Page 101 - Basic physical chemistry for the atmospheric sciences

Acids and bases 1<7

Lowry to propose a more general view of acids and bases, in whid1
acids tend to donate protons and bases tend to accept protons. From
this viewpoint, in both Reactions (5.5) and (5.6) HCI acts as an acid,
and H20 in Reaction (5.5) and NH3 in Reaction (5.6) act as bases. As
indicated by the two-way arrows in Reaction (5.5), H30 +(aq) may
-
donate a proton and Cl ( aq) may accept a proton. In this case,
c1 - (aq) is the base and H30 + (aq) the acid. Therefore, we could write
HCl(aq) + H20(1) � H 0 +(aq) + Cl - (aq) (5. 7 )
3
acid I + base 2� acid 2 + base 1
where, HCl(aq)) and c1 -(aq), which differ only by a proton, are called
the conjugate acid-base pair for the forward reaction (indicated by l ) ,
and H30+ (aq) and H20(1) are the conjugate acid-base pair fo r the
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reverse reaction (indicated by 2). In Reaction ( . 6), HCl and c1- are
the conjugate acid-base pair for the forward reaction, and N H 1 and
NH3 are the con u gate acid-base pair for the reverse reaction.
j

5.4 The Lewis theory
An even more general theory of acids and bases was given by the
American chemist G. N . Lewis in 1 9 23 . In this theory, an acid is an
3
electron acceptor and a base is an electron donor. This is a more
general theory than the Bqzlnsted-Lowry theory, because it allows the
acid-base classification to be applied to reactions in which neither
H + (aq) nor OH - ( aq) play a role, or even to reactions in which there
is no solvent. For example, the following are acid-base reactions in
the Lewis theory
Ag+(aq) + 2CN - (aq)� Ag(CN) 2 (aq)
acid base
2
Zn(s) + Cu + (aq)� Zn 2 + (aq) + Cu(s)
base acid

5.5 Strengths of acids and bases; acid-dissociation
(or ionization) constant
The BrS1)nsted-Lowry view of acids and bases suggests that the
strengths of acids can be compared by measuring their relative tenden­
s
cies to release a proton to a common base (taken to be water). Thu , if
we represent an acid by HA and consider its reaction with water