化学专业英语之路易斯酸碱理论
BRONSTED'S AND LEWIS' ACID-BASE CONCEPTS
Bronsted—Lowry Concept (1923), According to this concept, an acid is a substance that releas protons (a proton donor)and a ba is a substance that combines with protons (a proton acceptor). Neutralization reactions involve the transfer of protons. Salts are merely aggregates of ions that are produced in some but not all neutralization reactions.
Upon loss of a proton, an acid forms a ba, since by the rever reaction the substance formed can gain a proton. In the same way a ba forms an acid upon gaining a proton. The relationships can be reprented by equations, and the sum is an equation for a neutralization reaction.
We e, then, that a neutralization reaction is a competition for protons between two bas. The acid and ba reprented in Equation 1 (and Equation 2)are a conjugate pair; that is an acid forms its conjugate ba upon loss of a proton and a ba forms its conjugate acid when it gains proton. Substances with the same subscript in Equation 3 are conjugate pairs. This concept can be illustrated by some examples.
The first equation is a neutralization reaction according to the Arrhenius concept as well as Bronsted — Lowry but, since Arrhenius recognized only the ba OH-, the other reacti
ons are excluded according to his concept1. Note that in the cond and third reactions that water acts as a ba and an acid, respectively. Water, like many other substances that can both donate and accept protons, is amphoteric according to this concept. It functions as an acid in the prence of bas stronger than itlf(NH3,for example)and as a ba in the prence of an acid stronger than itlf(HCl, for example). The positions of the equilibria given above depend upon the relative proton — donating ability of the two acids in each ca (or the relative proton-accepting ability of the two bas).
The relationships allow the establishment of scales of acid and ba strength. Relative acid strength of proton donors is measured by the extent to which reactions with a common ba proceed at equilibrium2. Relative ba strength of proton acceptors is established in similar fashion. Table 8. lists veral familiar substances arranged in the order of decreasing acid strength. Since the conjugate ba of a weak acid is strong, and vice versa, the bas formed in the reactions given are in the rever order by strength; that is, the strongest ba is at the bottom of the table.
Table 8. Acid—Ba Reactions in water
The relative strength of a ba, according to the Bronsted concept, is a measure of its ability to accept a proton. Between any two substances that compete for protons, the one that gains protons over the other is more basic. For example, water may compete with th
e chloride ion for a proton, but Cl~ is so weak as a ba that water easily accepts a proton and forms H3O+ in this reaction:
HCl + H2O------→H3O + +Cl-
On the other hand, water los out in the competition for protons when water competes with ammonia for protons. Ammonia takes protons from the water, forcing the water to act as an acid in the reaction
NH3+H2O-----→NH4+ + OH-
The relative strength of the competitors for protons can be derived from Table 8. The strongest proton acceptors are the bas at the bottom of the list, while the strongest acids or proton donors are acids at the top of the list.
Lewis Concept (1923). G. N. Lewis propod another acid-ba concept in the same year that the Bronsted-Lowry concept appeared. Lewis defined an acid as a molecule or i
on that can accept an electron pair from another molecule or ion, and a ba as a substance that can share its electron pair with an acid. Thus, an acid is an electron-pair acceptor and a ba is an electron-pair donor. When a Lewis acid reacts with a Lewis ba, conquently, a coordinate covalent bond is formed. This is esntially the neutralization reaction.
Electron dot formulas clarify the mechanism of forming a coordinate bond in the neutralization of a Lewis acid. Neutralization of a proton by an ammonia molecule is
The acid — ba pair Al3+ and H2O undergoes Lewis neutralization by forming six coordinate bonds.
Many displacement reactions illustrate the relative strength of the Lewis acid or ba. For example, the cyanide ion is a stronger ba than the fluoride ion by virtue of its ability to d
isplace the fluoride ion from the hexafluoroferrate (III) ion: FeF63- + 6CN- ------→ Fe(CN)63- + 6F-
There are many reactions that fit the Lewis acid-ba concept. In contrast to proton loss or gain in the Bronsted-Lowry concept, the Lewis concept emphasizes the electron pair ------a Lewis acid lacks an electron pair in an empty orbital, or has an orbital that can be vacated and a Lewis ba has a nonbonding electron pair and can supply this pair to another substance lacking an electron pair.
It is evident that the Lewis concept applies, not only to the chemical behavior correlated by the Bronsted-Lowry concept, but also to many chemical reactions that do not involve proton transfer, and for this reason it is most uful. Its generality precludes the establishment of a scale of acid and ba strengths for all Lewis acids and bas, but comparisons can be made between lected substances.
