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Salt (chemistry)

Contributed by: Alfred B. Garrett

Publication year: 2014

A compound formed when one or more of the hydrogen atoms of an acid are replaced by one or more cations of the base. The common example is sodium chloride in which the hydrogen ions of hydrochloric acid are replaced by the sodium ions (cations) of sodium hydroxide. There is a great variety of salts because of the large number of acids and bases which has become known.

Classification

Salts are classified in several ways. One method—normal, acid, and basic salts—depends upon whether all the hydrogen ions of the acid or all the hydroxide ions of the base have been replaced (see Table 1):

The other method—simple salts, double salts (including alums), and complex salts—depends upon the character of completeness of the ionization (see Table 2):

In general, all salts in solution will give ions of each of the metal ions; an exception is the complex type of salt such as K, 3 Fe(CN), 6 and K, 2 Cr, 2 O, 7 . In such salts the ionization is entirely as shown by reactions (1) and (2).

( 1 ) Image of Chem Equation 1

Image of Chem Equation 2 (2)

3+ 3+ No detectable quantities of Fe, or Cr, from these salts exist in solution because of the strong bonding of these ions in the complex ions. However, in those complex salts where the bonding is weak, ions of the metal can be detected; for example, in Na, 2 CdCl, 4 , the cadmium complex ion ionizes appreciably as in reaction (3).

Image of Chem Equation 3 (3)

The elements with unfilled inner electron shells form complex salts readily. The alum type of salt is a sulfate 3+ 3+ 3+ including the univalent cation of a relatively strong base and a trivalent metal ion such as Al, , Fe, , or Cr, . AccessScience from McGraw-Hill Education Page 2 of 4 www.accessscience.com

TABLE 1.

Class , Examples ,

Normal salts , NaCl, NH, 4 Cl, Na, 2 SO, 4 , Na, 2 CO, 3 , Na, 3 PO, 4 , Ca, 3 (PO, 4 ,) 2 ,

Acids salts , NaHCO, 3 , NaH, 2 PO, 4 , Na, 2 HPO, 4 , NaHSO, 4 ,

Basic salts , Pb(OH)Cl, Sn(OH)Cl ,

TABLE 2.

Class , Examples ,

Simple salts , NaCl, NaHCO, 3 , Pb(OH)Cl ,

⋅ Double salts , KCl MgCl, 2 ,

Alums , KAl(SO, 4 ,) 2 , NaFe(SO, 4 ,) 2 , NH, 4 Cr(SO, 4 ,) 2 ,

Complex salts , K, 3 Fe(CN), 6 , Cu(NH, 3 ,) 4 Cl, 2 , K, 2 Cr, 2 O, 7 ,

Double salts include ions of nearly enough the same size to fit into the same crystal lattice.

Hydrolysis

The solutions of some normal salts are neutral, but those of others are acidic or basic. This results from the reactor of the ions of salt with water. This reaction is called hydrolysis. Examples are shown in reactions (4)–(6).

Image of Chem Equation(4 4)

Image of Chem Equation(5) 5

Image of Chem Equation 6( 6)

The resulting solution will be acidic or basic, depending upon whether the hydrolysis produces an excess of hydrogen or hydroxide ion. See also: HYDROLYSIS . AccessScience from McGraw-Hill Education Page 3 of 4 www.accessscience.com

Theories of acids and definition of salts

The development of more general theories of acids and bases in the twentieth century has required a broadening of the concept of salts.

The Br onsted¨ theory of acids lays emphasis on the process of the reaction between acids and bases, and not so much on the product except that the products are other acids and bases, as in reactions (7) and (8).

Image of Chem Equation 7(7 )

Image of Chem Equation 8 (8)

Since the Br onsted¨ theory extends the proton theory of acids to solvents other than water, the original definition of a salt must be expanded as follows: A salt is an electrovalent compound that contains some cation other than the solvated proton and some anion other than the anion which is the conjugate base of the solvent. In the water ,+ ,− system the salt should not contain the H, 3 O and OH ions alone; in the liquid ammonia system it should not ,+ ,− contain the NH, 4 and NH, 2 ions alone.

In terms of the Lewis theory of acids and bases, compound ( 1 ) is an acid, and compound ( 2 ) is a base. Hence the salt should be compound ( 3 ).

( 1 )

Image of Chemical Structure

( 2 )

Image of Chemical Structure AccessScience from McGraw-Hill Education Page 4 of 4 www.accessscience.com

( 3 )

Image of Chemical Structure

+ Here the salt is not limited to replacement of the H, with a cation of a base. Rather, it is any aggregate of molecules, atoms, or ions joined together with a coordinate covalent bond. Such compounds correctly can be called salts; however, by common parlance, the term salt usually refers to an electrovalent compound, the classical example of which is sodium chloride. See also: ACID AND BASE ; CHEMICAL BONDING .

Alfred B. Garrett

Bibliography

R. Chang and J. Overby, General Chemistry: The Essential Concepts , 6th ed., McGraw-Hill, 2010

Additional Readings

C. L. Cooke et al., Binary and ternary phase diagrams as routes to salt discovery: Ephedrine and pimelic acid,

Cryst. Growth Des. , 10(12):5270–5278, 2010 DOI: http://doi.org/10.1021/cg1011296

D. D. Ebbing and S. D. Gammon, General Chemistry , 9th ed., Brooks ∕ Cole, 2011

N. Griffete et al., Preparation of water-soluble magnetic nanocrystals using aryl diazonium salt chemistry, J. Am.

Chem. Soc. , 133(6):1646–1649, 2011 DOI: http://doi.org/10.1021/ja108928b

S. L. Seager and M. R. Slabaugh, Introductory Chemistry for Today , Brooks ∕ Cole, 2011

A. Singhal, The Pearson Guide to Objective Chemistry for the AIEEE , 2d ed., Dorling Kindersley, 2011