Chapter 12 Acid-Base Chemistry

Combining Equation 12.1 and our rule that

Brønsted acid-base reactions for which K

10

3 are extensive and can be written with singl

e arrows, we can state that a reaction can

be written with a single arrow when the K

of the reacting acid is at least 10a

3 times greater

than that of the produced acid. As an example

of the application of Equation 12.1, consider

the following reaction:

HF(aq) + ClO

1-(aq)

U

1- F

(aq)+ HClO

(aq)

HF (K

= 7.2x10a

-4) is the reacting acid and HClO (K

= 3.5x10a

-8) is the produced acid.

Ka

(HF) > K

(HClO), so the reacting acid is stronger a

than the produced acid. We conclude

that the equilibrium constant for the reaction between HF and ClO

1- is large, and the

proton transfer is extensive. The value of the equilibrium constant can be determined with Equation 12.1 as follows:

K

(HF)

K =

a

Ka

(HOCl)

=

7.2

×^10

-4

3.5

×^10

-8

= 2.1

×^10

(^4)
K > 10
3 , so the reaction is extensive and could be written with a single arrow. That the
above is indeed the equilibrium constant for
the reaction can be verified by dividing the K
(^) a
expressions
1+
3
a
a
[H O ]
1
K = K (HF)

×K (HOCl)
[
1-
1+
3
[F ]
HOCl]
[HF]
×[H O ]
1-[][
1-
1-
FHOC
(^) l]

[OCl ]
[HF][OCl ]
The expression is indeed the equilibrium
constant expression for the reaction.
Example 12.10
Ka
values: HCN = 4.0x10
-10
; HNO
= 4.0x10 2
-4; HF = 7.2x10
-4; NH
1+ 4
= 5.6x10
-10.
a) What is the strongest acid? Which has the strongest conjugate base?
The acid with the largest K
is HF, so it is the strongest acid. The acid with the smallest Ka
(^) a
is HCN, so it is the weakest acid. C
onjugate base strengths ar
e opposite the acid
strengths, so F
1- ion would be the weakest base and CN
1- ion the strongest base.
b) Indicate whether each of the following aqueous reactions is extensive and could
be represented with a single arrow. i) HCN + NH
U 3
CN
1- + NH
1+ 4
We can use the K
values to see that NHa
1+ 4
ion, the produced acid, is a stronger acid than
HCN, the reacting acid, so the reaction is not
extensive. Using Equation 12.1 we obtain
-10
a
1+
-10
K (HCN)a4
4.0 10
K =

= 0.71
K(NH )
5.6 10
× ×
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