_end]) and their derivatives occur in nature. [blank_start]Acetic acid[blank_end] is present in vinegar. The sting of an ant bite is due to [blank_start]formic acid[blank_end]. [blank_start]Amino acids[blank_end] are carboxylic acids that also contain basic groups derived from ammonia, and are the building blocks of proteins, which are important materials in all forms of life. The pleasant odors and flavors of ripe fruit are due in large part to the presence of [blank_start]esters[blank_end] that are formed from organic acids in unripe fruit.

[blank_start]Protic acids[blank_end] are those containing acidic hydrogen atoms.

In 1680, [blank_start]Robert Boyle[blank_end] noted that acids (1) dissolve many substances, (2) change the colors of some natural dyes (indicators), and (3) lose their characteristic properties when mixed with alkalis (bases). By 1814, [blank_start]J. Gay-Lussac[blank_end] concluded that acids neutralize bases and that the two classes of substances should be defined in terms of their reactions with each other.

In 1884, [blank_start]Svante Arrhenius[blank_end] (1859–1927) presented his theory of electrolytic dissociation, which resulted in the Arrhenius theory of acid–base reactions.

According to [blank_start]Arrhenius[blank_end] theory an acid is a substance that contains hydrogen and produces H ions in aqueous solution. A base is a substance that contains the OH (hydroxyl) group and produces hydroxide ions in aqueous solution.

[blank_start]Neutralization[blank_end] is defined as the reaction of H ions with OH ions to form H2O molecules.

The [blank_start]hydrated hydrogen ion[blank_end] is the species that gives aqueous solutions of acids their characteristic acidic properties.

According to [blank_start]Brønsted–Lowry[blank_end] theory, an acid is defined as a proton donor (H ion), and a base is defined as a proton acceptor.

An [blank_start]acid–base reaction[blank_end] is the transfer of a proton from an acid to a base. The complete ionization of hydrogen chloride, HCl, a strong acid, in water is an example of this reaction in which water acts as a base or proton acceptor.

The indicator [blank_start]bromthymol blue[blank_end] is yellow in acidic solution and blue in basic solution.

Hydrogen fluoride (HF) is a [blank_start]weak[blank_end] acid.

We can describe Brønsted–Lowry acid–base reactions in terms of [blank_start]conjugate acid–base[blank_end] pairs. These are two species that differ by a proton.

The [blank_start]stronger[blank_end] the acid, the [blank_start]weaker[blank_end] is its conjugate base; the [blank_start]weaker[blank_end] the acid, the [blank_start]stronger[blank_end] is its conjugate base. Note: “Strong” and “weak,” like many adjectives, are used here in a relative sense. “Stronger” and “weaker,” are simply comparative terms that we use to compare the basicity (or acidity) of two (or more) ions or molecules.

Ammonia acts as a [blank_start]weak[blank_end] Brønsted–Lowry base, and water acts as an acid in the ionization of aqueous ammonia. Note: Be careful to avoid confusing solubility in water and extent of ionization. They are not necessarily related. Ammonia is very soluble in water (≈15 mol/L at 25˚ C). In a 0.10 M solution, NH3 is only 1.3% ionized to form NH4 ions and 98.7% nonionized.

Water can act either as an acid or as a base, depending on the other species present.

The ability of a substance to react as either an acid or a base is known as [blank_start]amphoterism[blank_end].

B-H, C-H, and N-H bonds in neutral molecules are usually too strong to ionize to produce H ions.

This [blank_start]autoionization[blank_end] of water is an acid–base reaction according to the Brønsted–Lowry theory. One H2O molecule (the acid) donates a proton to another H2O molecule (the base). The H2O molecule that donates a proton becomes an OH- ion, the conjugate base of water. The H2O molecule that accepts a proton becomes a hydronium ion.

Water is said to be [blank_start]amphiprotic[blank_end]; that is, H2O molecules can both donate and accept protons.

Whether a particular substance behaves as an acid or as a base is independent of its environment.

[blank_start]Amphoterism[blank_end] is a more general term that describes the ability of a substance to react either as an acid or as a base.

Several insoluble [blank_start]metal hydroxides[blank_end] are amphoteric; that is, they react with acids to form salts and water, but they also dissolve in and react with excess strong bases.

Only a few hydroxides containing small, highly charged metal ions are insoluble in water.

Generally, elements of [blank_start]intermediate[blank_end] electronegativity form amphoteric hydroxides. Those of [blank_start]high[blank_end] and [blank_start]low[blank_end] electronegativity form acidic and basic “hydroxides,” respectively.

The order of bond strengths for the hydrogen halides is (strongest bonds) [blank_start]HF[blank_end] >> [blank_start]HCl[blank_end] > [blank_start]HBr[blank_end] > [blank_start]HI[blank_end] (weakest bonds)

A weak acid may be very reactive. For example, [blank_start]hydrogen fluoride[blank_end] dissolves sand and glass.

The [blank_start]hydronium ion[blank_end] is the strongest acid that can exist in aqueous solution. All acids stronger than it will react completely with water to produce H3O+ (aq) and their conjugate bases.

The [blank_start]hydroxide ion[blank_end] is the strongest base that can exist in aqueous solution. Bases stronger than it will react completely with H2O to produce OH- and their conjugate acids.

[blank_start]Sulfuric acid[blank_end] is the only common polyprotic acid that is also a strong acid.

The first step in the ionization of a polyprotic acid always occurs to a [blank_start]greater[blank_end] extent than the second step because it is easier to remove a proton from a neutral acid molecule than from a negatively charged anion.

Acid strengths of most ternary acids containing the same central element [blank_start]increase[blank_end] with increasing oxidation state of the central element and with increasing numbers of oxygen atoms.

For most ternary acids containing different elements from the same periodic table group,in the same oxidation state, acid strengths [blank_start]increase[blank_end] with increasing electronegativity of the central element.

Most [blank_start]salts[blank_end] are ionic compounds that contain a cation other than H+ and an anion other than OH- or O2-.

[blank_start]Neutralization[blank_end] reactions involve the reaction of an acid with a base to form a salt that contains the cation characteristic of the base and the anion characteristic of the acid. Water is also usually formed.

[blank_start]Normal salts[blank_end] are salts that contain no ionizable H atoms or OH groups.

[blank_start]Baking soda[blank_end] is very handy for neutralizing chemical spills of acids or bases due to its ability to react with and neutralize both strong acids and strong bases. Containers of this substance are kept in many chemistry labs for this purpose.

If less than a stoichiometric amount of a base reacts with a polyprotic acid, the resulting salt is classified as an [blank_start]acidic salt[blank_end], because it can neutralize additional base.

[blank_start]Polyhydroxy bases[blank_end] are bases that contain more than one OH per formula unit.

The reaction of a polyhydroxy base with less than a stoichiometric amount of an acid forms a [blank_start]basic salt[blank_end], that is, a salt that contains unreacted OH groups.

Aqueous solutions of basic salts are not necessarily basic, but they can neutralize acids.

A few basic salts are rather insoluble in water.

According to [blank_start]Lewis[blank_end] theory, a/an acid is any species that can accept a share in an electron pair. A/An base is any species that can donate or share one or more lone pairs of electrons.

In Lewis theory, theoretically, any species that contains a lone pair of electrons could act as a/an [blank_start]base[blank_end].

[blank_start]Arrhenius acids[blank_end] and bases are also [blank_start]Brønsted–Lowry[blank_end] acids and bases; the reverse is not true. [blank_start]Brønsted–Lowry[blank_end] acids and bases are also [blank_start]Lewis[blank_end] acids and bases; the reverse is not true.

The reaction of concentrated sulfuric acid with solid [blank_start]sodium fluoride[blank_end] produces gaseous hydrogen fluoride, and with solid [blank_start]sodium chloride[blank_end] produces gaseous hydrogen fluoride.

Because concentrated sulfuric acid can act as an [blank_start]oxidizing[blank_end] agent, it cannot be used to prepare hydrogen bromide or hydrogen iodide; instead, the free halogens are produced.

Some high oxidation state transition metal oxides are [blank_start]acidic[blank_end] oxides; that is, they dissolve in water to give solutions of ternary [blank_start]acids[blank_end].

A nonmetal oxide that reacts with water to form an acid. [blank_start]Acid anhydride[blank_end]

A metal oxide that reacts with water to form a base. [blank_start]Basic anhydride[blank_end]

In aqueous solution, the process in which a solid ionic compound separates into its ions. [blank_start]Dissociation[blank_end]

A substance whose aqueous solutions conduct electricity. [blank_start]Electrolyte[blank_end]

A chemical equation in which all compounds are represented by complete formulas. [blank_start]Formula unit equation[blank_end]

The effect by which all acids stronger than the acid that is characteristic of the solvent react with the solvent to produce that acid; a similar statement applies to bases. The strongest acid (base) that can exist in a given solvent is the acid (base) characteristic of that solvent. [blank_start]Leveling effect[blank_end]

A proton donor. [blank_start]Brønsted–Lowry acid[blank_end] A proton acceptor. [blank_start]Brønsted–Lowry base[blank_end] A substance that produces OH- (aq) ions in aqueous solution. Strong bases are soluble in water and are completely dissociated. Weak bases ionize only slightly. [blank_start]Arrhenius base[blank_end] A substance that produces H+ (aq) ions in aqueous solution. Strong acids ionize completely or almost completely in dilute aqueous solution; weak acids ionize only slightly. [blank_start]Arrhenius acid[blank_end] Any species that can accept a share in an electron pair to form a coordinate covalent bond. [blank_start]Lewis acid[blank_end] Any species that can make available a share in an electron pair to form a coordinate covalent bond. [blank_start]Lewis base[blank_end]