Chemistry - A Molecular Science

Chapter 12 Acid-Base Chemistry

282

Any acid (HA) that is above H

O 3

1+ in Table 12.3 is a strong acid (K

1. and reacts a

extensively with water to produce H

O 3

1+ and its conjugate base (A

1-

), which is why

solutions of strong acids are strong electrolytes. Just as a solution of sodium chloride is represented as Na

1+ + Cl

1-, hydrochloric acid is represented as H

O 3

1+ + Cl

1-. Hydrofluoric

acid is a weak acid (K

<< 1) and a weak electrolyte (onla

y about 4% of the molecules in a

0.1 M solution of HF are in the form of H

O 3

1+ and F

1- ions), so it is written as HF,

not

as

H^3

1+O

+ F

1- because HF represents the predominant species in an aqueous solution.

A

solution of a strong acid is represented by the hydronium ion and its conjugate base, but a solution of a weak acid is written as the unreacted (undissociated) acid.

Note that in each

case, the solution is represented by the principl

e species present; that is, the one(s) present

with the greatest concentration(s).

Weaker Bases Stronger Bases

Stronger Acids Weaker Acids

Table 12.3

An Acid-Base Table

Acid

Ka

Base

HClO

>>1 ClO 4

1- 4

HX

>>1

1-X

(X=I,Br,Cl)

H^2

SO

>>1 HSO 4

1- 4

HNO

>>1 NO 3

1- 3

H^3

1+O

1.0 H

O 2

H^2

SO

1.5x10 3

-2 HSO

1- 3

HSO

1- 4

1.2x10

-2 SO

2- 4

H^3

PO

7.5x10 4

-3 H

PO 2

1- 4

HF 7.2x10

-4 F

1-^

HNO

4.0x10 2

-4 NO

1- 2

HC

H 2

O 3

1.8x10 2

-5 C

H 2

O 3

1- 2

H^2

CO

4.3x10 3

-7 HCO

1- 3

HSO

1- 3

1.0x10

-7 SO

2- 3

H^2

S 1.0x10

-7 HS

1-^

H^2

PO

1- 4

6.2x10

-8 HPO

2- 4

HClO 3.5x10

-8 ClO

1-^

NH

1+ 4

5.6x10

-10

NH

(^3)
HCN 4.0x10
-10
CN
1-^
HCO
1- 3
4.7x10
-11
CO
2- 3
HPO
2- 4
4.8x10
-13
PO
3- 4
HS
1- 1.3x10
-13
S
2-^
H^2
O 1.0x10
-14
OH
1-^
NH
<<10 3
-14
NH
1- 2
OH
1-^
<<1
-14 0
2-O
Example 12.11
Write net equations for the following
reactions. Determine the value of the
equilibrium constant. Use a single arrow for reactions in which K
≥^10
3.
a) Solutions of barium hydroxide and nitric acid are mixed.
Ba
2+ + OH
1- + H
O 3
1+ + NO
1- 3
1
Nitric acid is a strong acid, so it must be written as H
O 3
1+ + NO
1- 3
.
2

1. All nitrates are soluble (Solubility Rule 2), so Ba

2+ and NO

1- 3

do not react; they are

spectator ions.

2. H

O 3

1+ is the strongest acid that can exist in water (all of the acids above H

O 3

1+ react

with water to produce H

O 3

1+), and OH

1- is the strongest base in water (any base

below OH

1- reacts with water to produce OH

1-). Consequently, they react extensively.

H

O 3

1+ + OH

1-^

→

H

O + H 2

O 2

Acids and bases that lie between the lines drawn above water as a base and below water as an acid exist in aqueous solution,

i.e.

,

they are weak acids and bases. Those above or below the lines react extensively with water to produce either H

O 3

1+ or OH

1-;

i.e.

,

they are strong acids and bases. The arrows on the sides indicate the result of extensive proton transfer: stronger acids are co

nverted into weaker acids and

stronger bases are converted into weaker bases.

The preceding chemical equation is the general equation for the reaction of any strong acid and any strong base.

H

O 3

1+ is the reacting acid and H

O is the produced acid, so K 2

= (1.0)/(1.0x10

-14

) = 1.0x10

+14

, which is so large that the reaction is usually written with a

single arrow rather than double, equilibrium arro

ws; but it is still an equilibrium process.

In the remainder of this example, the ions and/or molecules that comprise the solutions are shown in the margin.

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North

Carolina

State

University