Tuesday, April 7, 2015

Hard and Soft Acids and Bases

The ease with which an acid-base reaction occurs depends on the strength of both the acid and the base. Strong acids and bases are generally more reactive than weak acids and bases. However, the direction of the reaction and the stability of the products often depend on another quality—the hardness or softness of the acid and base. Although chemists have not created a quantitative measure to describe the qualities that makes an acid or base hard or soft, they do describe them qualitatively. As you look at the following list of characteristics that describe hard and soft acids and bases, remember that an acid has an empty orbital and an unfilled valence shell, and a base has in its valence shell a pair of nonbonding electrons that is available for donation.


Soft Acids. For soft acids, the electron-pair acceptor atoms are large, have a low positive charge density, and contain unshared pairs of electrons in their valence shells. The unshared pairs of electrons are in the p or d orbitals. Also, soft acids have a high polarizability and a low electronegativity. In organic chemistry, the soft acids usually include only the halogens, phosphorus, and sulfur compounds.


Hard Acids. For hard acids, the acceptor atoms are small, have a high positive charge density, and contain no unshared pairs of electrons in their valence shells. They have a low polarizability and a high electronegativity. The hydrogen ion is a good example of a hard acid.

Soft Bases. For soft bases, the donor atoms hold their valence electrons loosely. They have high polarizability, low negative charge density, and low electronegativity. Common soft bases are the cyanide (-CN) and iodide (I-) ions.
Hard Bases. For hard bases, the donor atoms are small, have a high negative charge density, and hold their valence electrons tightly. They have a low polarizability and a high electronegativity. The hydroxide ion is a good example of a hard base.
To visualize a polarizable atom, imagine that an atom is a large floppy ball and you are holding it cupped in both hands. The ball tends to be spherical, but, as you shift one hand higher than the other, it easily deforms. If you raise your left hand a little, the portion of the ball in your right hand becomes larger. Then, if you raise your right hand a little, the portion of the ball in your left hand becomes larger. A polarizable atom shifts its electron density from one part of the atom to another: at one instant, one portion of the atom has the higher electron density; then the next instant, another portion has the higher electron density.


For the concepts of hardness or softness of acids and bases to be of value to you, you must be able to differentiate between them. To do this, your most useful tool is the periodic table. A general rule is that hardness goes to softness moving from the top to the bottom on the periodic table because the size of the atoms increases with increasing numbers of electrons. A larger acid or base has a lower charge density and is more polarizable. For example, base softness in Group VII A on the periodic table decreases in this order: I- > Br- > Cl- > F-. Also, the elements on the left side tend to be acids, and elements on the right side tend to be bases. In this way, chemists approximately rank acids and bases in order of hardness or softness. Base softness within a period on the periodic table decreases in order of increasing electronegativity; for example, -CH3 > -NH2 > -OH > F.
Hardness and softness are difficult to quantify. Rather than relying specifically on these types of sequences, chemists divide acids and bases into three groups: (1) hard acids or bases, (2) soft acids or

bases, and (3) borderline acids or bases. Table 5.2 lists a few examples of each category.

H⊕ is a hard acid because it has no electrons and has a high positive charge density. The HO- ion is a soft base because it has a pair of electrons and only one proton, so it holds the electrons rather loosely. Thus, it is quite polarizable and soft.






No comments:

Post a Comment