Acid-Base Theories

In 1887, a Swedish chemist, Svante Arrhenius, published a paper concerning acids and bases. In 1887, a Swedish chemist, Svante Arrhenius, published a paper concerning acids and bases. He knew that solutions containing acids or bases conducted an electric current. He knew that solutions containing acids or bases conducted an electric current. He concluded that these substances released charged particles when dissolved. He concluded that these substances released charged particles when dissolved. He called these charged particles ions (wanderers). He called these charged particles ions (wanderers). He concluded that acids were substances which separated (ionized) in water solution to produce hydrogen ions, H +, or free protons). He concluded that acids were substances which separated (ionized) in water solution to produce hydrogen ions, H +, or free protons). He also believed that bases were substances which ionized to produce hydroxide ions, OH - in water solution. He also believed that bases were substances which ionized to produce hydroxide ions, OH - in water solution. HCl --> H + + Cl - HCl --> H + + Cl - NaOH --> Na + + OH - NaOH --> Na + + OH - Arrhenius Theory

Bronsted-Lowry Theory As the knowledge of catalysts and nonaqueous solutions increased, it became necessary to redefine the terms acid base. As the knowledge of catalysts and nonaqueous solutions increased, it became necessary to redefine the terms acid base. In l923, an English scientist, T. M. Lowry, and a Danish scientist, J. N. Bronsted, independently proposed a new definition. In l923, an English scientist, T. M. Lowry, and a Danish scientist, J. N. Bronsted, independently proposed a new definition. They stated that in a chemical reaction, any substance which donates a proton is an acid and any substance which accepts a proton is a base. They stated that in a chemical reaction, any substance which donates a proton is an acid and any substance which accepts a proton is a base. –E.g. When hydrogen chloride gas is dissolved in water, ions are formed. –HCl(aq) (acid) + H 2 0(l) (base) ---> H 3 O + (aq) + Cl - (aq) In this reaction, hydrogen chloride is an acid, and water is a base. Notice that the hydrogen ion has combined with a water molecule to form the polyatomic ion H30 +, which is called the hydronium. In this reaction, hydrogen chloride is an acid, and water is a base. Notice that the hydrogen ion has combined with a water molecule to form the polyatomic ion H30 +, which is called the hydronium. There is strong evidence that the hydrogen ion is never found free as H +. There is strong evidence that the hydrogen ion is never found free as H +. It is possible to have isolated H+ ions in the gas phase. But if a free hydrogen ion encounters a water molecule, it attacks the unshared electron pairs on the oxygen in the water molecule and forms a hydronium ion, H3O+. The chemical bond that forms between the water and hydrogen ion is covalent and very strong. In an aqueous solution, essentially all of the H+ exists as H3O+. It is possible to have isolated H+ ions in the gas phase. But if a free hydrogen ion encounters a water molecule, it attacks the unshared electron pairs on the oxygen in the water molecule and forms a hydronium ion, H3O+. The chemical bond that forms between the water and hydrogen ion is covalent and very strong. In an aqueous solution, essentially all of the H+ exists as H3O+. H (aq) + Cl - (aq) ---> HCl(aq) + H 2 O(l) H (aq) + Cl - (aq) ---> HCl(aq) + H 2 O(l) In this reaction, the H 3 O + ion is an acid. It acts as an acid because it donates a proton to the chloride ion, which is a base. In this reaction, the H 3 O + ion is an acid. It acts as an acid because it donates a proton to the chloride ion, which is a base.

Conjugates The hydronium ion is said to be the conjugate acid of the base, water. The hydronium ion is said to be the conjugate acid of the base, water. The chloride ion is called the conjugate base of the acid, hydrochloric acid. The chloride ion is called the conjugate base of the acid, hydrochloric acid. In general, any acid-base reaction is described as: In general, any acid-base reaction is described as: acid + base ---> conjugate base + conjugate acid acid + base ---> conjugate base + conjugate acid The conjugate base of an acid is the particle that remains after a proton has been released by the acid. The conjugate acid of a base is formed when the base acquires a proton from the acid. The conjugate base of an acid is the particle that remains after a proton has been released by the acid. The conjugate acid of a base is formed when the base acquires a proton from the acid. Table 24-1 contains a list of some anions and their conjugate acids. Table 24-1 contains a list of some anions and their conjugate acids.

Sample reaction Consider what happens when ammonia gas is added to water. Consider what happens when ammonia gas is added to water. NH 3 (ag) + H 2 0(l) ---> NH 4 + (aq) + OH - (aq) NH 3 (ag) + H 2 0(l) ---> NH 4 + (aq) + OH - (aq) base + acid ---> conjugate acid + conjugate base base + acid ---> conjugate acid + conjugate base In this reaction, water acts as an acid because it donates a proton to the ammonia molecule. The ammonium ion is the conjugate acid of ammonia, a base, which receives a proton from water. Hydroxide ion is the conjugate base. In this reaction, water acts as an acid because it donates a proton to the ammonia molecule. The ammonium ion is the conjugate acid of ammonia, a base, which receives a proton from water. Hydroxide ion is the conjugate base.

Lewis Theory In 1923, the same year that Bronsted and Lowry proposed their theories, another new idea appeared. In 1923, the same year that Bronsted and Lowry proposed their theories, another new idea appeared. Gilbert Newton Lewis, an American chemist, proposed an even broader definition of acids and bases. Gilbert Newton Lewis, an American chemist, proposed an even broader definition of acids and bases. The same type of reasoning as Bronsted's and Lowry's led to his proposals. The same type of reasoning as Bronsted's and Lowry's led to his proposals. However, Lewis focused on electron transfer instead of proton transfer. However, Lewis focused on electron transfer instead of proton transfer. He defined an acid as an electron-pair acceptor, and a base as an electron-pair donor. He defined an acid as an electron-pair acceptor, and a base as an electron-pair donor. This definition is more general than Bronsted's. It applies to solutions and reactions which do not even involve hydrogen or hydrogen ions. This definition is more general than Bronsted's. It applies to solutions and reactions which do not even involve hydrogen or hydrogen ions.

Acid-Base Behavior Consider a compound having the formula HOX. Consider a compound having the formula HOX. If the element X is highly electronegative, it will have a strong attraction for the electrons it is sharing with the oxygen. If the element X is highly electronegative, it will have a strong attraction for the electrons it is sharing with the oxygen. –As these electrons are pulled toward X, the oxygen, in turn, will pull strongly on the electrons it is sharing with the hydrogen. –The hydrogen ion, or proton, would then be lost easily. In this case, HOX is behaving as an acid. If the element X has a low electronegativity, the oxygen will tend to pull the shared electrons away from X. If the element X has a low electronegativity, the oxygen will tend to pull the shared electrons away from X. –The hydrogen will remain joined to the oxygen. –Since in this case the formation of hydroxide ion, OH-, is likely, HOX is behaving as a base. We know that nonmetals have high electronegativities and metals low electronegativities. We know that nonmetals have high electronegativities and metals low electronegativities. We can conclude, then, that nonmetals will tend to form acids, and metals will tend to form bases. We can conclude, then, that nonmetals will tend to form acids, and metals will tend to form bases.

Amphoteric Behavior Some substances can react as either an acid or a base. Some substances can react as either an acid or a base. If one of these substances is in the presence of a proton donor, then it reacts as a base. If one of these substances is in the presence of a proton donor, then it reacts as a base. In the presence of a proton acceptor, it acts as an acid. In the presence of a proton acceptor, it acts as an acid. Such a substance is said to be amphoteric. Such a substance is said to be amphoteric. Water is the most common amphoteric substance. Water is the most common amphoteric substance. HCl (proton donor) + H 2 O (base) ---> H Cl - HCl (proton donor) + H 2 O (base) ---> H Cl - NH 3 (proton acceptor) + H 2 O (acid) ---> NH OH - NH 3 (proton acceptor) + H 2 O (acid) ---> NH OH -

Acid-Base Neutralization An acid is composed of positive hydrogen ions combined with negative nonmetallic ions. An acid is composed of positive hydrogen ions combined with negative nonmetallic ions. Metallic bases are composed of negative hydroxide ions combined with positive metallic ions. Metallic bases are composed of negative hydroxide ions combined with positive metallic ions. An acid reacts with a base to form a salt and water. An acid reacts with a base to form a salt and water. –The water is formed from the hydrogen ion of the acid and the hydroxide ion of the base. –If the water is evaporated, the negative ions of the acid will unite with the positive ions of the base to form a new compound called a salt. It would appear such a reaction should result in removal of all hydrogen and hydroxide ions from solution. It would appear such a reaction should result in removal of all hydrogen and hydroxide ions from solution. –The resulting solution should neither an acid nor a base. –We could say that the solution is neutral (neither acidic nor basic). The reaction of an acid and a base is called a neutralization reaction. The reaction of an acid and a base is called a neutralization reaction.

Definition of a Salt A salt is a crystalline compound composed of the negative ion of an acid and the positive ion of a base. A salt is a crystalline compound composed of the negative ion of an acid and the positive ion of a base. For example, if equivalent amounts of chloric acid and sodium hydroxide react, sodium chlorate and water are formed. For example, if equivalent amounts of chloric acid and sodium hydroxide react, sodium chlorate and water are formed. HCl0 3 (aq) + NaOH(aq) ---> NaClO 3 (aq) + H 2 0(l) HCl0 3 (aq) + NaOH(aq) ---> NaClO 3 (aq) + H 2 0(l) acid + base ---> a salt + water acid + base ---> a salt + water Recall: Binary acids (prefix hydro-, suffix -ic) form salts ending in -ide. As an example, hydrochloric acid forms chloride salts. Ternary acids form salts in which -ic acids form -ate salts and -ous acids form -ite salts. Prefixes from the acid names remain in the salt names. Recall: Binary acids (prefix hydro-, suffix -ic) form salts ending in -ide. As an example, hydrochloric acid forms chloride salts. Ternary acids form salts in which -ic acids form -ate salts and -ous acids form -ite salts. Prefixes from the acid names remain in the salt names.