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Grade: 11


Plz explain hard and soft acids and bases.

11 years ago

Answers : (1)

askIITIians Expert
21 Points

The Lewis defination recognizes acids and bases in terms of their ability to accept or donate electron pairs. as such the strength of any acid or base is determined by the very nature of the reation involved in any particular electron transfer process. Accordingly, assignment of any single consistent crirerion for aid base strength becomes very difficult in th eLewis defination. Attempts have been made to correlate various factors from enthaly changes for acid base combinations.

The criterion of hardness (or softness) is ascribed to the "hardness" of the electron cloud associated with any species. A firmly held electron-cloud with low polarizability makes a species "hard", while an easily polarizable electron cloud characterizes the species as "soft". A third category with intermediate character appears in the border line.

The principal distinguishing features of hard and soft acid and bases may then be summarized as follows:

Hard acids

Soft acids

(i)                 Small size

(i)                 Large size

(ii)               High positive oxidation state, absence of any outer electron which are easily excited to higher states.

(iii)             Zero or low positive oxidation state, several easily excitable valence electrons.


11 years ago
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