Chemistry - A Molecular Science

b) Solutions of sodium acetate and hydrochloric acid are mixed.

(^1) Na
1+ + C
H 2
O 3
1- 2

• H
  O 3
  1+ + Cl
  1-^
  2
  Hydrochloric acid is a strong acid and is written as H
  O 3
  1+ + Cl
  1-.

1. Sodium chloride is soluble (Solubility Rule 1), so Na

1+ and Cl

1- are spectator ions.

2. Acetate ion is a weak bas

e, but it is well below H

O 3

1+ on the acid-base chart.

Consequently, an extensive proton transfer reaction occurs.

OH

HH

O C

OCH

3

HO

H

HO

C

CH

3

O

+

H^3

1+ O

+ C

H 2

O 3

1- 2

→

H

O + HC 2

H 2

O 3

(^2)
This reaction is representative
of a strong acid reacting with a weak base. These types of
reactions are extensive and are, therefore, usually written with single arrows. For the above reaction, K = (1.0)/(1.8x10
-5) = 5.6x10
4.
c) Solutions of sodium hydroxide and hydrofluoric acid are mixed.
(^1) Na
1+ + OH
1- + HF
2
HF is a weak acid, so it is written in molecular form. 1. NaF is soluble (Solubility Rule 1), so Na
1+ is a spectator ion.

2. HF is a weak acid, but OH

1- is a strong base, located well below the weak acid, so an

extensive reaction is predicted. FH

O

H

F

H

O

H

+

HF + OH

1-^

→

F

1- + H

O 2

This is a reaction between a weak acid and a

strong base. Reactions of weak acids with

hydroxide ion are extensive because hydroxide ion is a very strong base. Consequently, they are typically written with a single arrow. K = (7.2x10

-4)/(1.0x10

-14) = 7.2x10

10.

d) Solutions of ammonium chloride and sodium cyanide are mixed.

(^1) NH
1+ 4

• Cl
  1- + Na
  1+ + CN
  1-^
  2

1. NH

CN is soluble (Solubility Rule 1), but NH 4

1+ 4

is also a weak acid and CN

1- is a weak

base. Checking the acid-bas

e table, we see that NH

1+ 4

and CN

1- are close to one

another, so we expect the equilibr

ium constant to be close to 1.

2.^

NaCl is soluble, so Na

1+ and Cl

1- are spectator ions.

H

HHN H

CN

HH N H

CN

H

NH

1+ 4

+ CN

1-^

U

NH

+ HCN 3

This is an example of a reaction of a weak

acid and a weak base. Double arrows are

used because the reacting acid (NH

1+ 4

) and the produced acid (HCN) are of comparable

strengths, so the reverse reaction is important

in determining the amount of product. For

this reaction, K = (5.6x10

-10)/4.0x10

-10

) = 1.4, so [NH

][HCN] ~ [NH 3

1+ 4

][CN

1-] at equilibrium.

Chapter 12 Acid-Base Chemistry

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State

University